Type subsumption and unification
\begin{code}
-{-# OPTIONS_GHC -w #-}
--- The above warning supression flag is a temporary kludge.
--- While working on this module you are encouraged to remove it and fix
--- any warnings in the module. See
--- http://hackage.haskell.org/trac/ghc/wiki/WorkingConventions#Warnings
--- for details
-
module TcUnify (
- -- Full-blown subsumption
- tcSubExp, tcFunResTy, tcGen,
- checkSigTyVars, checkSigTyVarsWrt, bleatEscapedTvs, sigCtxt,
+ -- Full-blown subsumption
+ tcWrapResult, tcSubType, tcGen,
+ checkConstraints, newImplication, sigCtxt,
- -- Various unifications
- unifyType, unifyTypeList, unifyTheta,
- unifyKind, unifyKinds, unifyFunKind,
- checkExpectedKind,
- preSubType, boxyMatchTypes,
+ -- Various unifications
+ unifyType, unifyTypeList, unifyTheta, unifyKind,
--------------------------------
-- Holes
- tcInfer, subFunTys, unBox, refineBox, refineBoxToTau, withBox,
- boxyUnify, boxyUnifyList, zapToMonotype,
- boxySplitListTy, boxySplitTyConApp, boxySplitAppTy,
- wrapFunResCoercion
+ tcInfer,
+ matchExpectedListTy, matchExpectedPArrTy,
+ matchExpectedTyConApp, matchExpectedAppTy,
+ matchExpectedFunTys, matchExpectedFunKind,
+ wrapFunResCoercion, failWithMisMatch
) where
#include "HsVersions.h"
import HsSyn
import TypeRep
-
+import CoreUtils( mkPiTypes )
+import TcErrors ( unifyCtxt )
import TcMType
-import TcSimplify
-import TcEnv
-import TcTyFuns
import TcIface
-import TcRnMonad -- TcType, amongst others
+import TcRnMonad
import TcType
import Type
import Coercion
-import TysPrim
import Inst
import TyCon
import TysWiredIn
import VarEnv
import Name
import ErrUtils
-import Maybes
import BasicTypes
+import Maybes ( allMaybes )
import Util
import Outputable
-import Unique
-\end{code}
-
-%************************************************************************
-%* *
-\subsection{'hole' type variables}
-%* *
-%************************************************************************
+import FastString
-\begin{code}
-tcInfer :: (BoxyType -> TcM a) -> TcM (a, TcType)
-tcInfer tc_infer
- = do { box <- newBoxyTyVar openTypeKind
- ; res <- tc_infer (mkTyVarTy box)
- ; res_ty <- {- pprTrace "tcInfer" (ppr (mkTyVarTy box)) $ -} readFilledBox box -- Guaranteed filled-in by now
- ; return (res, res_ty) }
+import Control.Monad
\end{code}
%************************************************************************
-%* *
- subFunTys
-%* *
+%* *
+ matchExpected functions
+%* *
%************************************************************************
-\begin{code}
-subFunTys :: SDoc -- Somthing like "The function f has 3 arguments"
- -- or "The abstraction (\x.e) takes 1 argument"
- -> Arity -- Expected # of args
- -> BoxyRhoType -- res_ty
- -> ([BoxySigmaType] -> BoxyRhoType -> TcM a)
- -> TcM (HsWrapper, a)
--- Attempt to decompse res_ty to have enough top-level arrows to
--- match the number of patterns in the match group
---
--- If (subFunTys n_args res_ty thing_inside) = (co_fn, res)
--- and the inner call to thing_inside passes args: [a1,...,an], b
--- then co_fn :: (a1 -> ... -> an -> b) ~ res_ty
---
--- Note that it takes a BoxyRho type, and guarantees to return a BoxyRhoType
+Note [Herald for matchExpectedFunTys]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The 'herald' always looks like:
+ "The equation(s) for 'f' have"
+ "The abstraction (\x.e) takes"
+ "The section (+ x) expects"
+ "The function 'f' is applied to"
+This is used to construct a message of form
-{- Error messages from subFunTys
-
- The abstraction `\Just 1 -> ...' has two arguments
+ The abstraction `\Just 1 -> ...' takes two arguments
but its type `Maybe a -> a' has only one
The equation(s) for `f' have two arguments
The function 'f' is applied to two arguments
but its type `Int -> Int' has only one
--}
+Note [matchExpectedFunTys]
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+matchExpectedFunTys checks that an (Expected rho) has the form
+of an n-ary function. It passes the decomposed type to the
+thing_inside, and returns a wrapper to coerce between the two types
+
+It's used wherever a language construct must have a functional type,
+namely:
+ A lambda expression
+ A function definition
+ An operator section
+
+This is not (currently) where deep skolemisation occurs;
+matchExpectedFunTys does not skolmise nested foralls in the
+expected type, becuase it expects that to have been done already
-subFunTys error_herald n_pats res_ty thing_inside
- = loop n_pats [] res_ty
- where
- -- In 'loop', the parameter 'arg_tys' accumulates
- -- the arg types so far, in *reverse order*
- -- INVARIANT: res_ty :: *
- loop n args_so_far res_ty
- | Just res_ty' <- tcView res_ty = loop n args_so_far res_ty'
-
- loop n args_so_far res_ty
- | isSigmaTy res_ty -- Do this before checking n==0, because we
- -- guarantee to return a BoxyRhoType, not a BoxySigmaType
- = do { (gen_fn, (co_fn, res)) <- tcGen res_ty emptyVarSet $ \ _ res_ty' ->
- loop n args_so_far res_ty'
- ; return (gen_fn <.> co_fn, res) }
-
- loop 0 args_so_far res_ty
- = do { res <- thing_inside (reverse args_so_far) res_ty
- ; return (idHsWrapper, res) }
-
- loop n args_so_far (FunTy arg_ty res_ty)
- = do { (co_fn, res) <- loop (n-1) (arg_ty:args_so_far) res_ty
- ; co_fn' <- wrapFunResCoercion [arg_ty] co_fn
- ; return (co_fn', res) }
-
- -- res_ty might have a type variable at the head, such as (a b c),
- -- in which case we must fill in with (->). Simplest thing to do
- -- is to use boxyUnify, but we catch failure and generate our own
- -- error message on failure
- loop n args_so_far res_ty@(AppTy _ _)
- = do { [arg_ty',res_ty'] <- newBoxyTyVarTys [argTypeKind, openTypeKind]
- ; (_, mb_coi) <- tryTcErrs $ boxyUnify res_ty (FunTy arg_ty' res_ty')
- ; if isNothing mb_coi then bale_out args_so_far
- else do { case expectJust "subFunTys" mb_coi of
- IdCo -> return ()
- ACo co -> traceTc (text "you're dropping a coercion: " <+> ppr co)
- ; loop n args_so_far (FunTy arg_ty' res_ty')
- }
- }
-
- loop n args_so_far (TyVarTy tv)
- | isTyConableTyVar tv
- = do { cts <- readMetaTyVar tv
- ; case cts of
- Indirect ty -> loop n args_so_far ty
- Flexi -> do { (res_ty:arg_tys) <- withMetaTvs tv kinds mk_res_ty
- ; res <- thing_inside (reverse args_so_far ++ arg_tys) res_ty
- ; return (idHsWrapper, res) } }
- where
- mk_res_ty (res_ty' : arg_tys') = mkFunTys arg_tys' res_ty'
- mk_res_ty [] = panic "TcUnify.mk_res_ty1"
- kinds = openTypeKind : take n (repeat argTypeKind)
- -- Note argTypeKind: the args can have an unboxed type,
- -- but not an unboxed tuple.
-
- loop n args_so_far res_ty = bale_out args_so_far
-
- bale_out args_so_far
- = do { env0 <- tcInitTidyEnv
- ; res_ty' <- zonkTcType res_ty
- ; let (env1, res_ty'') = tidyOpenType env0 res_ty'
- ; failWithTcM (env1, mk_msg res_ty'' (length args_so_far)) }
-
- mk_msg res_ty n_actual
- = error_herald <> comma $$
- sep [ptext SLIT("but its type") <+> quotes (pprType res_ty),
- if n_actual == 0 then ptext SLIT("has none")
- else ptext SLIT("has only") <+> speakN n_actual]
-\end{code}
\begin{code}
-----------------------
-boxySplitTyConApp :: TyCon -- T :: k1 -> ... -> kn -> *
- -> BoxyRhoType -- Expected type (T a b c)
- -> TcM [BoxySigmaType] -- Element types, a b c
- -- It's used for wired-in tycons, so we call checkWiredInTyCOn
- -- Precondition: never called with FunTyCon
- -- Precondition: input type :: *
-
-boxySplitTyConApp tc orig_ty
- = do { checkWiredInTyCon tc
- ; loop (tyConArity tc) [] orig_ty }
+matchExpectedFunTys :: SDoc -- See Note [Herald for matchExpectedFunTys]
+ -> Arity
+ -> TcRhoType
+ -> TcM (Coercion, [TcSigmaType], TcRhoType)
+
+-- If matchExpectFunTys n ty = (co, [t1,..,tn], ty_r)
+-- then co : ty ~ (t1 -> ... -> tn -> ty_r)
+--
+-- Does not allocate unnecessary meta variables: if the input already is
+-- a function, we just take it apart. Not only is this efficient,
+-- it's important for higher rank: the argument might be of form
+-- (forall a. ty) -> other
+-- If allocated (fresh-meta-var1 -> fresh-meta-var2) and unified, we'd
+-- hide the forall inside a meta-variable
+
+matchExpectedFunTys herald arity orig_ty
+ = go arity orig_ty
where
- loop n_req args_so_far ty
- | Just ty' <- tcView ty = loop n_req args_so_far ty'
+ -- If go n ty = (co, [t1,..,tn], ty_r)
+ -- then co : ty ~ t1 -> .. -> tn -> ty_r
- loop n_req args_so_far (TyConApp tycon args)
- | tc == tycon
- = ASSERT( n_req == length args) -- ty::*
- return (args ++ args_so_far)
+ go n_req ty
+ | n_req == 0 = return (mkReflCo ty, [], ty)
- loop n_req args_so_far (AppTy fun arg)
- | n_req > 0
- = loop (n_req - 1) (arg:args_so_far) fun
+ go n_req ty
+ | Just ty' <- tcView ty = go n_req ty'
+
+ go n_req (FunTy arg_ty res_ty)
+ | not (isPredTy arg_ty)
+ = do { (coi, tys, ty_r) <- go (n_req-1) res_ty
+ ; return (mkFunCo (mkReflCo arg_ty) coi, arg_ty:tys, ty_r) }
+
+ go _ (TyConApp tc _) -- A common case
+ | not (isSynFamilyTyCon tc)
+ = do { (env,msg) <- mk_ctxt emptyTidyEnv
+ ; failWithTcM (env,msg) }
- loop n_req args_so_far (TyVarTy tv)
- | isTyConableTyVar tv
- , res_kind `isSubKind` tyVarKind tv
+ go n_req ty@(TyVarTy tv)
+ | ASSERT( isTcTyVar tv) isMetaTyVar tv
= do { cts <- readMetaTyVar tv
; case cts of
- Indirect ty -> loop n_req args_so_far ty
- Flexi -> do { arg_tys <- withMetaTvs tv arg_kinds mk_res_ty
- ; return (arg_tys ++ args_so_far) }
- }
- where
- mk_res_ty arg_tys' = mkTyConApp tc arg_tys'
- (arg_kinds, res_kind) = splitKindFunTysN n_req (tyConKind tc)
+ Indirect ty' -> go n_req ty'
+ Flexi -> defer n_req ty }
+
+ -- In all other cases we bale out into ordinary unification
+ go n_req ty = defer n_req ty
+
+ ------------
+ defer n_req fun_ty
+ = addErrCtxtM mk_ctxt $
+ do { arg_tys <- newFlexiTyVarTys n_req argTypeKind
+ ; res_ty <- newFlexiTyVarTy openTypeKind
+ ; coi <- unifyType fun_ty (mkFunTys arg_tys res_ty)
+ ; return (coi, arg_tys, res_ty) }
+
+ ------------
+ mk_ctxt :: TidyEnv -> TcM (TidyEnv, Message)
+ mk_ctxt env = do { orig_ty1 <- zonkTcType orig_ty
+ ; let (env', orig_ty2) = tidyOpenType env orig_ty1
+ (args, _) = tcSplitFunTys orig_ty2
+ n_actual = length args
+ ; return (env', mk_msg orig_ty2 n_actual) }
+
+ mk_msg ty n_args
+ = herald <+> speakNOf arity (ptext (sLit "argument")) <> comma $$
+ sep [ptext (sLit "but its type") <+> quotes (pprType ty),
+ if n_args == 0 then ptext (sLit "has none")
+ else ptext (sLit "has only") <+> speakN n_args]
+\end{code}
- loop _ _ _ = boxySplitFailure (mkTyConApp tc (mkTyVarTys (tyConTyVars tc))) orig_ty
+\begin{code}
----------------------
-boxySplitListTy :: BoxyRhoType -> TcM BoxySigmaType -- Special case for lists
-boxySplitListTy exp_ty = do { [elt_ty] <- boxySplitTyConApp listTyCon exp_ty
- ; return elt_ty }
+matchExpectedListTy :: TcRhoType -> TcM (Coercion, TcRhoType)
+-- Special case for lists
+matchExpectedListTy exp_ty
+ = do { (coi, [elt_ty]) <- matchExpectedTyConApp listTyCon exp_ty
+ ; return (coi, elt_ty) }
+----------------------
+matchExpectedPArrTy :: TcRhoType -> TcM (Coercion, TcRhoType)
+-- Special case for parrs
+matchExpectedPArrTy exp_ty
+ = do { (coi, [elt_ty]) <- matchExpectedTyConApp parrTyCon exp_ty
+ ; return (coi, elt_ty) }
----------------------
-boxySplitAppTy :: BoxyRhoType -- Type to split: m a
- -> TcM (BoxySigmaType, BoxySigmaType) -- Returns m, a
--- If the incoming type is a mutable type variable of kind k, then
--- boxySplitAppTy returns a new type variable (m: * -> k); note the *.
--- If the incoming type is boxy, then so are the result types; and vice versa
-
-boxySplitAppTy orig_ty
- = loop orig_ty
+matchExpectedTyConApp :: TyCon -- T :: k1 -> ... -> kn -> *
+ -> TcRhoType -- orig_ty
+ -> TcM (Coercion, -- T a b c ~ orig_ty
+ [TcSigmaType]) -- Element types, a b c
+
+-- It's used for wired-in tycons, so we call checkWiredInTyCon
+-- Precondition: never called with FunTyCon
+-- Precondition: input type :: *
+
+matchExpectedTyConApp tc orig_ty
+ = do { checkWiredInTyCon tc
+ ; go (tyConArity tc) orig_ty [] }
where
- loop ty
- | Just ty' <- tcView ty = loop ty'
+ go :: Int -> TcRhoType -> [TcSigmaType] -> TcM (Coercion, [TcSigmaType])
+ -- If go n ty tys = (co, [t1..tn] ++ tys)
+ -- then co : T t1..tn ~ ty
- loop ty
- | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
- = return (fun_ty, arg_ty)
+ go n_req ty tys
+ | Just ty' <- tcView ty = go n_req ty' tys
- loop (TyVarTy tv)
- | isTyConableTyVar tv
+ go n_req ty@(TyVarTy tv) tys
+ | ASSERT( isTcTyVar tv) isMetaTyVar tv
= do { cts <- readMetaTyVar tv
- ; case cts of
- Indirect ty -> loop ty
- Flexi -> do { [fun_ty,arg_ty] <- withMetaTvs tv kinds mk_res_ty
- ; return (fun_ty, arg_ty) } }
- where
- mk_res_ty [fun_ty', arg_ty'] = mkAppTy fun_ty' arg_ty'
- mk_res_ty other = panic "TcUnify.mk_res_ty2"
- tv_kind = tyVarKind tv
- kinds = [mkArrowKind liftedTypeKind (defaultKind tv_kind),
- -- m :: * -> k
- liftedTypeKind] -- arg type :: *
- -- The defaultKind is a bit smelly. If you remove it,
- -- try compiling f x = do { x }
- -- and you'll get a kind mis-match. It smells, but
- -- not enough to lose sleep over.
-
- loop _ = boxySplitFailure (mkAppTy alphaTy betaTy) orig_ty
-
-------------------
-boxySplitFailure actual_ty expected_ty
- = unifyMisMatch False False actual_ty expected_ty
- -- "outer" is False, so we don't pop the context
- -- which is what we want since we have not pushed one!
-\end{code}
+ ; case cts of
+ Indirect ty -> go n_req ty tys
+ Flexi -> defer n_req ty tys }
+ go n_req ty@(TyConApp tycon args) tys
+ | tc == tycon
+ = ASSERT( n_req == length args) -- ty::*
+ return (mkReflCo ty, args ++ tys)
---------------------------------
--- withBoxes: the key utility function
---------------------------------
+ go n_req (AppTy fun arg) tys
+ | n_req > 0
+ = do { (coi, args) <- go (n_req - 1) fun (arg : tys)
+ ; return (mkAppCo coi (mkReflCo arg), args) }
-\begin{code}
-withMetaTvs :: TcTyVar -- An unfilled-in, non-skolem, meta type variable
- -> [Kind] -- Make fresh boxes (with the same BoxTv/TauTv setting as tv)
- -> ([BoxySigmaType] -> BoxySigmaType)
- -- Constructs the type to assign
- -- to the original var
- -> TcM [BoxySigmaType] -- Return the fresh boxes
-
--- It's entirely possible for the [kind] to be empty.
--- For example, when pattern-matching on True,
--- we call boxySplitTyConApp passing a boolTyCon
-
--- Invariant: tv is still Flexi
-
-withMetaTvs tv kinds mk_res_ty
- | isBoxyTyVar tv
- = do { box_tvs <- mapM (newMetaTyVar BoxTv) kinds
- ; let box_tys = mkTyVarTys box_tvs
- ; writeMetaTyVar tv (mk_res_ty box_tys)
- ; return box_tys }
-
- | otherwise -- Non-boxy meta type variable
- = do { tau_tys <- mapM newFlexiTyVarTy kinds
- ; writeMetaTyVar tv (mk_res_ty tau_tys) -- Write it *first*
- -- Sure to be a tau-type
- ; return tau_tys }
-
-withBox :: Kind -> (BoxySigmaType -> TcM a) -> TcM (a, TcType)
--- Allocate a *boxy* tyvar
-withBox kind thing_inside
- = do { box_tv <- newMetaTyVar BoxTv kind
- ; res <- thing_inside (mkTyVarTy box_tv)
- ; ty <- {- pprTrace "with_box" (ppr (mkTyVarTy box_tv)) $ -} readFilledBox box_tv
- ; return (res, ty) }
-\end{code}
+ go n_req ty tys = defer n_req ty tys
+ ----------
+ defer n_req ty tys
+ = do { tau_tys <- mapM newFlexiTyVarTy arg_kinds
+ ; coi <- unifyType (mkTyConApp tc tau_tys) ty
+ ; return (coi, tau_tys ++ tys) }
+ where
+ (arg_kinds, _) = splitKindFunTysN n_req (tyConKind tc)
-%************************************************************************
-%* *
- Approximate boxy matching
-%* *
-%************************************************************************
+----------------------
+matchExpectedAppTy :: TcRhoType -- orig_ty
+ -> TcM (Coercion, -- m a ~ orig_ty
+ (TcSigmaType, TcSigmaType)) -- Returns m, a
+-- If the incoming type is a mutable type variable of kind k, then
+-- matchExpectedAppTy returns a new type variable (m: * -> k); note the *.
-\begin{code}
-preSubType :: [TcTyVar] -- Quantified type variables
- -> TcTyVarSet -- Subset of quantified type variables
- -- see Note [Pre-sub boxy]
- -> TcType -- The rho-type part; quantified tyvars scopes over this
- -> BoxySigmaType -- Matching type from the context
- -> TcM [TcType] -- Types to instantiate the tyvars
--- Perform pre-subsumption, and return suitable types
--- to instantiate the quantified type varibles:
--- info from the pre-subsumption, if there is any
--- a boxy type variable otherwise
---
--- Note [Pre-sub boxy]
--- The 'btvs' are a subset of 'qtvs'. They are the ones we can
--- instantiate to a boxy type variable, because they'll definitely be
--- filled in later. This isn't always the case; sometimes we have type
--- variables mentioned in the context of the type, but not the body;
--- f :: forall a b. C a b => a -> a
--- Then we may land up with an unconstrained 'b', so we want to
--- instantiate it to a monotype (non-boxy) type variable
---
--- The 'qtvs' that are *neither* fixed by the pre-subsumption, *nor* are in 'btvs',
--- are instantiated to TauTv meta variables.
-
-preSubType qtvs btvs qty expected_ty
- = do { tys <- mapM inst_tv qtvs
- ; traceTc (text "preSubType" <+> (ppr qtvs $$ ppr btvs $$ ppr qty $$ ppr expected_ty $$ ppr pre_subst $$ ppr tys))
- ; return tys }
- where
- pre_subst = boxySubMatchType (mkVarSet qtvs) qty expected_ty
- inst_tv tv
- | Just boxy_ty <- lookupTyVar pre_subst tv = return boxy_ty
- | tv `elemVarSet` btvs = do { tv' <- tcInstBoxyTyVar tv
- ; return (mkTyVarTy tv') }
- | otherwise = do { tv' <- tcInstTyVar tv
- ; return (mkTyVarTy tv') }
-
-boxySubMatchType
- :: TcTyVarSet -> TcType -- The "template"; the tyvars are skolems
- -> BoxyRhoType -- Type to match (note a *Rho* type)
- -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
-
--- boxySubMatchType implements the Pre-subsumption judgement, in Fig 5 of the paper
--- "Boxy types: inference for higher rank types and impredicativity"
-
-boxySubMatchType tmpl_tvs tmpl_ty boxy_ty
- = go tmpl_tvs tmpl_ty emptyVarSet boxy_ty
+matchExpectedAppTy orig_ty
+ = go orig_ty
where
- go t_tvs t_ty b_tvs b_ty
- | Just t_ty' <- tcView t_ty = go t_tvs t_ty' b_tvs b_ty
- | Just b_ty' <- tcView b_ty = go t_tvs t_ty b_tvs b_ty'
-
- go t_tvs (TyVarTy _) b_tvs b_ty = emptyTvSubst -- Rule S-ANY; no bindings
- -- Rule S-ANY covers (a) type variables and (b) boxy types
- -- in the template. Both look like a TyVarTy.
- -- See Note [Sub-match] below
-
- go t_tvs t_ty b_tvs b_ty
- | isSigmaTy t_ty, (tvs, _, t_tau) <- tcSplitSigmaTy t_ty
- = go (t_tvs `delVarSetList` tvs) t_tau b_tvs b_ty -- Rule S-SPEC
- -- Under a forall on the left, if there is shadowing,
- -- do not bind! Hence the delVarSetList.
- | isSigmaTy b_ty, (tvs, _, b_tau) <- tcSplitSigmaTy b_ty
- = go t_tvs t_ty (extendVarSetList b_tvs tvs) b_tau -- Rule S-SKOL
- -- Add to the variables we must not bind to
- -- NB: it's *important* to discard the theta part. Otherwise
- -- consider (forall a. Eq a => a -> b) ~<~ (Int -> Int -> Bool)
- -- and end up with a completely bogus binding (b |-> Bool), by lining
- -- up the (Eq a) with the Int, whereas it should be (b |-> (Int->Bool)).
- -- This pre-subsumption stuff can return too few bindings, but it
- -- must *never* return bogus info.
-
- go t_tvs (FunTy arg1 res1) b_tvs (FunTy arg2 res2) -- Rule S-FUN
- = boxy_match t_tvs arg1 b_tvs arg2 (go t_tvs res1 b_tvs res2)
- -- Match the args, and sub-match the results
-
- go t_tvs t_ty b_tvs b_ty = boxy_match t_tvs t_ty b_tvs b_ty emptyTvSubst
- -- Otherwise defer to boxy matching
- -- This covers TyConApp, AppTy, PredTy
-\end{code}
+ go ty
+ | Just ty' <- tcView ty = go ty'
-Note [Sub-match]
-~~~~~~~~~~~~~~~~
-Consider this
- head :: [a] -> a
- |- head xs : <rhobox>
-We will do a boxySubMatchType between a ~ <rhobox>
-But we *don't* want to match [a |-> <rhobox>] because
- (a) The box should be filled in with a rho-type, but
- but the returned substitution maps TyVars to boxy
- *sigma* types
- (b) In any case, the right final answer might be *either*
- instantiate 'a' with a rho-type or a sigma type
- head xs : Int vs head xs : forall b. b->b
-So the matcher MUST NOT make a choice here. In general, we only
-bind a template type variable in boxyMatchType, not in boxySubMatchType.
-
-
-\begin{code}
-boxyMatchTypes
- :: TcTyVarSet -> [TcType] -- The "template"; the tyvars are skolems
- -> [BoxySigmaType] -- Type to match
- -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
-
--- boxyMatchTypes implements the Pre-matching judgement, in Fig 5 of the paper
--- "Boxy types: inference for higher rank types and impredicativity"
-
--- Find a *boxy* substitution that makes the template look as much
--- like the BoxySigmaType as possible.
--- It's always ok to return an empty substitution;
--- anything more is jam on the pudding
---
--- NB1: This is a pure, non-monadic function.
--- It does no unification, and cannot fail
---
--- Precondition: the arg lengths are equal
--- Precondition: none of the template type variables appear anywhere in the [BoxySigmaType]
---
-
-------------
-boxyMatchTypes tmpl_tvs tmpl_tys boxy_tys
- = ASSERT( length tmpl_tys == length boxy_tys )
- boxy_match_s tmpl_tvs tmpl_tys emptyVarSet boxy_tys emptyTvSubst
- -- ToDo: add error context?
-
-boxy_match_s tmpl_tvs [] boxy_tvs [] subst
- = subst
-boxy_match_s tmpl_tvs (t_ty:t_tys) boxy_tvs (b_ty:b_tys) subst
- = boxy_match tmpl_tvs t_ty boxy_tvs b_ty $
- boxy_match_s tmpl_tvs t_tys boxy_tvs b_tys subst
-boxy_match_s tmpl_tvs _ boxy_tvs _ subst
- = panic "boxy_match_s" -- Lengths do not match
-
-
-------------
-boxy_match :: TcTyVarSet -> TcType -- Template
- -> TcTyVarSet -- boxy_tvs: do not bind template tyvars to any of these
- -> BoxySigmaType -- Match against this type
- -> TvSubst
- -> TvSubst
-
--- The boxy_tvs argument prevents this match:
--- [a] forall b. a ~ forall b. b
--- We don't want to bind the template variable 'a'
--- to the quantified type variable 'b'!
-
-boxy_match tmpl_tvs orig_tmpl_ty boxy_tvs orig_boxy_ty subst
- = go orig_tmpl_ty orig_boxy_ty
- where
- go t_ty b_ty
- | Just t_ty' <- tcView t_ty = go t_ty' b_ty
- | Just b_ty' <- tcView b_ty = go t_ty b_ty'
-
- go ty1 ty2 -- C.f. the isSigmaTy case for boxySubMatchType
- | isSigmaTy ty1
- , (tvs1, _, tau1) <- tcSplitSigmaTy ty1
- , (tvs2, _, tau2) <- tcSplitSigmaTy ty2
- , equalLength tvs1 tvs2
- = boxy_match (tmpl_tvs `delVarSetList` tvs1) tau1
- (boxy_tvs `extendVarSetList` tvs2) tau2 subst
+ | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
+ = return (mkReflCo orig_ty, (fun_ty, arg_ty))
- go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
- | tc1 == tc2
- , not $ isOpenSynTyCon tc1
- = go_s tys1 tys2
-
- go (FunTy arg1 res1) (FunTy arg2 res2)
- = go_s [arg1,res1] [arg2,res2]
-
- go t_ty b_ty
- | Just (s1,t1) <- tcSplitAppTy_maybe t_ty,
- Just (s2,t2) <- tcSplitAppTy_maybe b_ty,
- typeKind t2 `isSubKind` typeKind t1 -- Maintain invariant
- = go_s [s1,t1] [s2,t2]
-
- go (TyVarTy tv) b_ty
- | tv `elemVarSet` tmpl_tvs -- Template type variable in the template
- , boxy_tvs `disjointVarSet` tyVarsOfType orig_boxy_ty
- , typeKind b_ty `isSubKind` tyVarKind tv -- See Note [Matching kinds]
- = extendTvSubst subst tv boxy_ty'
- | otherwise
- = subst -- Ignore others
- where
- boxy_ty' = case lookupTyVar subst tv of
- Nothing -> orig_boxy_ty
- Just ty -> ty `boxyLub` orig_boxy_ty
-
- go _ _ = emptyTvSubst -- It's important to *fail* by returning the empty substitution
- -- Example: Tree a ~ Maybe Int
- -- We do not want to bind (a |-> Int) in pre-matching, because that can give very
- -- misleading error messages. An even more confusing case is
- -- a -> b ~ Maybe Int
- -- Then we do not want to bind (b |-> Int)! It's always safe to discard bindings
- -- from this pre-matching phase.
-
- --------
- go_s tys1 tys2 = boxy_match_s tmpl_tvs tys1 boxy_tvs tys2 subst
-
-
-boxyLub :: BoxySigmaType -> BoxySigmaType -> BoxySigmaType
--- Combine boxy information from the two types
--- If there is a conflict, return the first
-boxyLub orig_ty1 orig_ty2
- = go orig_ty1 orig_ty2
- where
- go (AppTy f1 a1) (AppTy f2 a2) = AppTy (boxyLub f1 f2) (boxyLub a1 a2)
- go (FunTy f1 a1) (FunTy f2 a2) = FunTy (boxyLub f1 f2) (boxyLub a1 a2)
- go (TyConApp tc1 ts1) (TyConApp tc2 ts2)
- | tc1 == tc2, length ts1 == length ts2
- = TyConApp tc1 (zipWith boxyLub ts1 ts2)
-
- go (TyVarTy tv1) ty2 -- This is the whole point;
- | isTcTyVar tv1, isBoxyTyVar tv1 -- choose ty2 if ty2 is a box
- = orig_ty2
-
- -- Look inside type synonyms, but only if the naive version fails
- go ty1 ty2 | Just ty1' <- tcView ty1 = go ty1' ty2
- | Just ty2' <- tcView ty1 = go ty1 ty2'
-
- -- For now, we don't look inside ForAlls, PredTys
- go ty1 ty2 = orig_ty1 -- Default
+ go (TyVarTy tv)
+ | ASSERT( isTcTyVar tv) isMetaTyVar tv
+ = do { cts <- readMetaTyVar tv
+ ; case cts of
+ Indirect ty -> go ty
+ Flexi -> defer }
+
+ go _ = defer
+
+ -- Defer splitting by generating an equality constraint
+ defer = do { ty1 <- newFlexiTyVarTy kind1
+ ; ty2 <- newFlexiTyVarTy kind2
+ ; coi <- unifyType (mkAppTy ty1 ty2) orig_ty
+ ; return (coi, (ty1, ty2)) }
+
+ orig_kind = typeKind orig_ty
+ kind1 = mkArrowKind liftedTypeKind (defaultKind orig_kind)
+ kind2 = liftedTypeKind -- m :: * -> k
+ -- arg type :: *
+ -- The defaultKind is a bit smelly. If you remove it,
+ -- try compiling f x = do { x }
+ -- and you'll get a kind mis-match. It smells, but
+ -- not enough to lose sleep over.
\end{code}
-Note [Matching kinds]
-~~~~~~~~~~~~~~~~~~~~~
-The target type might legitimately not be a sub-kind of template.
-For example, suppose the target is simply a box with an OpenTypeKind,
-and the template is a type variable with LiftedTypeKind.
-Then it's ok (because the target type will later be refined).
-We simply don't bind the template type variable.
-
-It might also be that the kind mis-match is an error. For example,
-suppose we match the template (a -> Int) against (Int# -> Int),
-where the template type variable 'a' has LiftedTypeKind. This
-matching function does not fail; it simply doesn't bind the template.
-Later stuff will fail.
%************************************************************************
-%* *
- Subsumption checking
-%* *
+%* *
+ Subsumption checking
+%* *
%************************************************************************
All the tcSub calls have the form
-
- tcSub expected_ty offered_ty
+ tcSub actual_ty expected_ty
which checks
- offered_ty <= expected_ty
+ actual_ty <= expected_ty
-That is, that a value of type offered_ty is acceptable in
+That is, that a value of type actual_ty is acceptable in
a place expecting a value of type expected_ty.
-It returns a coercion function
- co_fn :: offered_ty ~ expected_ty
-which takes an HsExpr of type offered_ty into one of type
+It returns a coercion function
+ co_fn :: actual_ty ~ expected_ty
+which takes an HsExpr of type actual_ty into one of type
expected_ty.
\begin{code}
------------------
-tcSubExp :: BoxySigmaType -> BoxySigmaType -> TcM HsWrapper -- Locally used only
- -- (tcSub act exp) checks that
- -- act <= exp
-tcSubExp actual_ty expected_ty
- = -- addErrCtxtM (unifyCtxt actual_ty expected_ty) $
- -- Adding the error context here leads to some very confusing error
- -- messages, such as "can't match forall a. a->a with forall a. a->a"
- -- Example is tcfail165:
- -- do var <- newEmptyMVar :: IO (MVar (forall a. Show a => a -> String))
- -- putMVar var (show :: forall a. Show a => a -> String)
- -- Here the info does not flow from the 'var' arg of putMVar to its 'show' arg
- -- but after zonking it looks as if it does!
- --
- -- So instead I'm adding the error context when moving from tc_sub to u_tys
-
- traceTc (text "tcSubExp" <+> ppr actual_ty <+> ppr expected_ty) >>
- tc_sub SubOther actual_ty actual_ty False expected_ty expected_ty
-
-tcFunResTy :: Name -> BoxySigmaType -> BoxySigmaType -> TcM HsWrapper -- Locally used only
-tcFunResTy fun actual_ty expected_ty
- = traceTc (text "tcFunResTy" <+> ppr actual_ty <+> ppr expected_ty) >>
- tc_sub (SubFun fun) actual_ty actual_ty False expected_ty expected_ty
-
------------------
-data SubCtxt = SubDone -- Error-context already pushed
- | SubFun Name -- Context is tcFunResTy
- | SubOther -- Context is something else
-
-tc_sub :: SubCtxt -- How to add an error-context
- -> BoxySigmaType -- actual_ty, before expanding synonyms
- -> BoxySigmaType -- ..and after
- -> InBox -- True <=> expected_ty is inside a box
- -> BoxySigmaType -- expected_ty, before
- -> BoxySigmaType -- ..and after
- -> TcM HsWrapper
- -- The acual_ty is never inside a box
--- IMPORTANT pre-condition: if the args contain foralls, the bound type
--- variables are visible non-monadically
--- (i.e. tha args are sufficiently zonked)
--- This invariant is needed so that we can "see" the foralls, ad
--- e.g. in the SPEC rule where we just use splitSigmaTy
-
-tc_sub sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
- = traceTc (text "tc_sub" <+> ppr act_ty $$ ppr exp_ty) >>
- tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
- -- This indirection is just here to make
- -- it easy to insert a debug trace!
-
-tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
- | Just exp_ty' <- tcView exp_ty = tc_sub sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty'
-tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
- | Just act_ty' <- tcView act_ty = tc_sub sub_ctxt act_sty act_ty' exp_ib exp_sty exp_ty
-
------------------------------------
--- Rule SBOXY, plus other cases when act_ty is a type variable
--- Just defer to boxy matching
--- This rule takes precedence over SKOL!
-tc_sub1 sub_ctxt act_sty (TyVarTy tv) exp_ib exp_sty exp_ty
- = do { traceTc (text "tc_sub1 - case 1")
- ; coi <- addSubCtxt sub_ctxt act_sty exp_sty $
- uVar True False tv exp_ib exp_sty exp_ty
- ; traceTc (case coi of
- IdCo -> text "tc_sub1 (Rule SBOXY) IdCo"
- ACo co -> text "tc_sub1 (Rule SBOXY) ACo" <+> ppr co)
- ; return $ case coi of
- IdCo -> idHsWrapper
- ACo co -> WpCo co
- }
+tcSubType :: CtOrigin -> UserTypeCtxt -> TcSigmaType -> TcSigmaType -> TcM HsWrapper
+-- Check that ty_actual is more polymorphic than ty_expected
+-- Both arguments might be polytypes, so we must instantiate and skolemise
+-- Returns a wrapper of shape ty_actual ~ ty_expected
+tcSubType origin ctxt ty_actual ty_expected
+ | isSigmaTy ty_actual
+ = do { (sk_wrap, inst_wrap)
+ <- tcGen ctxt ty_expected $ \ _ sk_rho -> do
+ { (in_wrap, in_rho) <- deeplyInstantiate origin ty_actual
+ ; coi <- unifyType in_rho sk_rho
+ ; return (coToHsWrapper coi <.> in_wrap) }
+ ; return (sk_wrap <.> inst_wrap) }
+
+ | otherwise -- Urgh! It seems deeply weird to have equality
+ -- when actual is not a polytype, and it makes a big
+ -- difference e.g. tcfail104
+ = do { coi <- unifyType ty_actual ty_expected
+ ; return (coToHsWrapper coi) }
+
+tcInfer :: (TcType -> TcM a) -> TcM (a, TcType)
+tcInfer tc_infer = do { ty <- newFlexiTyVarTy openTypeKind
+ ; res <- tc_infer ty
+ ; return (res, ty) }
------------------------------------
--- Skolemisation case (rule SKOL)
--- actual_ty: d:Eq b => b->b
--- expected_ty: forall a. Ord a => a->a
--- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
-
--- It is essential to do this *before* the specialisation case
--- Example: f :: (Eq a => a->a) -> ...
--- g :: Ord b => b->b
--- Consider f g !
-
-tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
- | isSigmaTy exp_ty
- = do { traceTc (text "tc_sub1 - case 2") ;
- if exp_ib then -- SKOL does not apply if exp_ty is inside a box
- defer_to_boxy_matching sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
- else do
- { (gen_fn, co_fn) <- tcGen exp_ty act_tvs $ \ _ body_exp_ty ->
- tc_sub sub_ctxt act_sty act_ty False body_exp_ty body_exp_ty
- ; return (gen_fn <.> co_fn) }
- }
- where
- act_tvs = tyVarsOfType act_ty
- -- It's really important to check for escape wrt
- -- the free vars of both expected_ty *and* actual_ty
-
------------------------------------
--- Specialisation case (rule ASPEC):
--- actual_ty: forall a. Ord a => a->a
--- expected_ty: Int -> Int
--- co_fn e = e Int dOrdInt
-
-tc_sub1 sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
--- Implements the new SPEC rule in the Appendix of the paper
--- "Boxy types: inference for higher rank types and impredicativity"
--- (This appendix isn't in the published version.)
--- The idea is to *first* do pre-subsumption, and then full subsumption
--- Example: forall a. a->a <= Int -> (forall b. Int)
--- Pre-subsumpion finds a|->Int, and that works fine, whereas
--- just running full subsumption would fail.
- | isSigmaTy actual_ty
- = do { traceTc (text "tc_sub1 - case 3")
- ; -- Perform pre-subsumption, and instantiate
- -- the type with info from the pre-subsumption;
- -- boxy tyvars if pre-subsumption gives no info
- let (tyvars, theta, tau) = tcSplitSigmaTy actual_ty
- tau_tvs = exactTyVarsOfType tau
- ; inst_tys <- if exp_ib then -- Inside a box, do not do clever stuff
- do { tyvars' <- mapM tcInstBoxyTyVar tyvars
- ; return (mkTyVarTys tyvars') }
- else -- Outside, do clever stuff
- preSubType tyvars tau_tvs tau expected_ty
- ; let subst' = zipOpenTvSubst tyvars inst_tys
- tau' = substTy subst' tau
-
- -- Perform a full subsumption check
- ; traceTc (text "tc_sub_spec" <+> vcat [ppr actual_ty,
- ppr tyvars <+> ppr theta <+> ppr tau,
- ppr tau'])
- ; co_fn2 <- tc_sub sub_ctxt tau' tau' exp_ib exp_sty expected_ty
-
- -- Deal with the dictionaries
- -- The origin gives a helpful origin when we have
- -- a function with type f :: Int -> forall a. Num a => ...
- -- This way the (Num a) dictionary gets an OccurrenceOf f origin
- ; let orig = case sub_ctxt of
- SubFun n -> OccurrenceOf n
- other -> InstSigOrigin -- Unhelpful
- ; co_fn1 <- instCall orig inst_tys (substTheta subst' theta)
- ; return (co_fn2 <.> co_fn1) }
-
------------------------------------
--- Function case (rule F1)
-tc_sub1 sub_ctxt act_sty (FunTy act_arg act_res) exp_ib exp_sty (FunTy exp_arg exp_res)
- = do { traceTc (text "tc_sub1 - case 4")
- ; addSubCtxt sub_ctxt act_sty exp_sty $
- tc_sub_funs act_arg act_res exp_ib exp_arg exp_res
- }
-
--- Function case (rule F2)
-tc_sub1 sub_ctxt act_sty act_ty@(FunTy act_arg act_res) _ exp_sty (TyVarTy exp_tv)
- | isBoxyTyVar exp_tv
- = addSubCtxt sub_ctxt act_sty exp_sty $
- do { traceTc (text "tc_sub1 - case 5")
- ; cts <- readMetaTyVar exp_tv
- ; case cts of
- Indirect ty -> tc_sub SubDone act_sty act_ty True exp_sty ty
- Flexi -> do { [arg_ty,res_ty] <- withMetaTvs exp_tv fun_kinds mk_res_ty
- ; tc_sub_funs act_arg act_res True arg_ty res_ty } }
- where
- mk_res_ty [arg_ty', res_ty'] = mkFunTy arg_ty' res_ty'
- mk_res_ty other = panic "TcUnify.mk_res_ty3"
- fun_kinds = [argTypeKind, openTypeKind]
-
--- Everything else: defer to boxy matching
-tc_sub1 sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty@(TyVarTy exp_tv)
- = do { traceTc (text "tc_sub1 - case 6a" <+> ppr [isBoxyTyVar exp_tv, isMetaTyVar exp_tv, isSkolemTyVar exp_tv, isExistentialTyVar exp_tv,isSigTyVar exp_tv] )
- ; defer_to_boxy_matching sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
- }
-
-tc_sub1 sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
- = do { traceTc (text "tc_sub1 - case 6")
- ; defer_to_boxy_matching sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
- }
-
------------------------------------
-defer_to_boxy_matching sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
- = do { coi <- addSubCtxt sub_ctxt act_sty exp_sty $
- u_tys outer False act_sty actual_ty exp_ib exp_sty expected_ty
- ; return $ case coi of
- IdCo -> idHsWrapper
- ACo co -> WpCo co
- }
- where
- outer = case sub_ctxt of -- Ugh
- SubDone -> False
- other -> True
-
------------------------------------
-tc_sub_funs act_arg act_res exp_ib exp_arg exp_res
- = do { arg_coi <- uTys False act_arg exp_ib exp_arg
- ; co_fn_res <- tc_sub SubDone act_res act_res exp_ib exp_res exp_res
- ; wrapper1 <- wrapFunResCoercion [exp_arg] co_fn_res
- ; let wrapper2 = case arg_coi of
- IdCo -> idHsWrapper
- ACo co -> WpCo $ FunTy co act_res
- ; return (wrapper1 <.> wrapper2)
- }
+-----------------
+tcWrapResult :: HsExpr TcId -> TcRhoType -> TcRhoType -> TcM (HsExpr TcId)
+tcWrapResult expr actual_ty res_ty
+ = do { coi <- unifyType actual_ty res_ty
+ -- Both types are deeply skolemised
+ ; return (mkHsWrapCo coi expr) }
-----------------------------------
-wrapFunResCoercion
- :: [TcType] -- Type of args
- -> HsWrapper -- HsExpr a -> HsExpr b
- -> TcM HsWrapper -- HsExpr (arg_tys -> a) -> HsExpr (arg_tys -> b)
+wrapFunResCoercion
+ :: [TcType] -- Type of args
+ -> HsWrapper -- HsExpr a -> HsExpr b
+ -> TcM HsWrapper -- HsExpr (arg_tys -> a) -> HsExpr (arg_tys -> b)
wrapFunResCoercion arg_tys co_fn_res
- | isIdHsWrapper co_fn_res
+ | isIdHsWrapper co_fn_res
= return idHsWrapper
- | null arg_tys
+ | null arg_tys
= return co_fn_res
- | otherwise
- = do { arg_ids <- newSysLocalIds FSLIT("sub") arg_tys
- ; return (mkWpLams arg_ids <.> co_fn_res <.> mkWpApps arg_ids) }
+ | otherwise
+ = do { arg_ids <- newSysLocalIds (fsLit "sub") arg_tys
+ ; return (mkWpLams arg_ids <.> co_fn_res <.> mkWpEvVarApps arg_ids) }
\end{code}
%************************************************************************
-%* *
+%* *
\subsection{Generalisation}
-%* *
+%* *
%************************************************************************
\begin{code}
-tcGen :: BoxySigmaType -- expected_ty
- -> TcTyVarSet -- Extra tyvars that the universally
- -- quantified tyvars of expected_ty
- -- must not be unified
- -> ([TcTyVar] -> BoxyRhoType -> TcM result)
+tcGen :: UserTypeCtxt -> TcType
+ -> ([TcTyVar] -> TcRhoType -> TcM result)
-> TcM (HsWrapper, result)
- -- The expression has type: spec_ty -> expected_ty
-
-tcGen expected_ty extra_tvs thing_inside -- We expect expected_ty to be a forall-type
- -- If not, the call is a no-op
- = do { traceTc (text "tcGen")
- -- We want the GenSkol info in the skolemised type variables to
- -- mention the *instantiated* tyvar names, so that we get a
- -- good error message "Rigid variable 'a' is bound by (forall a. a->a)"
- -- Hence the tiresome but innocuous fixM
- ; ((tvs', theta', rho'), skol_info) <- fixM (\ ~(_, skol_info) ->
- do { (forall_tvs, theta, rho_ty) <- tcInstSkolType skol_info expected_ty
- -- Get loation from monad, not from expected_ty
- ; let skol_info = GenSkol forall_tvs (mkPhiTy theta rho_ty)
- ; return ((forall_tvs, theta, rho_ty), skol_info) })
-
-#ifdef DEBUG
- ; traceTc (text "tcGen" <+> vcat [text "extra_tvs" <+> ppr extra_tvs,
- text "expected_ty" <+> ppr expected_ty,
- text "inst ty" <+> ppr tvs' <+> ppr theta' <+> ppr rho',
- text "free_tvs" <+> ppr free_tvs])
-#endif
-
- -- Type-check the arg and unify with poly type
- ; (result, lie) <- getLIE (thing_inside tvs' rho')
-
- -- Check that the "forall_tvs" havn't been constrained
- -- The interesting bit here is that we must include the free variables
- -- of the expected_ty. Here's an example:
- -- runST (newVar True)
- -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
- -- for (newVar True), with s fresh. Then we unify with the runST's arg type
- -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
- -- So now s' isn't unconstrained because it's linked to a.
- -- Conclusion: include the free vars of the expected_ty in the
- -- list of "free vars" for the signature check.
-
- ; loc <- getInstLoc (SigOrigin skol_info)
- ; dicts <- newDictBndrs loc theta'
- ; inst_binds <- tcSimplifyCheck loc tvs' dicts lie
-
- ; checkSigTyVarsWrt free_tvs tvs'
- ; traceTc (text "tcGen:done")
-
- ; let
- -- The WpLet binds any Insts which came out of the simplification.
- dict_ids = map instToId dicts
- co_fn = mkWpTyLams tvs' <.> mkWpLams dict_ids <.> WpLet inst_binds
- ; returnM (co_fn, result) }
- where
- free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
-\end{code}
+ -- The expression has type: spec_ty -> expected_ty
+
+tcGen ctxt expected_ty thing_inside
+ -- We expect expected_ty to be a forall-type
+ -- If not, the call is a no-op
+ = do { traceTc "tcGen" empty
+ ; (wrap, tvs', given, rho') <- deeplySkolemise expected_ty
+
+ ; when debugIsOn $
+ traceTc "tcGen" $ vcat [
+ text "expected_ty" <+> ppr expected_ty,
+ text "inst ty" <+> ppr tvs' <+> ppr rho' ]
+
+ -- Generally we must check that the "forall_tvs" havn't been constrained
+ -- The interesting bit here is that we must include the free variables
+ -- of the expected_ty. Here's an example:
+ -- runST (newVar True)
+ -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
+ -- for (newVar True), with s fresh. Then we unify with the runST's arg type
+ -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
+ -- So now s' isn't unconstrained because it's linked to a.
+ --
+ -- However [Oct 10] now that the untouchables are a range of
+ -- TcTyVars, all this is handled automatically with no need for
+ -- extra faffing around
+
+ -- Use the *instantiated* type in the SkolemInfo
+ -- so that the names of displayed type variables line up
+ ; let skol_info = SigSkol ctxt (mkPiTypes given rho')
+
+ ; (ev_binds, result) <- checkConstraints skol_info tvs' given $
+ thing_inside tvs' rho'
+
+ ; return (wrap <.> mkWpLet ev_binds, result) }
+ -- The ev_binds returned by checkConstraints is very
+ -- often empty, in which case mkWpLet is a no-op
+
+checkConstraints :: SkolemInfo
+ -> [TcTyVar] -- Skolems
+ -> [EvVar] -- Given
+ -> TcM result
+ -> TcM (TcEvBinds, result)
+
+checkConstraints skol_info skol_tvs given thing_inside
+ | null skol_tvs && null given
+ = do { res <- thing_inside; return (emptyTcEvBinds, res) }
+ -- Just for efficiency. We check every function argument with
+ -- tcPolyExpr, which uses tcGen and hence checkConstraints.
-
+ | otherwise
+ = newImplication skol_info skol_tvs given thing_inside
+
+newImplication :: SkolemInfo -> [TcTyVar]
+ -> [EvVar] -> TcM result
+ -> TcM (TcEvBinds, result)
+newImplication skol_info skol_tvs given thing_inside
+ = ASSERT2( all isTcTyVar skol_tvs, ppr skol_tvs )
+ ASSERT2( all isSkolemTyVar skol_tvs, ppr skol_tvs )
+ do { ((result, untch), wanted) <- captureConstraints $
+ captureUntouchables $
+ thing_inside
+
+ ; if isEmptyWC wanted && not (hasEqualities given)
+ -- Optimisation : if there are no wanteds, and the givens
+ -- are sufficiently simple, don't generate an implication
+ -- at all. Reason for the hasEqualities test:
+ -- we don't want to lose the "inaccessible alternative"
+ -- error check
+ then
+ return (emptyTcEvBinds, result)
+ else do
+ { ev_binds_var <- newTcEvBinds
+ ; lcl_env <- getLclTypeEnv
+ ; loc <- getCtLoc skol_info
+ ; emitImplication $ Implic { ic_untch = untch
+ , ic_env = lcl_env
+ , ic_skols = mkVarSet skol_tvs
+ , ic_given = given
+ , ic_wanted = wanted
+ , ic_insol = insolubleWC wanted
+ , ic_binds = ev_binds_var
+ , ic_loc = loc }
+
+ ; return (TcEvBinds ev_binds_var, result) } }
+\end{code}
%************************************************************************
-%* *
- Boxy unification
-%* *
+%* *
+ Boxy unification
+%* *
%************************************************************************
The exported functions are all defined as versions of some
non-exported generic functions.
\begin{code}
-boxyUnify :: BoxyType -> BoxyType -> TcM CoercionI
--- Acutal and expected, respectively
-boxyUnify ty1 ty2
- = addErrCtxtM (unifyCtxt ty1 ty2) $
- uTysOuter False ty1 False ty2
-
---------------
-boxyUnifyList :: [BoxyType] -> [BoxyType] -> TcM [CoercionI]
--- Arguments should have equal length
--- Acutal and expected types
-boxyUnifyList tys1 tys2 = uList boxyUnify tys1 tys2
+unifyType :: TcTauType -> TcTauType -> TcM Coercion
+-- Actual and expected types
+-- Returns a coercion : ty1 ~ ty2
+unifyType ty1 ty2 = uType [] ty1 ty2
---------------
-unifyType :: TcTauType -> TcTauType -> TcM CoercionI
--- No boxes expected inside these types
--- Acutal and expected types
-unifyType ty1 ty2 -- ty1 expected, ty2 inferred
- = ASSERT2( not (isBoxyTy ty1), ppr ty1 )
- ASSERT2( not (isBoxyTy ty2), ppr ty2 )
- addErrCtxtM (unifyCtxt ty1 ty2) $
- uTysOuter True ty1 True ty2
+unifyPred :: PredType -> PredType -> TcM Coercion
+-- Actual and expected types
+unifyPred p1 p2 = uPred [UnifyOrigin (mkPredTy p1) (mkPredTy p2)] p1 p2
---------------
-unifyPred :: PredType -> PredType -> TcM CoercionI
--- Acutal and expected types
-unifyPred p1 p2 = addErrCtxtM (unifyCtxt (mkPredTy p1) (mkPredTy p2)) $
- uPred True True p1 True p2
-
-unifyTheta :: TcThetaType -> TcThetaType -> TcM [CoercionI]
--- Acutal and expected types
+unifyTheta :: TcThetaType -> TcThetaType -> TcM [Coercion]
+-- Actual and expected types
unifyTheta theta1 theta2
- = do { checkTc (equalLength theta1 theta2)
- (vcat [ptext SLIT("Contexts differ in length"),
- nest 2 $ parens $ ptext SLIT("Use -fglasgow-exts to allow this")])
- ; uList unifyPred theta1 theta2
- }
-
----------------
-uList :: (a -> a -> TcM b)
- -> [a] -> [a] -> TcM [b]
--- Unify corresponding elements of two lists of types, which
--- should be of equal length. We charge down the list explicitly so that
--- we can complain if their lengths differ.
-uList unify [] [] = return []
-uList unify (ty1:tys1) (ty2:tys2) = do { x <- unify ty1 ty2;
- ; xs <- uList unify tys1 tys2
- ; return (x:xs)
- }
-uList unify ty1s ty2s = panic "Unify.uList: mismatched type lists!"
+ = do { checkTc (equalLength theta1 theta2)
+ (vcat [ptext (sLit "Contexts differ in length"),
+ nest 2 $ parens $ ptext (sLit "Use -XRelaxedPolyRec to allow this")])
+ ; zipWithM unifyPred theta1 theta2 }
\end{code}
@unifyTypeList@ takes a single list of @TauType@s and unifies them
\begin{code}
unifyTypeList :: [TcTauType] -> TcM ()
-unifyTypeList [] = returnM ()
-unifyTypeList [ty] = returnM ()
-unifyTypeList (ty1:tys@(ty2:_)) = do { unifyType ty1 ty2
- ; unifyTypeList tys }
+unifyTypeList [] = return ()
+unifyTypeList [_] = return ()
+unifyTypeList (ty1:tys@(ty2:_)) = do { _ <- unifyType ty1 ty2
+ ; unifyTypeList tys }
\end{code}
%************************************************************************
-%* *
-\subsection[Unify-uTys]{@uTys@: getting down to business}
-%* *
+%* *
+ uType and friends
+%* *
%************************************************************************
-@uTys@ is the heart of the unifier. Each arg occurs twice, because
+uType is the heart of the unifier. Each arg occurs twice, because
we want to report errors in terms of synomyms if possible. The first of
the pair is used in error messages only; it is always the same as the
second, except that if the first is a synonym then the second may be a
de-synonym'd version. This way we get better error messages.
-We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
-
\begin{code}
-type SwapFlag = Bool
- -- False <=> the two args are (actual, expected) respectively
- -- True <=> the two args are (expected, actual) respectively
-
-type InBox = Bool -- True <=> we are inside a box
- -- False <=> we are outside a box
- -- The importance of this is that if we get "filled-box meets
- -- filled-box", we'll look into the boxes and unify... but
- -- we must not allow polytypes. But if we are in a box on
- -- just one side, then we can allow polytypes
-
-type Outer = Bool -- True <=> this is the outer level of a unification
- -- so that the types being unified are the
- -- very ones we began with, not some sub
- -- component or synonym expansion
--- The idea is that if Outer is true then unifyMisMatch should
--- pop the context to remove the "Expected/Acutal" context
-
-uTysOuter, uTys
- :: InBox -> TcType -- ty1 is the *actual* type
- -> InBox -> TcType -- ty2 is the *expected* type
- -> TcM CoercionI
-uTysOuter nb1 ty1 nb2 ty2
- = do { traceTc (text "uTysOuter" <+> ppr ty1 <+> ppr ty2)
- ; u_tys True nb1 ty1 ty1 nb2 ty2 ty2 }
-uTys nb1 ty1 nb2 ty2
- = do { traceTc (text "uTys" <+> ppr ty1 <+> ppr ty2)
- ; u_tys False nb1 ty1 ty1 nb2 ty2 ty2 }
+data SwapFlag
+ = NotSwapped -- Args are: actual, expected
+ | IsSwapped -- Args are: expected, actual
+
+instance Outputable SwapFlag where
+ ppr IsSwapped = ptext (sLit "Is-swapped")
+ ppr NotSwapped = ptext (sLit "Not-swapped")
+unSwap :: SwapFlag -> (a->a->b) -> a -> a -> b
+unSwap NotSwapped f a b = f a b
+unSwap IsSwapped f a b = f b a
+
+------------
+uType, uType_np, uType_defer
+ :: [EqOrigin]
+ -> TcType -- ty1 is the *actual* type
+ -> TcType -- ty2 is the *expected* type
+ -> TcM Coercion
--------------
-uTys_s :: InBox -> [TcType] -- tys1 are the *actual* types
- -> InBox -> [TcType] -- tys2 are the *expected* types
- -> TcM [CoercionI]
-uTys_s nb1 [] nb2 [] = returnM []
-uTys_s nb1 (ty1:tys1) nb2 (ty2:tys2) = do { coi <- uTys nb1 ty1 nb2 ty2
- ; cois <- uTys_s nb1 tys1 nb2 tys2
- ; return (coi:cois)
- }
-uTys_s nb1 ty1s nb2 ty2s = panic "Unify.uTys_s: mismatched type lists!"
+-- It is always safe to defer unification to the main constraint solver
+-- See Note [Deferred unification]
+uType_defer (item : origin) ty1 ty2
+ = wrapEqCtxt origin $
+ do { co_var <- newCoVar ty1 ty2
+ ; loc <- getCtLoc (TypeEqOrigin item)
+ ; emitFlat (mkEvVarX co_var loc)
+
+ -- Error trace only
+ ; ctxt <- getErrCtxt
+ ; doc <- mkErrInfo emptyTidyEnv ctxt
+ ; traceTc "utype_defer" (vcat [ppr co_var, ppr ty1, ppr ty2, ppr origin, doc])
+
+ ; return $ mkCoVarCo co_var }
+uType_defer [] _ _
+ = panic "uType_defer"
--------------
-u_tys :: Outer
- -> InBox -> TcType -> TcType -- ty1 is the *actual* type
- -> InBox -> TcType -> TcType -- ty2 is the *expected* type
- -> TcM CoercionI
-
-u_tys outer nb1 orig_ty1 ty1 nb2 orig_ty2 ty2
- = do { traceTc (text "u_tys " <+> ppr ty1 <+> text " " <+> ppr ty2)
- ; coi <- go outer ty1 ty2
- ; traceTc (case coi of
- ACo co -> text "u_tys yields coercion: " <+> ppr co
- IdCo -> text "u_tys yields no coercion")
- ; return coi
- }
- where
-
- go :: Outer -> TcType -> TcType -> TcM CoercionI
- go outer ty1 ty2 =
- do { traceTc (text "go " <+> ppr orig_ty1 <+> text "/" <+> ppr ty1
- <+> ppr orig_ty2 <+> text "/" <+> ppr ty2)
- ; go1 outer ty1 ty2
- }
-
- go1 :: Outer -> TcType -> TcType -> TcM CoercionI
- -- Always expand synonyms: see Note [Unification and synonyms]
- -- (this also throws away FTVs)
- go1 outer ty1 ty2
- | Just ty1' <- tcView ty1 = go False ty1' ty2
- | Just ty2' <- tcView ty2 = go False ty1 ty2'
-
- -- Variables; go for uVar
- go1 outer (TyVarTy tyvar1) ty2 = uVar outer False tyvar1 nb2 orig_ty2 ty2
- go1 outer ty1 (TyVarTy tyvar2) = uVar outer True tyvar2 nb1 orig_ty1 ty1
- -- "True" means args swapped
-
- -- The case for sigma-types must *follow* the variable cases
- -- because a boxy variable can be filed with a polytype;
- -- but must precede FunTy, because ((?x::Int) => ty) look
- -- like a FunTy; there isn't necy a forall at the top
- go1 _ ty1 ty2
- | isSigmaTy ty1 || isSigmaTy ty2
- = do { traceTc (text "We have sigma types: equalLength" <+> ppr tvs1 <+> ppr tvs2)
- ; checkM (equalLength tvs1 tvs2)
- (unifyMisMatch outer False orig_ty1 orig_ty2)
- ; traceTc (text "We're past the first length test")
- ; tvs <- tcInstSkolTyVars UnkSkol tvs1 -- Not a helpful SkolemInfo
- -- Get location from monad, not from tvs1
- ; let tys = mkTyVarTys tvs
- in_scope = mkInScopeSet (mkVarSet tvs)
- phi1 = substTy (mkTvSubst in_scope (zipTyEnv tvs1 tys)) body1
- phi2 = substTy (mkTvSubst in_scope (zipTyEnv tvs2 tys)) body2
- (theta1,tau1) = tcSplitPhiTy phi1
- (theta2,tau2) = tcSplitPhiTy phi2
-
- ; addErrCtxtM (unifyForAllCtxt tvs phi1 phi2) $ do
- { checkM (equalLength theta1 theta2)
- (unifyMisMatch outer False orig_ty1 orig_ty2)
-
- ; cois <- uPreds False nb1 theta1 nb2 theta2 -- TOMDO: do something with these pred_cois
- ; traceTc (text "TOMDO!")
- ; coi <- uTys nb1 tau1 nb2 tau2
-
- -- Check for escape; e.g. (forall a. a->b) ~ (forall a. a->a)
- ; free_tvs <- zonkTcTyVarsAndFV (varSetElems (tyVarsOfType ty1 `unionVarSet` tyVarsOfType ty2))
- ; ifM (any (`elemVarSet` free_tvs) tvs)
- (bleatEscapedTvs free_tvs tvs tvs)
-
- -- If both sides are inside a box, we are in a "box-meets-box"
- -- situation, and we should not have a polytype at all.
- -- If we get here we have two boxes, already filled with
- -- the same polytype... but it should be a monotype.
- -- This check comes last, because the error message is
- -- extremely unhelpful.
- ; ifM (nb1 && nb2) (notMonoType ty1)
- ; return coi
- }}
- where
- (tvs1, body1) = tcSplitForAllTys ty1
- (tvs2, body2) = tcSplitForAllTys ty2
-
- -- Predicates
- go1 outer (PredTy p1) (PredTy p2)
- = uPred False nb1 p1 nb2 p2
-
- -- Type constructors must match
- go1 _ (TyConApp con1 tys1) (TyConApp con2 tys2)
- | con1 == con2 && not (isOpenSynTyCon con1)
- = do { cois <- uTys_s nb1 tys1 nb2 tys2
- ; return $ mkTyConAppCoI con1 tys1 cois
- }
- -- See Note [TyCon app]
- | con1 == con2 && identicalOpenSynTyConApp
- = do { cois <- uTys_s nb1 tys1' nb2 tys2'
- ; return $ mkTyConAppCoI con1 tys1 (replicate n IdCo ++ cois)
- }
- where
- n = tyConArity con1
- (idxTys1, tys1') = splitAt n tys1
- (idxTys2, tys2') = splitAt n tys2
- identicalOpenSynTyConApp = idxTys1 `tcEqTypes` idxTys2
- -- See Note [OpenSynTyCon app]
-
- -- Functions; just check the two parts
- go1 _ (FunTy fun1 arg1) (FunTy fun2 arg2)
- = do { coi_l <- uTys nb1 fun1 nb2 fun2
- ; coi_r <- uTys nb1 arg1 nb2 arg2
- ; return $ mkFunTyCoI fun1 coi_l arg1 coi_r
- }
-
- -- Applications need a bit of care!
- -- They can match FunTy and TyConApp, so use splitAppTy_maybe
- -- NB: we've already dealt with type variables and Notes,
- -- so if one type is an App the other one jolly well better be too
- go1 outer (AppTy s1 t1) ty2
+-- Push a new item on the origin stack (the most common case)
+uType origin ty1 ty2 -- Push a new item on the origin stack
+ = uType_np (pushOrigin ty1 ty2 origin) ty1 ty2
+
+--------------
+-- unify_np (short for "no push" on the origin stack) does the work
+uType_np origin orig_ty1 orig_ty2
+ = do { traceTc "u_tys " $ vcat
+ [ sep [ ppr orig_ty1, text "~", ppr orig_ty2]
+ , ppr origin]
+ ; coi <- go orig_ty1 orig_ty2
+ ; if isReflCo coi
+ then traceTc "u_tys yields no coercion" empty
+ else traceTc "u_tys yields coercion:" (ppr coi)
+ ; return coi }
+ where
+ bale_out :: [EqOrigin] -> TcM a
+ bale_out origin = failWithMisMatch origin
+
+ go :: TcType -> TcType -> TcM Coercion
+ -- The arguments to 'go' are always semantically identical
+ -- to orig_ty{1,2} except for looking through type synonyms
+
+ -- Variables; go for uVar
+ -- Note that we pass in *original* (before synonym expansion),
+ -- so that type variables tend to get filled in with
+ -- the most informative version of the type
+ go (TyVarTy tyvar1) ty2 = uVar origin NotSwapped tyvar1 ty2
+ go ty1 (TyVarTy tyvar2) = uVar origin IsSwapped tyvar2 ty1
+
+ -- Expand synonyms:
+ -- see Note [Unification and synonyms]
+ -- Do this after the variable case so that we tend to unify
+ -- variables with un-expanded type synonym
+ --
+ -- Also NB that we recurse to 'go' so that we don't push a
+ -- new item on the origin stack. As a result if we have
+ -- type Foo = Int
+ -- and we try to unify Foo ~ Bool
+ -- we'll end up saying "can't match Foo with Bool"
+ -- rather than "can't match "Int with Bool". See Trac #4535.
+ go ty1 ty2
+ | Just ty1' <- tcView ty1 = go ty1' ty2
+ | Just ty2' <- tcView ty2 = go ty1 ty2'
+
+ -- Predicates
+ go (PredTy p1) (PredTy p2) = uPred origin p1 p2
+
+ -- Functions (or predicate functions) just check the two parts
+ go (FunTy fun1 arg1) (FunTy fun2 arg2)
+ = do { coi_l <- uType origin fun1 fun2
+ ; coi_r <- uType origin arg1 arg2
+ ; return $ mkFunCo coi_l coi_r }
+
+ -- Always defer if a type synonym family (type function)
+ -- is involved. (Data families behave rigidly.)
+ go ty1@(TyConApp tc1 _) ty2
+ | isSynFamilyTyCon tc1 = uType_defer origin ty1 ty2
+ go ty1 ty2@(TyConApp tc2 _)
+ | isSynFamilyTyCon tc2 = uType_defer origin ty1 ty2
+
+ go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
+ | tc1 == tc2 -- See Note [TyCon app]
+ = do { cois <- uList origin uType tys1 tys2
+ ; return $ mkTyConAppCo tc1 cois }
+
+ -- See Note [Care with type applications]
+ go (AppTy s1 t1) ty2
| Just (s2,t2) <- tcSplitAppTy_maybe ty2
- = do { coi_s <- uTys nb1 s1 nb2 s2; coi_t <- uTys nb1 t1 nb2 t2
- ; return $ mkAppTyCoI s1 coi_s t1 coi_t }
+ = do { coi_s <- uType_np origin s1 s2 -- See Note [Unifying AppTy]
+ ; coi_t <- uType origin t1 t2
+ ; return $ mkAppCo coi_s coi_t }
- -- Now the same, but the other way round
- -- Don't swap the types, because the error messages get worse
- go1 outer ty1 (AppTy s2 t2)
+ go ty1 (AppTy s2 t2)
| Just (s1,t1) <- tcSplitAppTy_maybe ty1
- = do { coi_s <- uTys nb1 s1 nb2 s2; coi_t <- uTys nb1 t1 nb2 t2
- ; return $ mkAppTyCoI s1 coi_s t1 coi_t }
-
- -- One or both outermost constructors are type family applications.
- -- If we can normalise them away, proceed as usual; otherwise, we
- -- need to defer unification by generating a wanted equality constraint.
- go1 outer ty1 ty2
- | ty1_is_fun || ty2_is_fun
- = do { (coi1, ty1') <- if ty1_is_fun then tcNormalizeFamInst ty1
- else return (IdCo, ty1)
- ; (coi2, ty2') <- if ty2_is_fun then tcNormalizeFamInst ty2
- else return (IdCo, ty2)
- ; coi <- if isOpenSynTyConApp ty1' || isOpenSynTyConApp ty2'
- then do { -- One type family app can't be reduced yet
- -- => defer
- ; ty1'' <- zonkTcType ty1'
- ; ty2'' <- zonkTcType ty2'
- ; if tcEqType ty1'' ty2''
- then return IdCo
- else -- see [Deferred Unification]
- defer_unification outer False orig_ty1 orig_ty2
- }
- else -- unification can proceed
- go outer ty1' ty2'
- ; return $ coi1 `mkTransCoI` coi `mkTransCoI` (mkSymCoI coi2)
- }
- where
- ty1_is_fun = isOpenSynTyConApp ty1
- ty2_is_fun = isOpenSynTyConApp ty2
-
- -- Anything else fails
- go1 outer _ _ = unifyMisMatch outer False orig_ty1 orig_ty2
-
+ = do { coi_s <- uType_np origin s1 s2
+ ; coi_t <- uType origin t1 t2
+ ; return $ mkAppCo coi_s coi_t }
+
+ go ty1 ty2
+ | tcIsForAllTy ty1 || tcIsForAllTy ty2
+ = unifySigmaTy origin ty1 ty2
+
+ -- Anything else fails
+ go _ _ = bale_out origin
+
+unifySigmaTy :: [EqOrigin] -> TcType -> TcType -> TcM Coercion
+unifySigmaTy origin ty1 ty2
+ = do { let (tvs1, body1) = tcSplitForAllTys ty1
+ (tvs2, body2) = tcSplitForAllTys ty2
+ ; unless (equalLength tvs1 tvs2) (failWithMisMatch origin)
+ ; skol_tvs <- tcInstSkolTyVars tvs1
+ -- Get location from monad, not from tvs1
+ ; let tys = mkTyVarTys skol_tvs
+ in_scope = mkInScopeSet (mkVarSet skol_tvs)
+ phi1 = Type.substTy (mkTvSubst in_scope (zipTyEnv tvs1 tys)) body1
+ phi2 = Type.substTy (mkTvSubst in_scope (zipTyEnv tvs2 tys)) body2
+
+ ; ((coi, _untch), lie) <- captureConstraints $
+ captureUntouchables $
+ uType origin phi1 phi2
+ -- Check for escape; e.g. (forall a. a->b) ~ (forall a. a->a)
+ -- VERY UNSATISFACTORY; the constraint might be fine, but
+ -- we fail eagerly because we don't have any place to put
+ -- the bindings from an implication constraint
+ -- This only works because most constraints get solved on the fly
+ -- See Note [Avoid deferring]
+ ; when (any (`elemVarSet` tyVarsOfWC lie) skol_tvs)
+ (failWithMisMatch origin) -- ToDo: give details from bad_lie
+
+ ; emitConstraints lie
+ ; return (foldr mkForAllCo coi skol_tvs) }
----------
-uPred outer nb1 (IParam n1 t1) nb2 (IParam n2 t2)
- | n1 == n2 =
- do { coi <- uTys nb1 t1 nb2 t2
- ; return $ mkIParamPredCoI n1 coi
- }
-uPred outer nb1 (ClassP c1 tys1) nb2 (ClassP c2 tys2)
- | c1 == c2 =
- do { cois <- uTys_s nb1 tys1 nb2 tys2 -- Guaranteed equal lengths because the kinds check
- ; return $ mkClassPPredCoI c1 tys1 cois
- }
-uPred outer _ p1 _ p2 = unifyMisMatch outer False (mkPredTy p1) (mkPredTy p2)
-
-uPreds outer nb1 [] nb2 [] = return []
-uPreds outer nb1 (p1:ps1) nb2 (p2:ps2) =
- do { coi <- uPred outer nb1 p1 nb2 p2
- ; cois <- uPreds outer nb1 ps1 nb2 ps2
- ; return (coi:cois)
- }
-uPreds outer nb1 ps1 nb2 ps2 = panic "uPreds"
+uPred :: [EqOrigin] -> PredType -> PredType -> TcM Coercion
+uPred origin (IParam n1 t1) (IParam n2 t2)
+ | n1 == n2
+ = do { coi <- uType origin t1 t2
+ ; return $ mkPredCo $ IParam n1 coi }
+uPred origin (ClassP c1 tys1) (ClassP c2 tys2)
+ | c1 == c2
+ = do { cois <- uList origin uType tys1 tys2
+ -- Guaranteed equal lengths because the kinds check
+ ; return $ mkPredCo $ ClassP c1 cois }
+
+uPred origin (EqPred ty1a ty1b) (EqPred ty2a ty2b)
+ = do { coa <- uType origin ty1a ty2a
+ ; cob <- uType origin ty1b ty2b
+ ; return $ mkPredCo $ EqPred coa cob }
+
+uPred origin _ _ = failWithMisMatch origin
+
+---------------
+uList :: [EqOrigin]
+ -> ([EqOrigin] -> a -> a -> TcM b)
+ -> [a] -> [a] -> TcM [b]
+-- Unify corresponding elements of two lists of types, which
+-- should be of equal length. We charge down the list explicitly so that
+-- we can complain if their lengths differ.
+uList _ _ [] [] = return []
+uList origin unify (ty1:tys1) (ty2:tys2) = do { x <- unify origin ty1 ty2;
+ ; xs <- uList origin unify tys1 tys2
+ ; return (x:xs) }
+uList origin _ _ _ = failWithMisMatch origin
+ -- See Note [Mismatched type lists and application decomposition]
+
\end{code}
+Note [Care with type applications]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Note: type applications need a bit of care!
+They can match FunTy and TyConApp, so use splitAppTy_maybe
+NB: we've already dealt with type variables and Notes,
+so if one type is an App the other one jolly well better be too
+
+Note [Unifying AppTy]
+~~~~~~~~~~~~~~~~~~~~~
+Considerm unifying (m Int) ~ (IO Int) where m is a unification variable
+that is now bound to (say) (Bool ->). Then we want to report
+ "Can't unify (Bool -> Int) with (IO Int)
+and not
+ "Can't unify ((->) Bool) with IO"
+That is why we use the "_np" variant of uType, which does not alter the error
+message.
+
Note [TyCon app]
~~~~~~~~~~~~~~~~
When we find two TyConApps, the argument lists are guaranteed equal
length. Reason: intially the kinds of the two types to be unified is
the same. The only way it can become not the same is when unifying two
-AppTys (f1 a1):=:(f2 a2). In that case there can't be a TyConApp in
-the f1,f2 (because it'd absorb the app). If we unify f1:=:f2 first,
+AppTys (f1 a1)~(f2 a2). In that case there can't be a TyConApp in
+the f1,f2 (because it'd absorb the app). If we unify f1~f2 first,
which we do, that ensures that f1,f2 have the same kind; and that
means a1,a2 have the same kind. And now the argument repeats.
-Note [OpenSynTyCon app]
-~~~~~~~~~~~~~~~~~~~~~~~
-Given
+Note [Mismatched type lists and application decomposition]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+When we find two TyConApps, you might think that the argument lists
+are guaranteed equal length. But they aren't. Consider matching
+ w (T x) ~ Foo (T x y)
+We do match (w ~ Foo) first, but in some circumstances we simply create
+a deferred constraint; and then go ahead and match (T x ~ T x y).
+This came up in Trac #3950.
- type family T a :: * -> *
+So either
+ (a) either we must check for identical argument kinds
+ when decomposing applications,
+
+ (b) or we must be prepared for ill-kinded unification sub-problems
-the two types (T () a) and (T () Int) must unify, even if there are
-no type instances for T at all. Should we just turn them into an
-equality (T () a ~ T () Int)? I don't think so. We currently try to
-eagerly unify everything we can before generating equalities; otherwise,
-we could turn the unification of [Int] with [a] into an equality, too.
+Currently we adopt (b) since it seems more robust -- no need to maintain
+a global invariant.
Note [Unification and synonyms]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If you are tempted to make a short cut on synonyms, as in this
pseudocode...
-\begin{verbatim}
--- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
--- NO = if (con1 == con2) then
--- NO -- Good news! Same synonym constructors, so we can shortcut
--- NO -- by unifying their arguments and ignoring their expansions.
--- NO unifyTypepeLists args1 args2
--- NO else
--- NO -- Never mind. Just expand them and try again
--- NO uTys ty1 ty2
-\end{verbatim}
+ uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
+ = if (con1 == con2) then
+ -- Good news! Same synonym constructors, so we can shortcut
+ -- by unifying their arguments and ignoring their expansions.
+ unifyTypepeLists args1 args2
+ else
+ -- Never mind. Just expand them and try again
+ uTys ty1 ty2
then THINK AGAIN. Here is the whole story, as detected and reported
-by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
-\begin{quotation}
+by Chris Okasaki:
+
Here's a test program that should detect the problem:
-\begin{verbatim}
- type Bogus a = Int
- x = (1 :: Bogus Char) :: Bogus Bool
-\end{verbatim}
+ type Bogus a = Int
+ x = (1 :: Bogus Char) :: Bogus Bool
The problem with [the attempted shortcut code] is that
-\begin{verbatim}
- con1 == con2
-\end{verbatim}
+
+ con1 == con2
+
is not a sufficient condition to be able to use the shortcut!
You also need to know that the type synonym actually USES all
its arguments. For example, consider the following type synonym
which does not use all its arguments.
-\begin{verbatim}
- type Bogus a = Int
-\end{verbatim}
-If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
-the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
-would fail, even though the expanded forms (both \tr{Int}) should
-match.
+ type Bogus a = Int
-Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
-unnecessarily bind \tr{t} to \tr{Char}.
+If you ever tried unifying, say, (Bogus Char) with )Bogus Bool), the
+unifier would blithely try to unify Char with Bool and would fail,
+even though the expanded forms (both Int) should match. Similarly,
+unifying (Bogus Char) with (Bogus t) would unnecessarily bind t to
+Char.
... You could explicitly test for the problem synonyms and mark them
somehow as needing expansion, perhaps also issuing a warning to the
user.
-\end{quotation}
-
-
-%************************************************************************
-%* *
-\subsection[Unify-uVar]{@uVar@: unifying with a type variable}
-%* *
-%************************************************************************
-
-@uVar@ is called when at least one of the types being unified is a
-variable. It does {\em not} assume that the variable is a fixed point
-of the substitution; rather, notice that @uVar@ (defined below) nips
-back into @uTys@ if it turns out that the variable is already bound.
-
-\begin{code}
-uVar :: Outer
- -> SwapFlag -- False => tyvar is the "actual" (ty is "expected")
- -- True => ty is the "actual" (tyvar is "expected")
- -> TcTyVar
- -> InBox -- True <=> definitely no boxes in t2
- -> TcTauType -> TcTauType -- printing and real versions
- -> TcM CoercionI
-
-uVar outer swapped tv1 nb2 ps_ty2 ty2
- = do { let expansion | showSDoc (ppr ty2) == showSDoc (ppr ps_ty2) = empty
- | otherwise = brackets (equals <+> ppr ty2)
- ; traceTc (text "uVar" <+> ppr swapped <+>
- sep [ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1 ),
- nest 2 (ptext SLIT(" <-> ")),
- ppr ps_ty2 <+> dcolon <+> ppr (typeKind ty2) <+> expansion])
- ; details <- lookupTcTyVar tv1
- ; case details of
- IndirectTv ty1
- | swapped -> u_tys outer nb2 ps_ty2 ty2 True ty1 ty1 -- Swap back
- | otherwise -> u_tys outer True ty1 ty1 nb2 ps_ty2 ty2 -- Same order
- -- The 'True' here says that ty1 is now inside a box
- DoneTv details1 -> uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
- }
-
-----------------
-uUnfilledVar :: Outer
- -> SwapFlag
- -> TcTyVar -> TcTyVarDetails -- Tyvar 1
- -> TcTauType -> TcTauType -- Type 2
- -> TcM CoercionI
--- Invariant: tyvar 1 is not unified with anything
-
-uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
- | Just ty2' <- tcView ty2
- = -- Expand synonyms; ignore FTVs
- uUnfilledVar False swapped tv1 details1 ps_ty2 ty2'
-
-uUnfilledVar outer swapped tv1 details1 ps_ty2 (TyVarTy tv2)
- | tv1 == tv2 -- Same type variable => no-op (but watch out for the boxy case)
- = case details1 of
- MetaTv BoxTv ref1 -- A boxy type variable meets itself;
- -- this is box-meets-box, so fill in with a tau-type
- -> do { tau_tv <- tcInstTyVar tv1
- ; updateMeta tv1 ref1 (mkTyVarTy tau_tv)
- ; return IdCo
- }
- other -> returnM IdCo -- No-op
-
- | otherwise -- Distinct type variables
- = do { lookup2 <- lookupTcTyVar tv2
- ; case lookup2 of
- IndirectTv ty2' -> uUnfilledVar outer swapped tv1 details1 ty2' ty2'
- DoneTv details2 -> uUnfilledVars outer swapped tv1 details1 tv2 details2
- }
-
-uUnfilledVar outer swapped tv1 details1 ps_ty2 non_var_ty2
- = -- ty2 is not a type variable
- case details1 of
- MetaTv (SigTv _) _ -> rigid_variable
- MetaTv info ref1 ->
- do { uMetaVar swapped tv1 info ref1 ps_ty2 non_var_ty2
- ; return IdCo
- }
- SkolemTv _ -> rigid_variable
- where
- rigid_variable
- | isOpenSynTyConApp non_var_ty2
- = -- 'non_var_ty2's outermost constructor is a type family,
- -- which we may may be able to normalise
- do { (coi2, ty2') <- tcNormalizeFamInst non_var_ty2
- ; case coi2 of
- IdCo -> -- no progress, but maybe after other instantiations
- defer_unification outer swapped (TyVarTy tv1) ps_ty2
- ACo co -> -- progress: so lets try again
- do { traceTc $
- ppr co <+> text "::"<+> ppr non_var_ty2 <+> text "~" <+>
- ppr ty2'
- ; coi <- uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2'
- ; let coi2' = (if swapped then id else mkSymCoI) coi2
- ; return $ coi2' `mkTransCoI` coi
- }
- }
- | SkolemTv RuntimeUnkSkol <- details1
- -- runtime unknown will never match
- = unifyMisMatch outer swapped (TyVarTy tv1) ps_ty2
- | otherwise -- defer as a given equality may still resolve this
- = defer_unification outer swapped (TyVarTy tv1) ps_ty2
-\end{code}
Note [Deferred Unification]
-~~~~~~~~~~~~~~~~~~~~
-We may encounter a unification ty1 = ty2 that cannot be performed syntactically,
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+We may encounter a unification ty1 ~ ty2 that cannot be performed syntactically,
and yet its consistency is undetermined. Previously, there was no way to still
-make it consistent. So a mismatch error was issued.
+make it consistent. So a mismatch error was issued.
Now these unfications are deferred until constraint simplification, where type
family instances and given equations may (or may not) establish the consistency.
-Deferred unifications are of the form
- F ... ~ ...
-or x ~ ...
-where F is a type function and x is a type variable.
-E.g.
- id :: x ~ y => x -> y
- id e = e
+Deferred unifications are of the form
+ F ... ~ ...
+or x ~ ...
+where F is a type function and x is a type variable.
+E.g.
+ id :: x ~ y => x -> y
+ id e = e
involves the unfication x = y. It is deferred until we bring into account the
context x ~ y to establish that it holds.
If available, we defer original types (rather than those where closed type
-synonyms have already been expanded via tcCoreView). This is as usual, to
+synonyms have already been expanded via tcCoreView). This is, as usual, to
improve error messages.
+
+%************************************************************************
+%* *
+ uVar and friends
+%* *
+%************************************************************************
+
+@uVar@ is called when at least one of the types being unified is a
+variable. It does {\em not} assume that the variable is a fixed point
+of the substitution; rather, notice that @uVar@ (defined below) nips
+back into @uTys@ if it turns out that the variable is already bound.
+
\begin{code}
-defer_unification :: Bool -- pop innermost context?
- -> SwapFlag
- -> TcType
- -> TcType
- -> TcM CoercionI
-defer_unification outer True ty1 ty2
- = defer_unification outer False ty2 ty1
-defer_unification outer False ty1 ty2
- = do { ty1' <- zonkTcType ty1
- ; ty2' <- zonkTcType ty2
- ; traceTc $ text "deferring:" <+> ppr ty1 <+> text "~" <+> ppr ty2
- ; cotv <- newMetaTyVar TauTv (mkCoKind ty1' ty2')
- -- put ty1 ~ ty2 in LIE
- -- Left means "wanted"
- ; inst <- (if outer then popErrCtxt else id) $
- mkEqInst (EqPred ty1' ty2') (Left cotv)
- ; extendLIE inst
- ; return $ ACo $ TyVarTy cotv }
+uVar :: [EqOrigin] -> SwapFlag -> TcTyVar -> TcTauType -> TcM Coercion
+uVar origin swapped tv1 ty2
+ = do { traceTc "uVar" (vcat [ ppr origin
+ , ppr swapped
+ , ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1)
+ , nest 2 (ptext (sLit " ~ "))
+ , ppr ty2 <+> dcolon <+> ppr (typeKind ty2)])
+ ; details <- lookupTcTyVar tv1
+ ; case details of
+ Filled ty1 -> unSwap swapped (uType_np origin) ty1 ty2
+ Unfilled details1 -> uUnfilledVar origin swapped tv1 details1 ty2
+ }
----------------
-uMetaVar :: SwapFlag
- -> TcTyVar -> BoxInfo -> IORef MetaDetails
- -> TcType -> TcType
- -> TcM ()
--- tv1 is an un-filled-in meta type variable (maybe boxy, maybe tau)
--- ty2 is not a type variable
-
-uMetaVar swapped tv1 BoxTv ref1 ps_ty2 non_var_ty2
- = -- tv1 is a BoxTv. So we must unbox ty2, to ensure
- -- that any boxes in ty2 are filled with monotypes
- --
- -- It should not be the case that tv1 occurs in ty2
- -- (i.e. no occurs check should be needed), but if perchance
- -- it does, the unbox operation will fill it, and the DEBUG
- -- checks for that.
- do { final_ty <- unBox ps_ty2
-#ifdef DEBUG
- ; meta_details <- readMutVar ref1
- ; case meta_details of
- Indirect ty -> WARN( True, ppr tv1 <+> ppr ty )
- return () -- This really should *not* happen
- Flexi -> return ()
-#endif
- ; checkUpdateMeta swapped tv1 ref1 final_ty }
-
-uMetaVar swapped tv1 info1 ref1 ps_ty2 non_var_ty2
- = do { final_ty <- checkTauTvUpdate tv1 ps_ty2 -- Occurs check + monotype check
- ; checkUpdateMeta swapped tv1 ref1 final_ty }
+uUnfilledVar :: [EqOrigin]
+ -> SwapFlag
+ -> TcTyVar -> TcTyVarDetails -- Tyvar 1
+ -> TcTauType -- Type 2
+ -> TcM Coercion
+-- "Unfilled" means that the variable is definitely not a filled-in meta tyvar
+-- It might be a skolem, or untouchable, or meta
+
+uUnfilledVar origin swapped tv1 details1 (TyVarTy tv2)
+ | tv1 == tv2 -- Same type variable => no-op
+ = return (mkReflCo (mkTyVarTy tv1))
+
+ | otherwise -- Distinct type variables
+ = do { lookup2 <- lookupTcTyVar tv2
+ ; case lookup2 of
+ Filled ty2' -> uUnfilledVar origin swapped tv1 details1 ty2'
+ Unfilled details2 -> uUnfilledVars origin swapped tv1 details1 tv2 details2
+ }
+
+uUnfilledVar origin swapped tv1 details1 non_var_ty2 -- ty2 is not a type variable
+ = case details1 of
+ MetaTv TauTv ref1
+ -> do { mb_ty2' <- checkTauTvUpdate tv1 non_var_ty2
+ ; case mb_ty2' of
+ Nothing -> do { traceTc "Occ/kind defer" (ppr tv1); defer }
+ Just ty2' -> updateMeta tv1 ref1 ty2'
+ }
+
+ _other -> do { traceTc "Skolem defer" (ppr tv1); defer } -- Skolems of all sorts
+ where
+ defer | Just ty2' <- tcView non_var_ty2 -- Note [Avoid deferring]
+ -- non_var_ty2 isn't expanded yet
+ = uUnfilledVar origin swapped tv1 details1 ty2'
+ | otherwise
+ = unSwap swapped (uType_defer origin) (mkTyVarTy tv1) non_var_ty2
+ -- Occurs check or an untouchable: just defer
+ -- NB: occurs check isn't necessarily fatal:
+ -- eg tv1 occured in type family parameter
----------------
-uUnfilledVars :: Outer
- -> SwapFlag
- -> TcTyVar -> TcTyVarDetails -- Tyvar 1
- -> TcTyVar -> TcTyVarDetails -- Tyvar 2
- -> TcM CoercionI
--- Invarant: The type variables are distinct,
--- Neither is filled in yet
--- They might be boxy or not
-
-uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (SkolemTv _)
- = -- see [Deferred Unification]
- defer_unification outer swapped (mkTyVarTy tv1) (mkTyVarTy tv2)
-
-uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (SkolemTv _)
- = checkUpdateMeta swapped tv1 ref1 (mkTyVarTy tv2) >> return IdCo
-uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (MetaTv info2 ref2)
- = checkUpdateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1) >> return IdCo
-
--- ToDo: this function seems too long for what it acutally does!
-uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (MetaTv info2 ref2)
- = case (info1, info2) of
- (BoxTv, BoxTv) -> box_meets_box >> return IdCo
-
- -- If a box meets a TauTv, but the fomer has the smaller kind
- -- then we must create a fresh TauTv with the smaller kind
- (_, BoxTv) | k1_sub_k2 -> update_tv2 >> return IdCo
- | otherwise -> box_meets_box >> return IdCo
- (BoxTv, _ ) | k2_sub_k1 -> update_tv1 >> return IdCo
- | otherwise -> box_meets_box >> return IdCo
-
- -- Avoid SigTvs if poss
- (SigTv _, _ ) | k1_sub_k2 -> update_tv2 >> return IdCo
- (_, SigTv _) | k2_sub_k1 -> update_tv1 >> return IdCo
-
- (_, _) | k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1
- then update_tv1 >> return IdCo -- Same kinds
- else update_tv2 >> return IdCo
- | k2_sub_k1 -> update_tv1 >> return IdCo
- | otherwise -> kind_err >> return IdCo
-
- -- Update the variable with least kind info
- -- See notes on type inference in Kind.lhs
- -- The "nicer to" part only applies if the two kinds are the same,
- -- so we can choose which to do.
+uUnfilledVars :: [EqOrigin]
+ -> SwapFlag
+ -> TcTyVar -> TcTyVarDetails -- Tyvar 1
+ -> TcTyVar -> TcTyVarDetails -- Tyvar 2
+ -> TcM Coercion
+-- Invarant: The type variables are distinct,
+-- Neither is filled in yet
+
+uUnfilledVars origin swapped tv1 details1 tv2 details2
+ = case (details1, details2) of
+ (MetaTv i1 ref1, MetaTv i2 ref2)
+ | k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1 i1 i2
+ then updateMeta tv1 ref1 ty2
+ else updateMeta tv2 ref2 ty1
+ | k2_sub_k1 -> updateMeta tv1 ref1 ty2
+
+ (_, MetaTv _ ref2) | k1_sub_k2 -> updateMeta tv2 ref2 ty1
+ (MetaTv _ ref1, _) | k2_sub_k1 -> updateMeta tv1 ref1 ty2
+
+ (_, _) -> unSwap swapped (uType_defer origin) ty1 ty2
+ -- Defer for skolems of all sorts
where
- -- Kinds should be guaranteed ok at this point
- update_tv1 = updateMeta tv1 ref1 (mkTyVarTy tv2)
- update_tv2 = updateMeta tv2 ref2 (mkTyVarTy tv1)
-
- box_meets_box | k1_sub_k2 = if k2_sub_k1 && nicer_to_update_tv1
- then fill_from tv2
- else fill_from tv1
- | k2_sub_k1 = fill_from tv2
- | otherwise = kind_err
-
- -- Update *both* tyvars with a TauTv whose name and kind
- -- are gotten from tv (avoid losing nice names is poss)
- fill_from tv = do { tv' <- tcInstTyVar tv
- ; let tau_ty = mkTyVarTy tv'
- ; updateMeta tv1 ref1 tau_ty
- ; updateMeta tv2 ref2 tau_ty }
-
- kind_err = addErrCtxtM (unifyKindCtxt swapped tv1 (mkTyVarTy tv2)) $
- unifyKindMisMatch k1 k2
-
- k1 = tyVarKind tv1
- k2 = tyVarKind tv2
+ k1 = tyVarKind tv1
+ k2 = tyVarKind tv2
k1_sub_k2 = k1 `isSubKind` k2
k2_sub_k1 = k2 `isSubKind` k1
+ ty1 = mkTyVarTy tv1
+ ty2 = mkTyVarTy tv2
- nicer_to_update_tv1 = isSystemName (Var.varName tv1)
- -- Try to update sys-y type variables in preference to ones
- -- gotten (say) by instantiating a polymorphic function with
- -- a user-written type sig
-
-----------------
-checkUpdateMeta :: SwapFlag
- -> TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
--- Update tv1, which is flexi; occurs check is alrady done
--- The 'check' version does a kind check too
--- We do a sub-kind check here: we might unify (a b) with (c d)
--- where b::*->* and d::*; this should fail
-
-checkUpdateMeta swapped tv1 ref1 ty2
- = do { checkKinds swapped tv1 ty2
- ; updateMeta tv1 ref1 ty2 }
-
-updateMeta :: TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
-updateMeta tv1 ref1 ty2
- = ASSERT( isMetaTyVar tv1 )
- ASSERT( isBoxyTyVar tv1 || isTauTy ty2 )
- do { ASSERTM2( do { details <- readMetaTyVar tv1; return (isFlexi details) }, ppr tv1 )
- ; traceTc (text "updateMeta" <+> ppr tv1 <+> text ":=" <+> ppr ty2)
- ; writeMutVar ref1 (Indirect ty2)
- }
+ nicer_to_update_tv1 _ SigTv = True
+ nicer_to_update_tv1 SigTv _ = False
+ nicer_to_update_tv1 _ _ = isSystemName (Var.varName tv1)
+ -- Try not to update SigTvs; and try to update sys-y type
+ -- variables in preference to ones gotten (say) by
+ -- instantiating a polymorphic function with a user-written
+ -- type sig
----------------
-checkKinds swapped tv1 ty2
--- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
--- ty2 has been zonked at this stage, which ensures that
--- its kind has as much boxity information visible as possible.
- | tk2 `isSubKind` tk1 = returnM ()
-
- | otherwise
- -- Either the kinds aren't compatible
- -- (can happen if we unify (a b) with (c d))
- -- or we are unifying a lifted type variable with an
- -- unlifted type: e.g. (id 3#) is illegal
- = addErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
- unifyKindMisMatch k1 k2
- where
- (k1,k2) | swapped = (tk2,tk1)
- | otherwise = (tk1,tk2)
- tk1 = tyVarKind tv1
- tk2 = typeKind ty2
-
-----------------
-checkTauTvUpdate :: TcTyVar -> TcType -> TcM TcType
+checkTauTvUpdate :: TcTyVar -> TcType -> TcM (Maybe TcType)
-- (checkTauTvUpdate tv ty)
-- We are about to update the TauTv tv with ty.
-- Check (a) that tv doesn't occur in ty (occurs check)
--- (b) that ty is a monotype
--- Furthermore, in the interest of (b), if you find an
--- empty box (BoxTv that is Flexi), fill it in with a TauTv
+-- (b) that kind(ty) is a sub-kind of kind(tv)
+-- (c) that ty does not contain any type families, see Note [Type family sharing]
--
--- Returns the (non-boxy) type to update the type variable with, or fails
-
-checkTauTvUpdate orig_tv orig_ty
- = go orig_ty
- where
- go (TyConApp tc tys)
- | isSynTyCon tc = go_syn tc tys
- | otherwise = do { tys' <- mappM go tys; return (TyConApp tc tys') }
- go (NoteTy _ ty2) = go ty2 -- Discard free-tyvar annotations
- go (PredTy p) = do { p' <- go_pred p; return (PredTy p') }
- go (FunTy arg res) = do { arg' <- go arg; res' <- go res; return (FunTy arg' res') }
- go (AppTy fun arg) = do { fun' <- go fun; arg' <- go arg; return (mkAppTy fun' arg') }
- -- NB the mkAppTy; we might have instantiated a
- -- type variable to a type constructor, so we need
- -- to pull the TyConApp to the top.
- go (ForAllTy tv ty) = notMonoType orig_ty -- (b)
-
- go (TyVarTy tv)
- | orig_tv == tv = occurCheck tv orig_ty -- (a)
- | isTcTyVar tv = go_tyvar tv (tcTyVarDetails tv)
- | otherwise = return (TyVarTy tv)
- -- Ordinary (non Tc) tyvars
- -- occur inside quantified types
-
- go_pred (ClassP c tys) = do { tys' <- mapM go tys; return (ClassP c tys') }
- go_pred (IParam n ty) = do { ty' <- go ty; return (IParam n ty') }
- go_pred (EqPred t1 t2) = do { t1' <- go t1; t2' <- go t2; return (EqPred t1' t2') }
-
- go_tyvar tv (SkolemTv _) = return (TyVarTy tv)
- go_tyvar tv (MetaTv box ref)
- = do { cts <- readMutVar ref
- ; case cts of
- Indirect ty -> go ty
- Flexi -> case box of
- BoxTv -> fillBoxWithTau tv ref
- other -> return (TyVarTy tv)
- }
-
- -- go_syn is called for synonyms only
- -- See Note [Type synonyms and the occur check]
- go_syn tc tys
- | not (isTauTyCon tc)
- = notMonoType orig_ty -- (b) again
- | otherwise
- = do { (msgs, mb_tys') <- tryTc (mapM go tys)
- ; case mb_tys' of
- Just tys' -> return (TyConApp tc tys')
- -- Retain the synonym (the common case)
- Nothing | isOpenTyCon tc
- -> notMonoArgs (TyConApp tc tys)
- -- Synonym families must have monotype args
- | otherwise
- -> go (expectJust "checkTauTvUpdate"
- (tcView (TyConApp tc tys)))
- -- Try again, expanding the synonym
- }
-
-fillBoxWithTau :: BoxyTyVar -> IORef MetaDetails -> TcM TcType
--- (fillBoxWithTau tv ref) fills ref with a freshly allocated
--- tau-type meta-variable, whose print-name is the same as tv
--- Choosing the same name is good: when we instantiate a function
--- we allocate boxy tyvars with the same print-name as the quantified
--- tyvar; and then we often fill the box with a tau-tyvar, and again
--- we want to choose the same name.
-fillBoxWithTau tv ref
- = do { tv' <- tcInstTyVar tv -- Do not gratuitously forget
- ; let tau = mkTyVarTy tv' -- name of the type variable
- ; writeMutVar ref (Indirect tau)
- ; return tau }
+-- We have two possible outcomes:
+-- (1) Return the type to update the type variable with,
+-- [we know the update is ok]
+-- (2) Return Nothing,
+-- [the update might be dodgy]
+--
+-- Note that "Nothing" does not mean "definite error". For example
+-- type family F a
+-- type instance F Int = Int
+-- consider
+-- a ~ F a
+-- This is perfectly reasonable, if we later get a ~ Int. For now, though,
+-- we return Nothing, leaving it to the later constraint simplifier to
+-- sort matters out.
+
+checkTauTvUpdate tv ty
+ = do { ty' <- zonkTcType ty
+ ; if typeKind ty' `isSubKind` tyVarKind tv then
+ case ok ty' of
+ Nothing -> return Nothing
+ Just ty'' -> return (Just ty'')
+ else return Nothing }
+
+ where ok :: TcType -> Maybe TcType
+ ok (TyVarTy tv') | not (tv == tv') = Just (TyVarTy tv')
+ ok this_ty@(TyConApp tc tys)
+ | not (isSynFamilyTyCon tc), Just tys' <- allMaybes (map ok tys)
+ = Just (TyConApp tc tys')
+ | isSynTyCon tc, Just ty_expanded <- tcView this_ty
+ = ok ty_expanded -- See Note [Type synonyms and the occur check]
+ ok (PredTy sty) | Just sty' <- ok_pred sty = Just (PredTy sty')
+ ok (FunTy arg res) | Just arg' <- ok arg, Just res' <- ok res
+ = Just (FunTy arg' res')
+ ok (AppTy fun arg) | Just fun' <- ok fun, Just arg' <- ok arg
+ = Just (AppTy fun' arg')
+ ok (ForAllTy tv1 ty1) | Just ty1' <- ok ty1 = Just (ForAllTy tv1 ty1')
+ -- Fall-through
+ ok _ty = Nothing
+
+ ok_pred (IParam nm ty) | Just ty' <- ok ty = Just (IParam nm ty')
+ ok_pred (ClassP cl tys)
+ | Just tys' <- allMaybes (map ok tys)
+ = Just (ClassP cl tys')
+ ok_pred (EqPred ty1 ty2)
+ | Just ty1' <- ok ty1, Just ty2' <- ok ty2
+ = Just (EqPred ty1' ty2')
+ -- Fall-through
+ ok_pred _pty = Nothing
\end{code}
-Note [Type synonyms and the occur check]
-~~~~~~~~~~~~~~~~~~~~
-Basically we want to update tv1 := ps_ty2
-because ps_ty2 has type-synonym info, which improves later error messages
-
-But consider
- type A a = ()
+Note [Avoid deferring]
+~~~~~~~~~~~~~~~~~~~~~~
+We try to avoid creating deferred constraints for two reasons.
+ * First, efficiency.
+ * Second, currently we can only defer some constraints
+ under a forall. See unifySigmaTy.
+So expanding synonyms here is a good thing to do. Example (Trac #4917)
+ a ~ Const a b
+where type Const a b = a. We can solve this immediately, even when
+'a' is a skolem, just by expanding the synonym; and we should do so
+ in case this unification happens inside unifySigmaTy (sigh).
- f :: (A a -> a -> ()) -> ()
- f = \ _ -> ()
-
- x :: ()
- x = f (\ x p -> p x)
-
-In the application (p x), we try to match "t" with "A t". If we go
-ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
-an infinite loop later.
-But we should not reject the program, because A t = ().
-Rather, we should bind t to () (= non_var_ty2).
+Note [Type synonyms and the occur check]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Generally speaking we try to update a variable with type synonyms not
+expanded, which improves later error messages, unless looking
+inside a type synonym may help resolve a spurious occurs check
+error. Consider:
+ type A a = ()
+
+ f :: (A a -> a -> ()) -> ()
+ f = \ _ -> ()
+
+ x :: ()
+ x = f (\ x p -> p x)
+
+We will eventually get a constraint of the form t ~ A t. The ok function above will
+properly expand the type (A t) to just (), which is ok to be unified with t. If we had
+unified with the original type A t, we would lead the type checker into an infinite loop.
+
+Hence, if the occurs check fails for a type synonym application, then (and *only* then),
+the ok function expands the synonym to detect opportunities for occurs check success using
+the underlying definition of the type synonym.
+
+The same applies later on in the constraint interaction code; see TcInteract,
+function @occ_check_ok@.
+
+
+Note [Type family sharing]
+~~~~~~~~~~~~~~
+We must avoid eagerly unifying type variables to types that contain function symbols,
+because this may lead to loss of sharing, and in turn, in very poor performance of the
+constraint simplifier. Assume that we have a wanted constraint:
+{
+ m1 ~ [F m2],
+ m2 ~ [F m3],
+ m3 ~ [F m4],
+ D m1,
+ D m2,
+ D m3
+}
+where D is some type class. If we eagerly unify m1 := [F m2], m2 := [F m3], m3 := [F m2],
+then, after zonking, our constraint simplifier will be faced with the following wanted
+constraint:
+{
+ D [F [F [F m4]]],
+ D [F [F m4]],
+ D [F m4]
+}
+which has to be flattened by the constraint solver. However, because the sharing is lost,
+an polynomially larger number of flatten skolems will be created and the constraint sets
+we are working with will be polynomially larger.
+
+Instead, if we defer the unifications m1 := [F m2], etc. we will only be generating three
+flatten skolems, which is the maximum possible sharing arising from the original constraint.
\begin{code}
-refineBox :: TcType -> TcM TcType
--- Unbox the outer box of a boxy type (if any)
-refineBox ty@(TyVarTy box_tv)
- | isMetaTyVar box_tv
- = do { cts <- readMetaTyVar box_tv
- ; case cts of
- Flexi -> return ty
- Indirect ty -> return ty }
-refineBox other_ty = return other_ty
-
-refineBoxToTau :: TcType -> TcM TcType
--- Unbox the outer box of a boxy type, filling with a monotype if it is empty
--- Like refineBox except for the "fill with monotype" part.
-refineBoxToTau ty@(TyVarTy box_tv)
- | isMetaTyVar box_tv
- , MetaTv BoxTv ref <- tcTyVarDetails box_tv
- = do { cts <- readMutVar ref
- ; case cts of
- Flexi -> fillBoxWithTau box_tv ref
- Indirect ty -> return ty }
-refineBoxToTau other_ty = return other_ty
-
-zapToMonotype :: BoxySigmaType -> TcM TcTauType
--- Subtle... we must zap the boxy res_ty
--- to kind * before using it to instantiate a LitInst
--- Calling unBox instead doesn't do the job, because the box
--- often has an openTypeKind, and we don't want to instantiate
--- with that type.
-zapToMonotype res_ty
- = do { res_tau <- newFlexiTyVarTy liftedTypeKind
- ; boxyUnify res_tau res_ty
- ; return res_tau }
-
-unBox :: BoxyType -> TcM TcType
--- unBox implements the judgement
--- |- s' ~ box(s)
--- with input s', and result s
---
--- It removes all boxes from the input type, returning a non-boxy type.
--- A filled box in the type can only contain a monotype; unBox fails if not
--- The type can have empty boxes, which unBox fills with a monotype
---
--- Compare this wth checkTauTvUpdate
---
--- For once, it's safe to treat synonyms as opaque!
-
-unBox (NoteTy n ty) = do { ty' <- unBox ty; return (NoteTy n ty') }
-unBox (TyConApp tc tys) = do { tys' <- mapM unBox tys; return (TyConApp tc tys') }
-unBox (AppTy f a) = do { f' <- unBox f; a' <- unBox a; return (mkAppTy f' a') }
-unBox (FunTy f a) = do { f' <- unBox f; a' <- unBox a; return (FunTy f' a') }
-unBox (PredTy p) = do { p' <- unBoxPred p; return (PredTy p') }
-unBox (ForAllTy tv ty) = ASSERT( isImmutableTyVar tv )
- do { ty' <- unBox ty; return (ForAllTy tv ty') }
-unBox (TyVarTy tv)
- | isTcTyVar tv -- It's a boxy type variable
- , MetaTv BoxTv ref <- tcTyVarDetails tv -- NB: non-TcTyVars are possible
- = do { cts <- readMutVar ref -- under nested quantifiers
- ; case cts of
- Flexi -> fillBoxWithTau tv ref
- Indirect ty -> do { non_boxy_ty <- unBox ty
- ; if isTauTy non_boxy_ty
- then return non_boxy_ty
- else notMonoType non_boxy_ty }
- }
- | otherwise -- Skolems, and meta-tau-variables
- = return (TyVarTy tv)
-
-unBoxPred (ClassP cls tys) = do { tys' <- mapM unBox tys; return (ClassP cls tys') }
-unBoxPred (IParam ip ty) = do { ty' <- unBox ty; return (IParam ip ty') }
-unBoxPred (EqPred ty1 ty2) = do { ty1' <- unBox ty1; ty2' <- unBox ty2; return (EqPred ty1' ty2') }
+data LookupTyVarResult -- The result of a lookupTcTyVar call
+ = Unfilled TcTyVarDetails -- SkolemTv or virgin MetaTv
+ | Filled TcType
+
+lookupTcTyVar :: TcTyVar -> TcM LookupTyVarResult
+lookupTcTyVar tyvar
+ | MetaTv _ ref <- details
+ = do { meta_details <- readMutVar ref
+ ; case meta_details of
+ Indirect ty -> return (Filled ty)
+ Flexi -> do { is_untch <- isUntouchable tyvar
+ ; let -- Note [Unifying untouchables]
+ ret_details | is_untch = vanillaSkolemTv
+ | otherwise = details
+ ; return (Unfilled ret_details) } }
+ | otherwise
+ = return (Unfilled details)
+ where
+ details = ASSERT2( isTcTyVar tyvar, ppr tyvar )
+ tcTyVarDetails tyvar
+
+updateMeta :: TcTyVar -> TcRef MetaDetails -> TcType -> TcM Coercion
+updateMeta tv1 ref1 ty2
+ = do { writeMetaTyVarRef tv1 ref1 ty2
+ ; return (mkReflCo ty2) }
\end{code}
+Note [Unifying untouchables]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+We treat an untouchable type variable as if it was a skolem. That
+ensures it won't unify with anything. It's a slight had, because
+we return a made-up TcTyVarDetails, but I think it works smoothly.
%************************************************************************
-%* *
-\subsection[Unify-context]{Errors and contexts}
-%* *
+%* *
+ Errors and contexts
+%* *
%************************************************************************
-Errors
-~~~~~~
-
\begin{code}
-unifyCtxt act_ty exp_ty tidy_env
- = do { act_ty' <- zonkTcType act_ty
- ; exp_ty' <- zonkTcType exp_ty
- ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
- (env2, act_ty'') = tidyOpenType env1 act_ty'
- ; return (env2, mkExpectedActualMsg act_ty'' exp_ty'') }
+pushOrigin :: TcType -> TcType -> [EqOrigin] -> [EqOrigin]
+pushOrigin ty_act ty_exp origin
+ = UnifyOrigin { uo_actual = ty_act, uo_expected = ty_exp } : origin
-----------------
-mkExpectedActualMsg act_ty exp_ty
- = nest 2 (vcat [ text "Expected type" <> colon <+> ppr exp_ty,
- text "Inferred type" <> colon <+> ppr act_ty ])
+---------------
+wrapEqCtxt :: [EqOrigin] -> TcM a -> TcM a
+-- Build a suitable error context from the origin and do the thing inside
+-- The "couldn't match" error comes from the innermost item on the stack,
+-- and, if there is more than one item, the "Expected/inferred" part
+-- comes from the outermost item
+wrapEqCtxt [] thing_inside = thing_inside
+wrapEqCtxt items thing_inside = addErrCtxtM (unifyCtxt (last items)) thing_inside
+
+---------------
+failWithMisMatch :: [EqOrigin] -> TcM a
+-- Generate the message when two types fail to match,
+-- going to some trouble to make it helpful.
+-- We take the failing types from the top of the origin stack
+-- rather than reporting the particular ones we are looking
+-- at right now
+failWithMisMatch (item:origin)
+ = wrapEqCtxt origin $
+ do { ty_act <- zonkTcType (uo_actual item)
+ ; ty_exp <- zonkTcType (uo_expected item)
+ ; env0 <- tcInitTidyEnv
+ ; let (env1, pp_exp) = tidyOpenType env0 ty_exp
+ (env2, pp_act) = tidyOpenType env1 ty_act
+ ; failWithTcM (env2, misMatchMsg pp_act pp_exp) }
+failWithMisMatch []
+ = panic "failWithMisMatch"
+
+misMatchMsg :: TcType -> TcType -> SDoc
+misMatchMsg ty_act ty_exp
+ = sep [ ptext (sLit "Couldn't match expected type") <+> quotes (ppr ty_exp)
+ , nest 12 $ ptext (sLit "with actual type") <+> quotes (ppr ty_act)]
+\end{code}
+
+
+-----------------------------------------
+ UNUSED FOR NOW
+-----------------------------------------
----------------
+----------------
-- If an error happens we try to figure out whether the function
-- function has been given too many or too few arguments, and say so.
-addSubCtxt SubDone actual_res_ty expected_res_ty thing_inside
- = thing_inside
-addSubCtxt sub_ctxt actual_res_ty expected_res_ty thing_inside
+addSubCtxt :: InstOrigin -> TcType -> TcType -> TcM a -> TcM a
+addSubCtxt orig actual_res_ty expected_res_ty thing_inside
= addErrCtxtM mk_err thing_inside
where
mk_err tidy_env
= do { exp_ty' <- zonkTcType expected_res_ty
- ; act_ty' <- zonkTcType actual_res_ty
- ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
- (env2, act_ty'') = tidyOpenType env1 act_ty'
- (exp_args, _) = tcSplitFunTys exp_ty''
- (act_args, _) = tcSplitFunTys act_ty''
-
- len_act_args = length act_args
- len_exp_args = length exp_args
-
- message = case sub_ctxt of
- SubFun fun | len_exp_args < len_act_args -> wrongArgsCtxt "too few" fun
- | len_exp_args > len_act_args -> wrongArgsCtxt "too many" fun
- other -> mkExpectedActualMsg act_ty'' exp_ty''
- ; return (env2, message) }
-
- wrongArgsCtxt too_many_or_few fun
- = ptext SLIT("Probable cause:") <+> quotes (ppr fun)
- <+> ptext SLIT("is applied to") <+> text too_many_or_few
- <+> ptext SLIT("arguments")
-
-------------------
-unifyForAllCtxt tvs phi1 phi2 env
- = returnM (env2, msg)
- where
- (env', tvs') = tidyOpenTyVars env tvs -- NB: not tidyTyVarBndrs
- (env1, phi1') = tidyOpenType env' phi1
- (env2, phi2') = tidyOpenType env1 phi2
- msg = vcat [ptext SLIT("When matching") <+> quotes (ppr (mkForAllTys tvs' phi1')),
- ptext SLIT(" and") <+> quotes (ppr (mkForAllTys tvs' phi2'))]
-
-------------------
-unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
- -- tv1 and ty2 are zonked already
- = returnM msg
- where
- msg = (env2, ptext SLIT("When matching the kinds of") <+>
- sep [quotes pp_expected <+> ptext SLIT("and"), quotes pp_actual])
-
- (pp_expected, pp_actual) | swapped = (pp2, pp1)
- | otherwise = (pp1, pp2)
- (env1, tv1') = tidyOpenTyVar tidy_env tv1
- (env2, ty2') = tidyOpenType env1 ty2
- pp1 = ppr tv1' <+> dcolon <+> ppr (tyVarKind tv1)
- pp2 = ppr ty2' <+> dcolon <+> ppr (typeKind ty2)
+ ; act_ty' <- zonkTcType actual_res_ty
+ ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
+ (env2, act_ty'') = tidyOpenType env1 act_ty'
+ (exp_args, _) = tcSplitFunTys exp_ty''
+ (act_args, _) = tcSplitFunTys act_ty''
-unifyMisMatch outer swapped ty1 ty2
- = do { (env, msg) <- if swapped then misMatchMsg ty2 ty1
- else misMatchMsg ty1 ty2
+ len_act_args = length act_args
+ len_exp_args = length exp_args
- -- This is the whole point of the 'outer' stuff
- ; if outer then popErrCtxt (failWithTcM (env, msg))
- else failWithTcM (env, msg)
- }
-
------------------------
-notMonoType ty
- = do { ty' <- zonkTcType ty
- ; env0 <- tcInitTidyEnv
- ; let (env1, tidy_ty) = tidyOpenType env0 ty'
- msg = ptext SLIT("Cannot match a monotype with") <+> quotes (ppr tidy_ty)
- ; failWithTcM (env1, msg) }
-
-notMonoArgs ty
- = do { ty' <- zonkTcType ty
- ; env0 <- tcInitTidyEnv
- ; let (env1, tidy_ty) = tidyOpenType env0 ty'
- msg = ptext SLIT("Arguments of synonym family must be monotypes") <+> quotes (ppr tidy_ty)
- ; failWithTcM (env1, msg) }
-
-occurCheck tyvar ty
- = do { env0 <- tcInitTidyEnv
- ; ty' <- zonkTcType ty
- ; let (env1, tidy_tyvar) = tidyOpenTyVar env0 tyvar
- (env2, tidy_ty) = tidyOpenType env1 ty'
- extra = sep [ppr tidy_tyvar, char '=', ppr tidy_ty]
- ; failWithTcM (env2, hang msg 2 extra) }
- where
- msg = ptext SLIT("Occurs check: cannot construct the infinite type:")
-\end{code}
+ message = case orig of
+ OccurrenceOf fun
+ | len_exp_args < len_act_args -> wrongArgsCtxt "too few" fun
+ | len_exp_args > len_act_args -> wrongArgsCtxt "too many" fun
+ _ -> mkExpectedActualMsg act_ty'' exp_ty''
+ ; return (env2, message) }
%************************************************************************
-%* *
- Kind unification
-%* *
+%* *
+ Kind unification
+%* *
%************************************************************************
Unifying kinds is much, much simpler than unifying types.
\begin{code}
-unifyKind :: TcKind -- Expected
- -> TcKind -- Actual
- -> TcM ()
-unifyKind (TyConApp kc1 []) (TyConApp kc2 [])
- | isSubKindCon kc2 kc1 = returnM ()
+matchExpectedFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
+-- Like unifyFunTy, but does not fail; instead just returns Nothing
+
+matchExpectedFunKind (TyVarTy kvar) = do
+ maybe_kind <- readKindVar kvar
+ case maybe_kind of
+ Indirect fun_kind -> matchExpectedFunKind fun_kind
+ Flexi ->
+ do { arg_kind <- newKindVar
+ ; res_kind <- newKindVar
+ ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
+ ; return (Just (arg_kind,res_kind)) }
+
+matchExpectedFunKind (FunTy arg_kind res_kind) = return (Just (arg_kind,res_kind))
+matchExpectedFunKind _ = return Nothing
+
+-----------------
+unifyKind :: TcKind -- Expected
+ -> TcKind -- Actual
+ -> TcM ()
+
+unifyKind (TyConApp kc1 []) (TyConApp kc2 [])
+ | isSubKindCon kc2 kc1 = return ()
unifyKind (FunTy a1 r1) (FunTy a2 r2)
= do { unifyKind a2 a1; unifyKind r1 r2 }
- -- Notice the flip in the argument,
- -- so that the sub-kinding works right
+ -- Notice the flip in the argument,
+ -- so that the sub-kinding works right
unifyKind (TyVarTy kv1) k2 = uKVar False kv1 k2
unifyKind k1 (TyVarTy kv2) = uKVar True kv2 k1
unifyKind k1 k2 = unifyKindMisMatch k1 k2
-unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
-unifyKinds [] [] = returnM ()
-unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenM_`
- unifyKinds ks1 ks2
-unifyKinds _ _ = panic "unifyKinds: length mis-match"
-
----------------
uKVar :: Bool -> KindVar -> TcKind -> TcM ()
uKVar swapped kv1 k2
- = do { mb_k1 <- readKindVar kv1
- ; case mb_k1 of
- Flexi -> uUnboundKVar swapped kv1 k2
- Indirect k1 | swapped -> unifyKind k2 k1
- | otherwise -> unifyKind k1 k2 }
+ = do { mb_k1 <- readKindVar kv1
+ ; case mb_k1 of
+ Flexi -> uUnboundKVar swapped kv1 k2
+ Indirect k1 | swapped -> unifyKind k2 k1
+ | otherwise -> unifyKind k1 k2 }
----------------
uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
uUnboundKVar swapped kv1 k2@(TyVarTy kv2)
- | kv1 == kv2 = returnM ()
- | otherwise -- Distinct kind variables
- = do { mb_k2 <- readKindVar kv2
- ; case mb_k2 of
- Indirect k2 -> uUnboundKVar swapped kv1 k2
- Flexi -> writeKindVar kv1 k2 }
+ | kv1 == kv2 = return ()
+ | otherwise -- Distinct kind variables
+ = do { mb_k2 <- readKindVar kv2
+ ; case mb_k2 of
+ Indirect k2 -> uUnboundKVar swapped kv1 k2
+ Flexi -> writeKindVar kv1 k2 }
uUnboundKVar swapped kv1 non_var_k2
- = do { k2' <- zonkTcKind non_var_k2
- ; kindOccurCheck kv1 k2'
- ; k2'' <- kindSimpleKind swapped k2'
- -- KindVars must be bound only to simple kinds
- -- Polarities: (kindSimpleKind True ?) succeeds
- -- returning *, corresponding to unifying
- -- expected: ?
- -- actual: kind-ver
- ; writeKindVar kv1 k2'' }
+ = do { k2' <- zonkTcKind non_var_k2
+ ; kindOccurCheck kv1 k2'
+ ; k2'' <- kindSimpleKind swapped k2'
+ -- KindVars must be bound only to simple kinds
+ -- Polarities: (kindSimpleKind True ?) succeeds
+ -- returning *, corresponding to unifying
+ -- expected: ?
+ -- actual: kind-ver
+ ; writeKindVar kv1 k2'' }
----------------
-kindOccurCheck kv1 k2 -- k2 is zonked
+kindOccurCheck :: TyVar -> Type -> TcM ()
+kindOccurCheck kv1 k2 -- k2 is zonked
= checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
where
- not_in (TyVarTy kv2) = kv1 /= kv2
+ not_in (TyVarTy kv2) = kv1 /= kv2
not_in (FunTy a2 r2) = not_in a2 && not_in r2
- not_in other = True
+ not_in _ = True
kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
-- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
-- If the flag is False, it requires k <: sk
--- E.g. kindSimpleKind False ?? = *
--- What about (kv -> *) :=: ?? -> *
+-- E.g. kindSimpleKind False ?? = *
+-- What about (kv -> *) ~ ?? -> *
kindSimpleKind orig_swapped orig_kind
= go orig_swapped orig_kind
where
go sw (FunTy k1 k2) = do { k1' <- go (not sw) k1
- ; k2' <- go sw k2
- ; return (mkArrowKind k1' k2') }
+ ; k2' <- go sw k2
+ ; return (mkArrowKind k1' k2') }
go True k
| isOpenTypeKind k = return liftedTypeKind
| isArgTypeKind k = return liftedTypeKind
- go sw k
+ go _ k
| isLiftedTypeKind k = return liftedTypeKind
| isUnliftedTypeKind k = return unliftedTypeKind
- go sw k@(TyVarTy _) = return k -- KindVars are always simple
- go swapped kind = failWithTc (ptext SLIT("Unexpected kind unification failure:")
- <+> ppr orig_swapped <+> ppr orig_kind)
- -- I think this can't actually happen
-
--- T v = MkT v v must be a type
--- T v w = MkT (v -> w) v must not be an umboxed tuple
-
-----------------
-kindOccurCheckErr tyvar ty
- = hang (ptext SLIT("Occurs check: cannot construct the infinite kind:"))
- 2 (sep [ppr tyvar, char '=', ppr ty])
-
-unifyKindMisMatch ty1 ty2
- = zonkTcKind ty1 `thenM` \ ty1' ->
- zonkTcKind ty2 `thenM` \ ty2' ->
+ go _ k@(TyVarTy _) = return k -- KindVars are always simple
+ go _ _ = failWithTc (ptext (sLit "Unexpected kind unification failure:")
+ <+> ppr orig_swapped <+> ppr orig_kind)
+ -- I think this can't actually happen
+
+-- T v = MkT v v must be a type
+-- T v w = MkT (v -> w) v must not be an umboxed tuple
+
+unifyKindMisMatch :: TcKind -> TcKind -> TcM ()
+unifyKindMisMatch ty1 ty2 = do
+ ty1' <- zonkTcKind ty1
+ ty2' <- zonkTcKind ty2
let
- msg = hang (ptext SLIT("Couldn't match kind"))
+ msg = hang (ptext (sLit "Couldn't match kind"))
2 (sep [quotes (ppr ty1'),
- ptext SLIT("against"),
+ ptext (sLit "against"),
quotes (ppr ty2')])
- in
failWithTc msg
-\end{code}
-
-\begin{code}
-unifyFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
--- Like unifyFunTy, but does not fail; instead just returns Nothing
-unifyFunKind (TyVarTy kvar)
- = readKindVar kvar `thenM` \ maybe_kind ->
- case maybe_kind of
- Indirect fun_kind -> unifyFunKind fun_kind
- Flexi ->
- do { arg_kind <- newKindVar
- ; res_kind <- newKindVar
- ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
- ; returnM (Just (arg_kind,res_kind)) }
-
-unifyFunKind (FunTy arg_kind res_kind) = returnM (Just (arg_kind,res_kind))
-unifyFunKind other = returnM Nothing
-\end{code}
-
-%************************************************************************
-%* *
- Checking kinds
-%* *
-%************************************************************************
-
----------------------------
--- We would like to get a decent error message from
--- (a) Under-applied type constructors
--- f :: (Maybe, Maybe)
--- (b) Over-applied type constructors
--- f :: Int x -> Int x
---
-
-\begin{code}
-checkExpectedKind :: Outputable a => a -> TcKind -> TcKind -> TcM ()
--- A fancy wrapper for 'unifyKind', which tries
--- to give decent error messages.
--- (checkExpectedKind ty act_kind exp_kind)
--- checks that the actual kind act_kind is compatible
--- with the expected kind exp_kind
--- The first argument, ty, is used only in the error message generation
-checkExpectedKind ty act_kind exp_kind
- | act_kind `isSubKind` exp_kind -- Short cut for a very common case
- = returnM ()
- | otherwise
- = tryTc (unifyKind exp_kind act_kind) `thenM` \ (_errs, mb_r) ->
- case mb_r of {
- Just r -> returnM () ; -- Unification succeeded
- Nothing ->
-
- -- So there's definitely an error
- -- Now to find out what sort
- zonkTcKind exp_kind `thenM` \ exp_kind ->
- zonkTcKind act_kind `thenM` \ act_kind ->
-
- tcInitTidyEnv `thenM` \ env0 ->
- let (exp_as, _) = splitKindFunTys exp_kind
- (act_as, _) = splitKindFunTys act_kind
- n_exp_as = length exp_as
- n_act_as = length act_as
-
- (env1, tidy_exp_kind) = tidyKind env0 exp_kind
- (env2, tidy_act_kind) = tidyKind env1 act_kind
-
- err | n_exp_as < n_act_as -- E.g. [Maybe]
- = quotes (ppr ty) <+> ptext SLIT("is not applied to enough type arguments")
-
- -- Now n_exp_as >= n_act_as. In the next two cases,
- -- n_exp_as == 0, and hence so is n_act_as
- | isLiftedTypeKind exp_kind && isUnliftedTypeKind act_kind
- = ptext SLIT("Expecting a lifted type, but") <+> quotes (ppr ty)
- <+> ptext SLIT("is unlifted")
-
- | isUnliftedTypeKind exp_kind && isLiftedTypeKind act_kind
- = ptext SLIT("Expecting an unlifted type, but") <+> quotes (ppr ty)
- <+> ptext SLIT("is lifted")
-
- | otherwise -- E.g. Monad [Int]
- = ptext SLIT("Kind mis-match")
-
- more_info = sep [ ptext SLIT("Expected kind") <+>
- quotes (pprKind tidy_exp_kind) <> comma,
- ptext SLIT("but") <+> quotes (ppr ty) <+>
- ptext SLIT("has kind") <+> quotes (pprKind tidy_act_kind)]
- in
- failWithTcM (env2, err $$ more_info)
- }
+----------------
+kindOccurCheckErr :: Var -> Type -> SDoc
+kindOccurCheckErr tyvar ty
+ = hang (ptext (sLit "Occurs check: cannot construct the infinite kind:"))
+ 2 (sep [ppr tyvar, char '=', ppr ty])
\end{code}
%************************************************************************
-%* *
+%* *
\subsection{Checking signature type variables}
-%* *
+%* *
%************************************************************************
@checkSigTyVars@ checks that a set of universally quantified type varaibles
are not mentioned in the environment. In particular:
- (a) Not mentioned in the type of a variable in the envt
- eg the signature for f in this:
+ (a) Not mentioned in the type of a variable in the envt
+ eg the signature for f in this:
- g x = ... where
- f :: a->[a]
- f y = [x,y]
+ g x = ... where
+ f :: a->[a]
+ f y = [x,y]
- Here, f is forced to be monorphic by the free occurence of x.
+ Here, f is forced to be monorphic by the free occurence of x.
- (d) Not (unified with another type variable that is) in scope.
- eg f x :: (r->r) = (\y->y) :: forall a. a->r
- when checking the expression type signature, we find that
- even though there is nothing in scope whose type mentions r,
- nevertheless the type signature for the expression isn't right.
+ (d) Not (unified with another type variable that is) in scope.
+ eg f x :: (r->r) = (\y->y) :: forall a. a->r
+ when checking the expression type signature, we find that
+ even though there is nothing in scope whose type mentions r,
+ nevertheless the type signature for the expression isn't right.
- Another example is in a class or instance declaration:
- class C a where
- op :: forall b. a -> b
- op x = x
- Here, b gets unified with a
+ Another example is in a class or instance declaration:
+ class C a where
+ op :: forall b. a -> b
+ op x = x
+ Here, b gets unified with a
Before doing this, the substitution is applied to the signature type variable.
-\begin{code}
+-- \begin{code}
checkSigTyVars :: [TcTyVar] -> TcM ()
checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
--- The extra_tvs can include boxy type variables;
--- e.g. TcMatches.tcCheckExistentialPat
+-- The extra_tvs can include boxy type variables;
+-- e.g. TcMatches.tcCheckExistentialPat
checkSigTyVarsWrt extra_tvs sig_tvs
- = do { extra_tvs' <- zonkTcTyVarsAndFV (varSetElems extra_tvs)
- ; check_sig_tyvars extra_tvs' sig_tvs }
+ = do { extra_tvs' <- zonkTcTyVarsAndFV extra_tvs
+ ; check_sig_tyvars extra_tvs' sig_tvs }
check_sig_tyvars
- :: TcTyVarSet -- Global type variables. The universally quantified
- -- tyvars should not mention any of these
- -- Guaranteed already zonked.
- -> [TcTyVar] -- Universally-quantified type variables in the signature
- -- Guaranteed to be skolems
- -> TcM ()
-check_sig_tyvars extra_tvs []
- = returnM ()
-check_sig_tyvars extra_tvs sig_tvs
- = ASSERT( all isSkolemTyVar sig_tvs )
- do { gbl_tvs <- tcGetGlobalTyVars
- ; traceTc (text "check_sig_tyvars" <+> (vcat [text "sig_tys" <+> ppr sig_tvs,
- text "gbl_tvs" <+> ppr gbl_tvs,
- text "extra_tvs" <+> ppr extra_tvs]))
-
- ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
- ; ifM (any (`elemVarSet` env_tvs) sig_tvs)
- (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
- }
-
-bleatEscapedTvs :: TcTyVarSet -- The global tvs
- -> [TcTyVar] -- The possibly-escaping type variables
- -> [TcTyVar] -- The zonked versions thereof
- -> TcM ()
+ :: TcTyVarSet -- Global type variables. The universally quantified
+ -- tyvars should not mention any of these
+ -- Guaranteed already zonked.
+ -> [TcTyVar] -- Universally-quantified type variables in the signature
+ -- Guaranteed to be skolems
+ -> TcM ()
+check_sig_tyvars _ []
+ = return ()
+check_sig_tyvars extra_tvs sig_tvs
+ = ASSERT( all isTcTyVar sig_tvs && all isSkolemTyVar sig_tvs )
+ do { gbl_tvs <- tcGetGlobalTyVars
+ ; traceTc "check_sig_tyvars" $ vcat
+ [ text "sig_tys" <+> ppr sig_tvs
+ , text "gbl_tvs" <+> ppr gbl_tvs
+ , text "extra_tvs" <+> ppr extra_tvs]
+
+ ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
+ ; when (any (`elemVarSet` env_tvs) sig_tvs)
+ (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
+ }
+
+bleatEscapedTvs :: TcTyVarSet -- The global tvs
+ -> [TcTyVar] -- The possibly-escaping type variables
+ -> [TcTyVar] -- The zonked versions thereof
+ -> TcM ()
-- Complain about escaping type variables
-- We pass a list of type variables, at least one of which
-- escapes. The first list contains the original signature type variable,
-- while the second contains the type variable it is unified to (usually itself)
bleatEscapedTvs globals sig_tvs zonked_tvs
- = do { env0 <- tcInitTidyEnv
- ; let (env1, tidy_tvs) = tidyOpenTyVars env0 sig_tvs
- (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
+ = do { env0 <- tcInitTidyEnv
+ ; let (env1, tidy_tvs) = tidyOpenTyVars env0 sig_tvs
+ (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
- ; (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
- ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
+ ; (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
+ ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
where
- main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
+ main_msg = ptext (sLit "Inferred type is less polymorphic than expected")
check (tidy_env, msgs) (sig_tv, zonked_tv)
| not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
| otherwise
- = do { (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) tidy_env
- ; returnM (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
+ = do { lcl_env <- getLclTypeEnv
+ ; (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) lcl_env tidy_env
+ ; return (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
-----------------------
+escape_msg :: Var -> Var -> [SDoc] -> SDoc
escape_msg sig_tv zonked_tv globs
- | notNull globs
- = vcat [sep [msg, ptext SLIT("is mentioned in the environment:")],
- nest 2 (vcat globs)]
+ | notNull globs
+ = vcat [sep [msg, ptext (sLit "is mentioned in the environment:")],
+ nest 2 (vcat globs)]
| otherwise
- = msg <+> ptext SLIT("escapes")
- -- Sigh. It's really hard to give a good error message
- -- all the time. One bad case is an existential pattern match.
- -- We rely on the "When..." context to help.
+ = msg <+> ptext (sLit "escapes")
+ -- Sigh. It's really hard to give a good error message
+ -- all the time. One bad case is an existential pattern match.
+ -- We rely on the "When..." context to help.
where
- msg = ptext SLIT("Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
- is_bound_to
- | sig_tv == zonked_tv = empty
- | otherwise = ptext SLIT("is unified with") <+> quotes (ppr zonked_tv) <+> ptext SLIT("which")
-\end{code}
+ msg = ptext (sLit "Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
+ is_bound_to
+ | sig_tv == zonked_tv = empty
+ | otherwise = ptext (sLit "is unified with") <+> quotes (ppr zonked_tv) <+> ptext (sLit "which")
+-- \end{code}
These two context are used with checkSigTyVars
-
+
\begin{code}
sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
- -> TidyEnv -> TcM (TidyEnv, Message)
-sigCtxt id sig_tvs sig_theta sig_tau tidy_env
- = zonkTcType sig_tau `thenM` \ actual_tau ->
+ -> TidyEnv -> TcM (TidyEnv, Message)
+sigCtxt id sig_tvs sig_theta sig_tau tidy_env = do
+ actual_tau <- zonkTcType sig_tau
let
- (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
- (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
- (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
- sub_msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
- ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau
- ]
- msg = vcat [ptext SLIT("When trying to generalise the type inferred for") <+> quotes (ppr id),
- nest 2 sub_msg]
- in
- returnM (env3, msg)
+ (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
+ (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
+ (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
+ sub_msg = vcat [ptext (sLit "Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
+ ptext (sLit "Type to generalise:") <+> pprType tidy_actual_tau
+ ]
+ msg = vcat [ptext (sLit "When trying to generalise the type inferred for") <+> quotes (ppr id),
+ nest 2 sub_msg]
+
+ return (env3, msg)
\end{code}
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