TcSimplify
\begin{code}
+{-# OPTIONS -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/Commentary/CodingStyle#Warnings
+-- for details
+
module TcSimplify (
tcSimplifyInfer, tcSimplifyInferCheck,
tcSimplifyCheck, tcSimplifyRestricted,
tcSimplifyBracket, tcSimplifyCheckPat,
tcSimplifyDeriv, tcSimplifyDefault,
- bindInstsOfLocalFuns, bindIrreds,
+ bindInstsOfLocalFuns,
+
+ misMatchMsg
) where
#include "HsVersions.h"
import TcType
import TcMType
import TcIface
+import TcTyFuns
+import TypeRep
import Var
import Name
import NameSet
import BasicTypes
import VarSet
import VarEnv
+import Module
import FiniteMap
import Bag
import Outputable
+import Maybes
import ListSetOps
import Util
+import UniqSet
import SrcLoc
import DynFlags
Note [Implicit parameters and ambiguity]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-What type should we infer for this?
- f x = (show ?y, x::Int)
-Since we must quantify over the ?y, the most plausible type is
- f :: (Show a, ?y::a) => Int -> (String, Int)
-But notice that the type of the RHS is (String,Int), with no type
-varibables mentioned at all! The type of f looks ambiguous. But
-it isn't, because at a call site we might have
- let ?y = 5::Int in f 7
+Only a *class* predicate can give rise to ambiguity
+An *implicit parameter* cannot. For example:
+ foo :: (?x :: [a]) => Int
+ foo = length ?x
+is fine. The call site will suppply a particular 'x'
+
+Furthermore, the type variables fixed by an implicit parameter
+propagate to the others. E.g.
+ foo :: (Show a, ?x::[a]) => Int
+ foo = show (?x++?x)
+The type of foo looks ambiguous. But it isn't, because at a call site
+we might have
+ let ?x = 5::Int in foo
and all is well. In effect, implicit parameters are, well, parameters,
so we can take their type variables into account as part of the
"tau-tvs" stuff. This is done in the function 'FunDeps.grow'.
\begin{code}
tcSimplifyInfer doc tau_tvs wanted
- = do { tau_tvs' <- zonkTcTyVarsAndFV (varSetElems tau_tvs)
- ; wanted' <- mappM zonkInst wanted -- Zonk before deciding quantified tyvars
+ = do { tau_tvs1 <- zonkTcTyVarsAndFV (varSetElems tau_tvs)
+ ; wanted' <- mappM zonkInst wanted -- Zonk before deciding quantified tyvars
; gbl_tvs <- tcGetGlobalTyVars
- ; let preds = fdPredsOfInsts wanted'
- qtvs = grow preds tau_tvs' `minusVarSet` oclose preds gbl_tvs
+ ; let preds1 = fdPredsOfInsts wanted'
+ gbl_tvs1 = oclose preds1 gbl_tvs
+ qtvs = grow preds1 tau_tvs1 `minusVarSet` gbl_tvs1
-- See Note [Choosing which variables to quantify]
-- To maximise sharing, remove from consideration any
-- constraints that don't mention qtvs at all
- ; let (free1, bound) = partition (isFreeWhenInferring qtvs) wanted'
- ; extendLIEs free1
+ ; let (free, bound) = partition (isFreeWhenInferring qtvs) wanted'
+ ; extendLIEs free
-- To make types simple, reduce as much as possible
- ; traceTc (text "infer" <+> (ppr preds $$ ppr (grow preds tau_tvs') $$ ppr gbl_tvs $$
- ppr (oclose preds gbl_tvs) $$ ppr free1 $$ ppr bound))
+ ; traceTc (text "infer" <+> (ppr preds1 $$ ppr (grow preds1 tau_tvs1) $$ ppr gbl_tvs $$
+ ppr gbl_tvs1 $$ ppr free $$ ppr bound))
; (irreds1, binds1) <- tryHardCheckLoop doc bound
-- Note [Inference and implication constraints]
; let want_dict d = tyVarsOfInst d `intersectsVarSet` qtvs
; (irreds2, binds2) <- approximateImplications doc want_dict irreds1
- -- By now improvment may have taken place, and we must *not*
- -- quantify over any variable free in the environment
- -- tc137 (function h inside g) is an example
- ; gbl_tvs <- tcGetGlobalTyVars
- ; qtvs1 <- zonkTcTyVarsAndFV (varSetElems qtvs)
- ; qtvs2 <- zonkQuantifiedTyVars (varSetElems (qtvs1 `minusVarSet` gbl_tvs))
-
- -- Do not quantify over constraints that *now* do not
- -- mention quantified type variables, because they are
- -- simply ambiguous (or might be bound further out). Example:
- -- f :: Eq b => a -> (a, b)
- -- g x = fst (f x)
- -- From the RHS of g we get the MethodInst f77 :: alpha -> (alpha, beta)
- -- We decide to quantify over 'alpha' alone, but free1 does not include f77
- -- because f77 mentions 'alpha'. Then reducing leaves only the (ambiguous)
- -- constraint (Eq beta), which we dump back into the free set
- -- See test tcfail181
- ; let (free3, irreds3) = partition (isFreeWhenInferring (mkVarSet qtvs2)) irreds2
- ; extendLIEs free3
-
+ -- Now work out all over again which type variables to quantify,
+ -- exactly in the same way as before, but starting from irreds2. Why?
+ -- a) By now improvment may have taken place, and we must *not*
+ -- quantify over any variable free in the environment
+ -- tc137 (function h inside g) is an example
+ --
+ -- b) Do not quantify over constraints that *now* do not
+ -- mention quantified type variables, because they are
+ -- simply ambiguous (or might be bound further out). Example:
+ -- f :: Eq b => a -> (a, b)
+ -- g x = fst (f x)
+ -- From the RHS of g we get the MethodInst f77 :: alpha -> (alpha, beta)
+ -- We decide to quantify over 'alpha' alone, but free1 does not include f77
+ -- because f77 mentions 'alpha'. Then reducing leaves only the (ambiguous)
+ -- constraint (Eq beta), which we dump back into the free set
+ -- See test tcfail181
+ --
+ -- c) irreds may contain type variables not previously mentioned,
+ -- e.g. instance D a x => Foo [a]
+ -- wanteds = Foo [a]
+ -- Then after simplifying we'll get (D a x), and x is fresh
+ -- We must quantify over x else it'll be totally unbound
+ ; tau_tvs2 <- zonkTcTyVarsAndFV (varSetElems tau_tvs1)
+ ; gbl_tvs2 <- zonkTcTyVarsAndFV (varSetElems gbl_tvs1)
+ -- Note that we start from gbl_tvs1
+ -- We use tcGetGlobalTyVars, then oclose wrt preds2, because
+ -- we've already put some of the original preds1 into frees
+ -- E.g. wanteds = C a b (where a->b)
+ -- gbl_tvs = {a}
+ -- tau_tvs = {b}
+ -- Then b is fixed by gbl_tvs, so (C a b) will be in free, and
+ -- irreds2 will be empty. But we don't want to generalise over b!
+ ; let preds2 = fdPredsOfInsts irreds2 -- irreds2 is zonked
+ qtvs = grow preds2 tau_tvs2 `minusVarSet` oclose preds2 gbl_tvs2
+ ; let (free, irreds3) = partition (isFreeWhenInferring qtvs) irreds2
+ ; extendLIEs free
+
+ -- Turn the quantified meta-type variables into real type variables
+ ; qtvs2 <- zonkQuantifiedTyVars (varSetElems qtvs)
+
-- We can't abstract over any remaining unsolved
-- implications so instead just float them outwards. Ugh.
- ; let (q_dicts, implics) = partition isDict irreds3
+ ; let (q_dicts0, implics) = partition isAbstractableInst irreds3
; loc <- getInstLoc (ImplicOrigin doc)
- ; implic_bind <- bindIrreds loc qtvs2 q_dicts implics
+ ; implic_bind <- bindIrreds loc qtvs2 q_dicts0 implics
+
+ -- Prepare equality instances for quantification
+ ; let (q_eqs0,q_dicts) = partition isEqInst q_dicts0
+ ; q_eqs <- mappM finalizeEqInst q_eqs0
- ; return (qtvs2, q_dicts, binds1 `unionBags` binds2 `unionBags` implic_bind) }
+ ; return (qtvs2, q_eqs ++ q_dicts, binds1 `unionBags` binds2 `unionBags` implic_bind) }
-- NB: when we are done, we might have some bindings, but
-- the final qtvs might be empty. See Note [NO TYVARS] below.
= [ d | let tv_set = mkVarSet tvs
, d <- get_dicts wanteds
, not (tyVarsOfInst d `intersectsVarSet` tv_set)]
+ get_dict i@(EqInst {}) | want_dict i = [i]
+ | otherwise = []
get_dict other = pprPanic "approximateImplications" (ppr other)
\end{code}
So our best approximation is to make (D [a]) part of the inferred
context, so we can use that to discharge the implication. Hence
-the strange function getImplicWanteds.
+the strange function get_dicts in approximateImplications.
The common cases are more clear-cut, when we have things like
forall a. C a => C b
TcDictBinds) -- Bindings
tcSimplifyInferCheck loc tau_tvs givens wanteds
- = do { (irreds, binds) <- gentleCheckLoop loc givens wanteds
+ = do { traceTc (text "tcSimplifyInferCheck <-" <+> ppr wanteds)
+ ; (irreds, binds) <- gentleCheckLoop loc givens wanteds
-- Figure out which type variables to quantify over
-- You might think it should just be the signature tyvars,
-- Now we are back to normal (c.f. tcSimplCheck)
; implic_bind <- bindIrreds loc qtvs' givens irreds
+ ; traceTc (text "tcSimplifyInferCheck ->" <+> ppr (implic_bind))
; return (qtvs', binds `unionBags` implic_bind) }
\end{code}
-> TcM TcDictBinds -- Bindings
tcSimplifyCheck loc qtvs givens wanteds
= ASSERT( all isTcTyVar qtvs && all isSkolemTyVar qtvs )
- do { (irreds, binds) <- gentleCheckLoop loc givens wanteds
+ do { traceTc (text "tcSimplifyCheck")
+ ; (irreds, binds) <- gentleCheckLoop loc givens wanteds
; implic_bind <- bindIrreds loc qtvs givens irreds
; return (binds `unionBags` implic_bind) }
-> TcM TcDictBinds -- Bindings
tcSimplifyCheckPat loc co_vars reft qtvs givens wanteds
= ASSERT( all isTcTyVar qtvs && all isSkolemTyVar qtvs )
- do { (irreds, binds) <- gentleCheckLoop loc givens wanteds
+ do { traceTc (text "tcSimplifyCheckPat")
+ ; (irreds, binds) <- gentleCheckLoop loc givens wanteds
; implic_bind <- bindIrredsR loc qtvs co_vars reft
givens irreds
; return (binds `unionBags` implic_bind) }
| null irreds
= return emptyBag
| otherwise
- = do { let givens' = filter isDict givens
- -- The givens can include methods
+ = do { let givens' = filter isAbstractableInst givens
+ -- The givens can (redundantly) include methods
+ -- We want to retain both EqInsts and Dicts
+ -- There should be no implicadtion constraints
-- See Note [Pruning the givens in an implication constraint]
-- If there are no 'givens' *and* the refinement is empty
-- This binding must line up the 'rhs' in reduceImplication
makeImplicationBind loc all_tvs reft
givens -- Guaranteed all Dicts
+ -- or EqInsts
irreds
| null irreds -- If there are no irreds, we are done
= return ([], emptyBag)
| otherwise -- Otherwise we must generate a binding
= do { uniq <- newUnique
; span <- getSrcSpanM
+ ; let (eq_givens, dict_givens) = partition isEqInst givens
+ eq_tyvar_cos = mkTyVarTys (varSetElems $ tyVarsOfTypes $ map eqInstType eq_givens)
+ -- Urgh! See line 2187 or thereabouts. I believe that all these
+ -- 'givens' must be a simple CoVar. This MUST be cleaned up.
+
; let name = mkInternalName uniq (mkVarOcc "ic") span
implic_inst = ImplicInst { tci_name = name, tci_reft = reft,
tci_tyvars = all_tvs,
- tci_given = givens,
+ tci_given = (eq_givens ++ dict_givens),
tci_wanted = irreds, tci_loc = loc }
-
- ; let n_irreds = length irreds
- irred_ids = map instToId irreds
- tup_ty = mkTupleTy Boxed n_irreds (map idType irred_ids)
- pat = TuplePat (map nlVarPat irred_ids) Boxed tup_ty
+ ; let -- only create binder for dict_irreds
+ (eq_irreds, dict_irreds) = partition isEqInst irreds
+ n_dict_irreds = length dict_irreds
+ dict_irred_ids = map instToId dict_irreds
+ tup_ty = mkTupleTy Boxed n_dict_irreds (map idType dict_irred_ids)
+ pat = TuplePat (map nlVarPat dict_irred_ids) Boxed tup_ty
rhs = L span (mkHsWrap co (HsVar (instToId implic_inst)))
- co = mkWpApps (map instToId givens) <.> mkWpTyApps (mkTyVarTys all_tvs)
- bind | n_irreds==1 = VarBind (head irred_ids) rhs
- | otherwise = PatBind { pat_lhs = L span pat,
- pat_rhs = unguardedGRHSs rhs,
- pat_rhs_ty = tup_ty,
- bind_fvs = placeHolderNames }
- ; -- pprTrace "Make implic inst" (ppr implic_inst) $
- return ([implic_inst], unitBag (L span bind)) }
+ co = mkWpApps (map instToId dict_givens)
+ <.> mkWpTyApps eq_tyvar_cos
+ <.> mkWpTyApps (mkTyVarTys all_tvs)
+ bind | [dict_irred_id] <- dict_irred_ids = VarBind dict_irred_id rhs
+ | otherwise = PatBind { pat_lhs = L span pat,
+ pat_rhs = unguardedGRHSs rhs,
+ pat_rhs_ty = tup_ty,
+ bind_fvs = placeHolderNames }
+ ; traceTc $ text "makeImplicationBind" <+> ppr implic_inst
+ ; return ([implic_inst], unitBag (L span bind))
+ }
-----------------------------------------------------------
tryHardCheckLoop :: SDoc
-> TcM ([Inst], TcDictBinds)
tryHardCheckLoop doc wanteds
- = checkLoop (mkRedEnv doc try_me []) wanteds
+ = do { (irreds,binds,_) <- checkLoop (mkRedEnv doc try_me []) wanteds
+ ; return (irreds,binds)
+ }
where
try_me inst = ReduceMe AddSCs
-- Here's the try-hard bit
-> TcM ([Inst], TcDictBinds)
gentleCheckLoop inst_loc givens wanteds
- = checkLoop env wanteds
+ = do { (irreds,binds,_) <- checkLoop env wanteds
+ ; return (irreds,binds)
+ }
where
env = mkRedEnv (pprInstLoc inst_loc) try_me givens
| otherwise = Stop
-- When checking against a given signature
-- we MUST be very gentle: Note [Check gently]
+
+gentleInferLoop :: SDoc -> [Inst]
+ -> TcM ([Inst], TcDictBinds)
+gentleInferLoop doc wanteds
+ = do { (irreds, binds, _) <- checkLoop env wanteds
+ ; return (irreds, binds) }
+ where
+ env = mkRedEnv doc try_me []
+ try_me inst | isMethodOrLit inst = ReduceMe AddSCs
+ | otherwise = Stop
\end{code}
Note [Check gently]
Hence we have a dictionary for Show [a] available; and indeed we
need it. We are going to build an implication contraint
forall a. (b~[a]) => Show [a]
-Later, we will solve this constraint using the knowledg e(Show b)
+Later, we will solve this constraint using the knowledge (Show b)
But we MUST NOT reduce (Show [a]) to (Show a), else the whole
thing becomes insoluble. So we simplify gently (get rid of literals
-----------------------------------------------------------
checkLoop :: RedEnv
-> [Inst] -- Wanted
- -> TcM ([Inst], TcDictBinds)
+ -> TcM ([Inst], TcDictBinds,
+ [Inst]) -- needed givens
-- Precondition: givens are completely rigid
-- Postcondition: returned Insts are zonked
checkLoop env wanteds
- = do { -- Givens are skolems, so no need to zonk them
- wanteds' <- mappM zonkInst wanteds
+ = go env wanteds []
+ where go env wanteds needed_givens
+ = do { -- We do need to zonk the givens; cf Note [Zonking RedEnv]
+ ; env' <- zonkRedEnv env
+ ; wanteds' <- zonkInsts wanteds
+
+ ; (improved, binds, irreds, more_needed_givens) <- reduceContext env' wanteds'
- ; (improved, binds, irreds) <- reduceContext env wanteds'
+ ; let all_needed_givens = needed_givens ++ more_needed_givens
+
+ ; if not improved then
+ return (irreds, binds, all_needed_givens)
+ else do
+
+ -- If improvement did some unification, we go round again.
+ -- We start again with irreds, not wanteds
+ -- Using an instance decl might have introduced a fresh type variable
+ -- which might have been unified, so we'd get an infinite loop
+ -- if we started again with wanteds! See Note [LOOP]
+ { (irreds1, binds1, all_needed_givens1) <- go env' irreds all_needed_givens
+ ; return (irreds1, binds `unionBags` binds1, all_needed_givens1) } }
+\end{code}
- ; if not improved then
- return (irreds, binds)
- else do
+Note [Zonking RedEnv]
+~~~~~~~~~~~~~~~~~~~~~
+It might appear as if the givens in RedEnv are always rigid, but that is not
+necessarily the case for programs involving higher-rank types that have class
+contexts constraining the higher-rank variables. An example from tc237 in the
+testsuite is
+
+ class Modular s a | s -> a
+
+ wim :: forall a w. Integral a
+ => a -> (forall s. Modular s a => M s w) -> w
+ wim i k = error "urk"
+
+ test5 :: (Modular s a, Integral a) => M s a
+ test5 = error "urk"
+
+ test4 = wim 4 test4'
+
+Notice how the variable 'a' of (Modular s a) in the rank-2 type of wim is
+quantified further outside. When type checking test4, we have to check
+whether the signature of test5 is an instance of
+
+ (forall s. Modular s a => M s w)
+
+Consequently, we will get (Modular s t_a), where t_a is a TauTv into the
+givens.
+
+Given the FD of Modular in this example, class improvement will instantiate
+t_a to 'a', where 'a' is the skolem from test5's signatures (due to the
+Modular s a predicate in that signature). If we don't zonk (Modular s t_a) in
+the givens, we will get into a loop as improveOne uses the unification engine
+TcGadt.tcUnifyTys, which doesn't know about mutable type variables.
- -- If improvement did some unification, we go round again.
- -- We start again with irreds, not wanteds
- -- Using an instance decl might have introduced a fresh type variable
- -- which might have been unified, so we'd get an infinite loop
- -- if we started again with wanteds! See Note [LOOP]
- { (irreds1, binds1) <- checkLoop env irreds
- ; return (irreds1, binds `unionBags` binds1) } }
-\end{code}
Note [LOOP]
~~~~~~~~~~~
-> [Inst] -- Wanted
-> TcM TcDictBinds
tcSimplifySuperClasses loc givens sc_wanteds
- = do { (irreds, binds1) <- checkLoop env sc_wanteds
+ = do { traceTc (text "tcSimplifySuperClasses")
+ ; (irreds,binds1,_) <- checkLoop env sc_wanteds
; let (tidy_env, tidy_irreds) = tidyInsts irreds
; reportNoInstances tidy_env (Just (loc, givens)) tidy_irreds
; return binds1 }
tcSimplifyRestricted doc top_lvl bndrs tau_tvs wanteds
-- Zonk everything in sight
- = do { wanteds' <- mappM zonkInst wanteds
+ = do { traceTc (text "tcSimplifyRestricted")
+ ; wanteds' <- zonkInsts wanteds
-- 'ReduceMe': Reduce as far as we can. Don't stop at
-- dicts; the idea is to get rid of as many type
-- HOWEVER, some unification may take place, if we instantiate
-- a method Inst with an equality constraint
; let env = mkNoImproveRedEnv doc (\i -> ReduceMe AddSCs)
- ; (_imp, _binds, constrained_dicts) <- reduceContext env wanteds'
+ ; (_imp, _binds, constrained_dicts, _) <- reduceContext env wanteds'
-- Next, figure out the tyvars we will quantify over
; tau_tvs' <- zonkTcTyVarsAndFV (varSetElems tau_tvs)
; gbl_tvs' <- tcGetGlobalTyVars
- ; constrained_dicts' <- mappM zonkInst constrained_dicts
+ ; constrained_dicts' <- zonkInsts constrained_dicts
; let qtvs1 = tau_tvs' `minusVarSet` oclose (fdPredsOfInsts constrained_dicts) gbl_tvs'
-- As in tcSimplifyInfer
(is_nested_group || isDict inst) = Stop
| otherwise = ReduceMe AddSCs
env = mkNoImproveRedEnv doc try_me
- ; (_imp, binds, irreds) <- reduceContext env wanteds'
+ ; (_imp, binds, irreds, _) <- reduceContext env wanteds'
-- See "Notes on implicit parameters, Question 4: top level"
; ASSERT( all (isFreeWrtTyVars qtvs) irreds ) -- None should be captured
-- to fromInteger; this looks fragile to me
; lookup_result <- lookupSimpleInst w'
; case lookup_result of
- GenInst ws' rhs -> go dicts (addBind binds (instToId w) rhs) (ws' ++ ws)
+ GenInst ws' rhs ->
+ go dicts (addInstToDictBind binds w rhs) (ws' ++ ws)
NoInstance -> pprPanic "tcSimplifyRuleLhs" (ppr w)
}
\end{code}
-- makes them the same.
tcSimplifyIPs given_ips wanteds
- = do { wanteds' <- mappM zonkInst wanteds
- ; given_ips' <- mappM zonkInst given_ips
+ = do { wanteds' <- zonkInsts wanteds
+ ; given_ips' <- zonkInsts given_ips
-- Unusually for checking, we *must* zonk the given_ips
; let env = mkRedEnv doc try_me given_ips'
- ; (improved, binds, irreds) <- reduceContext env wanteds'
+ ; (improved, binds, irreds, _) <- reduceContext env wanteds'
; if not improved then
ASSERT( all is_free irreds )
returnM emptyLHsBinds
| otherwise
- = do { (irreds, binds) <- checkLoop env for_me
+ = do { (irreds, binds) <- gentleInferLoop doc for_me
; extendLIEs not_for_me
; extendLIEs irreds
; return binds }
where
- env = mkRedEnv doc try_me []
doc = text "bindInsts" <+> ppr local_ids
overloaded_ids = filter is_overloaded local_ids
is_overloaded id = isOverloadedTy (idType id)
overloaded_set = mkVarSet overloaded_ids -- There can occasionally be a lot of them
-- so it's worth building a set, so that
-- lookup (in isMethodFor) is faster
- try_me inst | isMethod inst = ReduceMe NoSCs
- | otherwise = Stop
\end{code}
, red_givens :: [Inst] -- All guaranteed rigid
-- Always dicts
-- but see Note [Rigidity]
+ , red_reft :: Refinement -- The refinement to apply to the 'givens'
+ -- You should think of it as 'given equalities'
, red_stack :: (Int, [Inst]) -- Recursion stack (for err msg)
-- See Note [RedStack]
}
mkRedEnv :: SDoc -> (Inst -> WhatToDo) -> [Inst] -> RedEnv
mkRedEnv doc try_me givens
= RedEnv { red_doc = doc, red_try_me = try_me,
- red_givens = givens, red_stack = (0,[]),
+ red_givens = givens,
+ red_reft = emptyRefinement,
+ red_stack = (0,[]),
red_improve = True }
mkNoImproveRedEnv :: SDoc -> (Inst -> WhatToDo) -> RedEnv
-- Do not do improvement; no givens
mkNoImproveRedEnv doc try_me
= RedEnv { red_doc = doc, red_try_me = try_me,
- red_givens = [], red_stack = (0,[]),
+ red_givens = [], red_reft = emptyRefinement,
+ red_stack = (0,[]),
red_improve = True }
data WhatToDo
-- of a predicate when adding it to the avails
-- The reason for this flag is entirely the super-class loop problem
-- Note [SUPER-CLASS LOOP 1]
+
+zonkRedEnv :: RedEnv -> TcM RedEnv
+zonkRedEnv env
+ = do { givens' <- mappM zonkInst (red_givens env)
+ ; return $ env {red_givens = givens'}
+ }
\end{code}
+
%************************************************************************
%* *
\subsection[reduce]{@reduce@}
%* *
%************************************************************************
+Note [Ancestor Equalities]
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+During context reduction, we add to the wanted equalities also those
+equalities that (transitively) occur in superclass contexts of wanted
+class constraints. Consider the following code
+
+ class a ~ Int => C a
+ instance C Int
+
+If (C a) is wanted, we want to add (a ~ Int), which will be discharged by
+substituting Int for a. Hence, we ultimately want (C Int), which we
+discharge with the explicit instance.
\begin{code}
reduceContext :: RedEnv
-> [Inst] -- Wanted
-> TcM (ImprovementDone,
TcDictBinds, -- Dictionary bindings
- [Inst]) -- Irreducible
+ [Inst], -- Irreducible
+ [Inst]) -- Needed givens
reduceContext env wanteds
= do { traceTc (text "reduceContext" <+> (vcat [
text "----------------------"
]))
- -- Build the Avail mapping from "givens"
- ; init_state <- foldlM addGiven emptyAvails (red_givens env)
-
- -- Do the real work
- -- Process non-implication constraints first, so that they are
- -- available to help solving the implication constraints
- -- ToDo: seems a bit inefficient and ad-hoc
- ; let (implics, rest) = partition isImplicInst wanteds
- ; avails <- reduceList env (rest ++ implics) init_state
- ; let improved = availsImproved avails
- ; (binds, irreds) <- extractResults avails wanteds
+ ; let givens = red_givens env
+ (given_eqs0, given_dicts0) = partition isEqInst givens
+ (wanted_eqs0, wanted_dicts0) = partition isEqInst wanteds
+
+ -- We want to add as wanted equalities those that (transitively)
+ -- occur in superclass contexts of wanted class constraints.
+ -- See Note [Ancestor Equalities]
+ ; ancestor_eqs <- ancestorEqualities wanted_dicts0
+ ; let wanted_eqs = wanted_eqs0 ++ ancestor_eqs
+ ; traceTc $ text "reduceContext: ancestor eqs" <+> ppr ancestor_eqs
+
+ -- 1. Normalise the *given* *equality* constraints
+ ; (given_eqs, eliminate_skolems) <- normaliseGivenEqs given_eqs0
+
+ -- 2. Normalise the *given* *dictionary* constraints
+ -- wrt. the toplevel and given equations
+ ; (given_dicts, given_binds) <- normaliseGivenDicts given_eqs
+ given_dicts0
+
+ -- 5. Build the Avail mapping from "given_dicts"
+ -- Add dicts refined by the current type refinement
+ ; (init_state, extra_givens) <- getLIE $ do
+ { init_state <- foldlM addGiven emptyAvails given_dicts
+ ; let reft = red_reft env
+ ; if isEmptyRefinement reft then return init_state
+ else foldlM (addRefinedGiven reft)
+ init_state given_dicts }
+
+ -- *** ToDo: what to do with the "extra_givens"? For the
+ -- moment I'm simply discarding them, which is probably wrong
+
+ -- 7. Normalise the *wanted* *dictionary* constraints
+ -- wrt. the toplevel and given equations
+ -- NB: normalisation includes zonking as part of what it does
+ -- so it's important to do it after any unifications
+ -- that happened as a result of the addGivens
+ ; (wanted_dicts,normalise_binds1) <- normaliseWantedDicts given_eqs wanted_dicts0
+
+ -- 6. Solve the *wanted* *dictionary* constraints
+ -- This may expose some further equational constraints...
+ ; (avails, extra_eqs) <- getLIE (reduceList env wanted_dicts init_state)
+ ; let (binds, irreds1, needed_givens) = extractResults avails wanted_dicts
+ ; traceTc $ text "reduceContext extractresults" <+> vcat
+ [ppr avails,ppr wanted_dicts,ppr binds,ppr needed_givens]
+
+ -- *** ToDo: what to do with the "extra_eqs"? For the
+ -- moment I'm simply discarding them, which is probably wrong
+
+ -- 3. Solve the *wanted* *equation* constraints
+ ; eq_irreds0 <- solveWantedEqs given_eqs wanted_eqs
+
+ -- 4. Normalise the *wanted* equality constraints with respect to
+ -- each other
+ ; eq_irreds <- normaliseWantedEqs eq_irreds0
+
+ -- 8. Substitute the wanted *equations* in the wanted *dictionaries*
+ ; (irreds,normalise_binds2) <- substEqInDictInsts eq_irreds irreds1
+
+ -- 9. eliminate the artificial skolem constants introduced in 1.
+ ; eliminate_skolems
+
+ -- Figure out whether we should go round again
+ -- My current plan is to see if any of the mutable tyvars in
+ -- givens or irreds has been filled in by improvement.
+ -- If so, there is merit in going around again, because
+ -- we may make further progress
+ --
+ -- ToDo: is it only mutable stuff? We may have exposed new
+ -- equality constraints and should probably go round again
+ -- then as well. But currently we are dropping them on the
+ -- floor anyway.
+
+ ; let all_irreds = irreds ++ eq_irreds
+ ; improved <- anyM isFilledMetaTyVar $ varSetElems $
+ tyVarsOfInsts (givens ++ all_irreds)
+
+ -- The old plan (fragile)
+ -- improveed = availsImproved avails
+ -- || (not $ isEmptyBag normalise_binds1)
+ -- || (not $ isEmptyBag normalise_binds2)
+ -- || (any isEqInst irreds)
; traceTc (text "reduceContext end" <+> (vcat [
text "----------------------",
red_doc env,
- text "given" <+> ppr (red_givens env),
+ text "given" <+> ppr givens,
+ text "given_eqs" <+> ppr given_eqs,
text "wanted" <+> ppr wanteds,
+ text "wanted_dicts" <+> ppr wanted_dicts,
text "----",
text "avails" <+> pprAvails avails,
text "improved =" <+> ppr improved,
- text "irreds = " <+> ppr irreds,
+ text "(all) irreds = " <+> ppr all_irreds,
+ text "binds = " <+> ppr binds,
+ text "needed givens = " <+> ppr needed_givens,
text "----------------------"
]))
- ; return (improved, binds, irreds) }
+ ; return (improved,
+ given_binds `unionBags` normalise_binds1
+ `unionBags` normalise_binds2
+ `unionBags` binds,
+ all_irreds,
+ needed_givens)
+ }
tcImproveOne :: Avails -> Inst -> TcM ImprovementDone
tcImproveOne avails inst
\begin{code}
reduceList :: RedEnv -> [Inst] -> Avails -> TcM Avails
reduceList env@(RedEnv {red_stack = (n,stk)}) wanteds state
- = do { dopts <- getDOpts
+ = do { traceTc (text "reduceList " <+> (ppr wanteds $$ ppr state))
+ ; dopts <- getDOpts
#ifdef DEBUG
; if n > 8 then
dumpTcRn (hang (ptext SLIT("Interesting! Context reduction stack depth") <+> int n)
reduce env wanted avails
-- It's the same as an existing inst, or a superclass thereof
| Just avail <- findAvail avails wanted
- = returnM avails
+ = do { traceTc (text "reduce: found " <+> ppr wanted)
+ ; returnM avails
+ }
| otherwise
- = case red_try_me env wanted of {
- ; Stop -> try_simple (addIrred NoSCs) -- See Note [No superclasses for Stop]
-
- ; ReduceMe want_scs -> -- It should be reduced
- reduceInst env avails wanted `thenM` \ (avails, lookup_result) ->
- case lookup_result of
- NoInstance -> -- No such instance!
+ = do { traceTc (text "reduce" <+> ppr wanted $$ ppr avails)
+ ; case red_try_me env wanted of {
+ Stop -> try_simple (addIrred NoSCs);
+ -- See Note [No superclasses for Stop]
+
+ ReduceMe want_scs -> do -- It should be reduced
+ { (avails, lookup_result) <- reduceInst env avails wanted
+ ; case lookup_result of
+ NoInstance -> addIrred want_scs avails wanted
-- Add it and its superclasses
- addIrred want_scs avails wanted
-
- GenInst [] rhs -> addWanted want_scs avails wanted rhs []
+
+ GenInst [] rhs -> addWanted want_scs avails wanted rhs []
- GenInst wanteds' rhs -> do { avails1 <- addIrred NoSCs avails wanted
- ; avails2 <- reduceList env wanteds' avails1
- ; addWanted want_scs avails2 wanted rhs wanteds' }
+ GenInst wanteds' rhs
+ -> do { avails1 <- addIrred NoSCs avails wanted
+ ; avails2 <- reduceList env wanteds' avails1
+ ; addWanted want_scs avails2 wanted rhs wanteds' } }
-- Temporarily do addIrred *before* the reduceList,
-- which has the effect of adding the thing we are trying
-- to prove to the database before trying to prove the things it
-- needs. See note [RECURSIVE DICTIONARIES]
-- NB: we must not do an addWanted before, because that adds the
- -- superclasses too, and thaat can lead to a spurious loop; see
+ -- superclasses too, and that can lead to a spurious loop; see
-- the examples in [SUPERCLASS-LOOP]
-- So we do an addIrred before, and then overwrite it afterwards with addWanted
-
- }
+ } }
where
-- First, see if the inst can be reduced to a constant in one step
-- Works well for literals (1::Int) and constant dictionaries (d::Num Int)
\begin{code}
---------------------------------------------
reduceInst :: RedEnv -> Avails -> Inst -> TcM (Avails, LookupInstResult)
-reduceInst env avails (ImplicInst { tci_tyvars = tvs, tci_reft = reft, tci_loc = loc,
+reduceInst env avails (ImplicInst { tci_name = name,
+ tci_tyvars = tvs, tci_reft = reft, tci_loc = loc,
tci_given = extra_givens, tci_wanted = wanteds })
- = reduceImplication env avails reft tvs extra_givens wanteds loc
+ = reduceImplication env avails name reft tvs extra_givens wanteds loc
reduceInst env avails other_inst
= do { result <- lookupSimpleInst other_inst
; return (avails, result) }
\end{code}
+Note [Equational Constraints in Implication Constraints]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+An implication constraint is of the form
+ Given => Wanted
+where Given and Wanted may contain both equational and dictionary
+constraints. The delay and reduction of these two kinds of constraints
+is distinct:
+
+-) In the generated code, wanted Dictionary constraints are wrapped up in an
+ implication constraint that is created at the code site where the wanted
+ dictionaries can be reduced via a let-binding. This let-bound implication
+ constraint is deconstructed at the use-site of the wanted dictionaries.
+
+-) While the reduction of equational constraints is also delayed, the delay
+ is not manifest in the generated code. The required evidence is generated
+ in the code directly at the use-site. There is no let-binding and deconstruction
+ necessary. The main disadvantage is that we cannot exploit sharing as the
+ same evidence may be generated at multiple use-sites. However, this disadvantage
+ is limited because it only concerns coercions which are erased.
+
+The different treatment is motivated by the different in representation. Dictionary
+constraints require manifest runtime dictionaries, while equations require coercions
+which are types.
+
\begin{code}
---------------------------------------------
reduceImplication :: RedEnv
-> Avails
+ -> Name
-> Refinement -- May refine the givens; often empty
-> [TcTyVar] -- Quantified type variables; all skolems
-> [Inst] -- Extra givens; all rigid
\begin{code}
-- ToDo: should we instantiate tvs? I think it's not necessary
--
- -- ToDo: what about improvement? There may be some improvement
- -- exposed as a result of the simplifications done by reduceList
- -- which are discarded if we back off.
- -- This is almost certainly Wrong, but we'll fix it when dealing
- -- better with equality constraints
-reduceImplication env orig_avails reft tvs extra_givens wanteds inst_loc
+ -- Note on coercion variables:
+ --
+ -- The extra given coercion variables are bound at two different sites:
+ -- -) in the creation context of the implication constraint
+ -- the solved equational constraints use these binders
+ --
+ -- -) at the solving site of the implication constraint
+ -- the solved dictionaries use these binders
+ -- these binders are generated by reduceImplication
+ --
+reduceImplication env orig_avails name reft tvs extra_givens wanteds inst_loc
= do { -- Add refined givens, and the extra givens
- (refined_red_givens, avails)
- <- if isEmptyRefinement reft then return (red_givens env, orig_avails)
- else foldlM (addRefinedGiven reft) ([], orig_avails) (red_givens env)
- ; avails <- foldlM addGiven avails extra_givens
+ -- Todo fix this
+-- (refined_red_givens,refined_avails)
+-- <- if isEmptyRefinement reft then return (red_givens env,orig_avails)
+-- else foldlM (addRefinedGiven reft) ([],orig_avails) (red_givens env)
+-- Commented out SLPJ Sept 07; see comment with extractLocalResults below
+ let refined_red_givens = []
-- Solve the sub-problem
; let try_me inst = ReduceMe AddSCs -- Note [Freeness and implications]
- env' = env { red_givens = refined_red_givens ++ extra_givens
+ env' = env { red_givens = extra_givens ++ availsInsts orig_avails
+ , red_reft = reft
+ , red_doc = sep [ptext SLIT("reduceImplication for") <+> ppr name,
+ nest 2 (parens $ ptext SLIT("within") <+> red_doc env)]
, red_try_me = try_me }
; traceTc (text "reduceImplication" <+> vcat
[ ppr orig_avails,
ppr (red_givens env), ppr extra_givens,
- ppr reft, ppr wanteds, ppr avails ])
- ; avails <- reduceList env' wanteds avails
+ ppr reft, ppr wanteds])
+ ; (irreds,binds,needed_givens0) <- checkLoop env' wanteds
+ ; let (extra_eq_givens, extra_dict_givens) = partition isEqInst extra_givens
+ -- SLPJ Sept 07: I think this is bogus; currently
+ -- there are no Eqinsts in extra_givens
+ dict_ids = map instToId extra_dict_givens
+
+ -- needed_givens0 is the free vars of the bindings
+ -- Remove the ones we are going to lambda-bind
+ -- Use the actual dictionary identity *not* equality on Insts
+ -- (Mind you, it should make no difference here.)
+ ; let needed_givens = [ng | ng <- needed_givens0
+ , instToVar ng `notElem` dict_ids]
+
+ -- Note [Reducing implication constraints]
+ -- Tom -- update note, put somewhere!
- -- Extract the binding
- ; (binds, irreds) <- extractResults avails wanteds
-
; traceTc (text "reduceImplication result" <+> vcat
- [ ppr irreds, ppr binds])
+ [ppr irreds, ppr binds, ppr needed_givens])
+
+ ; -- extract superclass binds
+ -- (sc_binds,_) <- extractResults avails []
+-- ; traceTc (text "reduceImplication sc_binds" <+> vcat
+-- [ppr sc_binds, ppr avails])
+--
-- We always discard the extra avails we've generated;
-- but we remember if we have done any (global) improvement
- ; let ret_avails = updateImprovement orig_avails avails
+-- ; let ret_avails = avails
+ ; let ret_avails = orig_avails
+-- ; let ret_avails = updateImprovement orig_avails avails
- ; if isEmptyLHsBinds binds then -- No progress
+ -- SLPJ Sept 07: what if improvement happened inside the checkLoop?
+ -- Then we must iterate the outer loop too!
+
+ ; traceTc (text "reduceImplication condition" <+> ppr ((isEmptyLHsBinds binds) || (null irreds)))
+
+-- Progress is no longer measered by the number of bindings
+ ; if (isEmptyLHsBinds binds) && (not $ null irreds) then -- No progress
+ -- If there are any irreds, we back off and return NoInstance
return (ret_avails, NoInstance)
else do
{ (implic_insts, bind) <- makeImplicationBind inst_loc tvs reft extra_givens irreds
-
- ; let dict_ids = map instToId extra_givens
- co = mkWpTyLams tvs <.> mkWpLams dict_ids <.> WpLet (binds `unionBags` bind)
+ -- This binding is useless if the recursive simplification
+ -- made no progress; but currently we don't try to optimise that
+ -- case. After all, we only try hard to reduce at top level, or
+ -- when inferring types.
+
+ ; let dict_wanteds = filter (not . isEqInst) wanteds
+ -- TOMDO: given equational constraints bug!
+ -- we need a different evidence for given
+ -- equations depending on whether we solve
+ -- dictionary constraints or equational constraints
+
+ eq_tyvars = varSetElems $ tyVarsOfTypes $ map eqInstType extra_eq_givens
+ -- SLPJ Sept07: this looks Utterly Wrong to me, but I think
+ -- that current extra_givens has no EqInsts, so
+ -- it makes no difference
+ co = wrap_inline -- Note [Always inline implication constraints]
+ <.> mkWpTyLams tvs
+ <.> mkWpTyLams eq_tyvars
+ <.> mkWpLams dict_ids
+ <.> WpLet (binds `unionBags` bind)
+ wrap_inline | null dict_ids = idHsWrapper
+ | otherwise = WpInline
rhs = mkHsWrap co payload
loc = instLocSpan inst_loc
- payload | [wanted] <- wanteds = HsVar (instToId wanted)
- | otherwise = ExplicitTuple (map (L loc . HsVar . instToId) wanteds) Boxed
+ payload | [dict_wanted] <- dict_wanteds = HsVar (instToId dict_wanted)
+ | otherwise = ExplicitTuple (map (L loc . HsVar . instToId) dict_wanteds) Boxed
- -- If there are any irreds, we back off and return NoInstance
- ; return (ret_avails, GenInst implic_insts (L loc rhs))
- } }
+
+ ; traceTc (vcat [text "reduceImplication" <+> ppr name,
+ ppr implic_insts,
+ text "->" <+> sep [ppr needed_givens, ppr rhs]])
+ ; return (ret_avails, GenInst (implic_insts ++ needed_givens) (L loc rhs))
+ }
+ }
\end{code}
+Note [Always inline implication constraints]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Suppose an implication constraint floats out of an INLINE function.
+Then although the implication has a single call site, it won't be
+inlined. And that is bad because it means that even if there is really
+*no* overloading (type signatures specify the exact types) there will
+still be dictionary passing in the resulting code. To avert this,
+we mark the implication constraints themselves as INLINE, at least when
+there is no loss of sharing as a result.
+
+Note [Reducing implication constraints]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Suppose we are trying to simplify
+ ( do: Ord a,
+ ic: (forall b. C a b => (W [a] b, D c b)) )
+where
+ instance (C a b, Ord a) => W [a] b
+When solving the implication constraint, we'll start with
+ Ord a -> Irred
+in the Avails. Then we add (C a b -> Given) and solve. Extracting
+the results gives us a binding for the (W [a] b), with an Irred of
+(Ord a, D c b). Now, the (Ord a) comes from "outside" the implication,
+but the (D d b) is from "inside". So we want to generate a GenInst
+like this
+
+ ic = GenInst
+ [ do :: Ord a,
+ ic' :: forall b. C a b => D c b]
+ (/\b \(dc:C a b). (df a b dc do, ic' b dc))
+
+The first arg of GenInst gives the free dictionary variables of the
+second argument -- the "needed givens". And that list in turn is
+vital because it's used to determine what other dicts must be solved.
+This very list ends up in the second field of the Rhs, and drives
+extractResults.
+
+The need for this field is why we have to return "needed givens"
+from extractResults, reduceContext, checkLoop, and so on.
+
+NB: the "needed givens" in a GenInst or Rhs, may contain two dicts
+with the same type but different Ids, e.g. [d12 :: Eq a, d81 :: Eq a]
+That says we must generate a binding for both d12 and d81.
+
+The "inside" and "outside" distinction is what's going on with 'inner' and
+'outer' in reduceImplication
+
+
Note [Freeness and implications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
It's hard to say when an implication constraint can be floated out. Consider
type AvailEnv = FiniteMap Inst AvailHow
data AvailHow
- = IsIrred TcId -- Used for irreducible dictionaries,
+ = IsIrred -- Used for irreducible dictionaries,
-- which are going to be lambda bound
- | Given TcId -- Used for dictionaries for which we have a binding
+ | Given Inst -- Used for dictionaries for which we have a binding
-- e.g. those "given" in a signature
| Rhs -- Used when there is a RHS
pprAvails (Avails imp avails)
= vcat [ ptext SLIT("Avails") <> (if imp then ptext SLIT("[improved]") else empty)
- , nest 2 (vcat [sep [ppr inst, nest 2 (equals <+> ppr avail)]
- | (inst,avail) <- fmToList avails ])]
+ , nest 2 $ braces $
+ vcat [ sep [ppr inst, nest 2 (equals <+> ppr avail)]
+ | (inst,avail) <- fmToList avails ]]
instance Outputable AvailHow where
ppr = pprAvail
-------------------------
pprAvail :: AvailHow -> SDoc
-pprAvail (IsIrred x) = text "Irred" <+> ppr x
+pprAvail IsIrred = text "Irred"
pprAvail (Given x) = text "Given" <+> ppr x
-pprAvail (Rhs rhs bs) = text "Rhs" <+> ppr rhs <+> braces (ppr bs)
+pprAvail (Rhs rhs bs) = sep [text "Rhs" <+> ppr bs,
+ nest 2 (ppr rhs)]
-------------------------
extendAvailEnv :: AvailEnv -> Inst -> AvailHow -> AvailEnv
dependency analyser can sort them out later
\begin{code}
+type DoneEnv = FiniteMap Inst [Id]
+-- Tracks which things we have evidence for
+
extractResults :: Avails
-> [Inst] -- Wanted
- -> TcM ( TcDictBinds, -- Bindings
- [Inst]) -- Irreducible ones
+ -> (TcDictBinds, -- Bindings
+ [Inst], -- Irreducible ones
+ [Inst]) -- Needed givens, i.e. ones used in the bindings
+ -- Postcondition: needed-givens = free vars( binds ) \ irreds
+ -- needed-gives is subset of Givens in incoming Avails
+ -- Note [Reducing implication constraints]
extractResults (Avails _ avails) wanteds
- = go avails emptyBag [] wanteds
+ = go emptyBag [] [] emptyFM wanteds
where
- go :: AvailEnv -> TcDictBinds -> [Inst] -> [Inst]
- -> TcM (TcDictBinds, [Inst])
- go avails binds irreds []
- = returnM (binds, irreds)
-
- go avails binds irreds (w:ws)
+ go :: TcDictBinds -- Bindings for dicts
+ -> [Inst] -- Irreds
+ -> [Inst] -- Needed givens
+ -> DoneEnv -- Has an entry for each inst in the above three sets
+ -> [Inst] -- Wanted
+ -> (TcDictBinds, [Inst], [Inst])
+ go binds irreds givens done []
+ = (binds, irreds, givens)
+
+ go binds irreds givens done (w:ws)
+ | Just done_ids@(done_id : rest_done_ids) <- lookupFM done w
+ = if w_id `elem` done_ids then
+ go binds irreds givens done ws
+ else
+ go (add_bind (nlHsVar done_id)) irreds givens
+ (addToFM done w (done_id : w_id : rest_done_ids)) ws
+
+ | otherwise -- Not yet done
= case findAvailEnv avails w of
- Nothing -> pprTrace "Urk: extractResults" (ppr w) $
- go avails binds irreds ws
-
- Just (Given id)
- | id == w_id -> go avails binds irreds ws
- | otherwise -> go avails (addBind binds w_id (nlHsVar id)) irreds ws
- -- The sought Id can be one of the givens, via a superclass chain
- -- and then we definitely don't want to generate an x=x binding!
-
- Just (IsIrred id)
- | id == w_id -> go (add_given avails w) binds (w:irreds) ws
- | otherwise -> go avails (addBind binds w_id (nlHsVar id)) irreds ws
- -- The add_given handles the case where we want (Ord a, Eq a), and we
- -- don't want to emit *two* Irreds for Ord a, one via the superclass chain
- -- This showed up in a dupliated Ord constraint in the error message for
- -- test tcfail043
-
- Just (Rhs rhs ws') -> go (add_given avails w) new_binds irreds (ws' ++ ws)
- where
- new_binds = addBind binds w_id rhs
- where
- w_id = instToId w
+ Nothing -> pprTrace "Urk: extractResults" (ppr w) $
+ go binds irreds givens done ws
+
+ Just IsIrred -> go binds (w:irreds) givens done' ws
- add_given avails w = extendAvailEnv avails w (Given (instToId w))
- -- Don't add the same binding twice
+ Just (Rhs rhs ws') -> go (add_bind rhs) irreds givens done' (ws' ++ ws)
-addBind binds id rhs = binds `unionBags` unitBag (L (getSrcSpan id) (VarBind id rhs))
+ Just (Given g) -> go binds' irreds (g:givens) (addToFM done w [g_id]) ws
+ where
+ g_id = instToId g
+ binds' | w_id == g_id = binds
+ | otherwise = add_bind (nlHsVar g_id)
+ where
+ w_id = instToId w
+ done' = addToFM done w [w_id]
+ add_bind rhs = addInstToDictBind binds w rhs
\end{code}
avail = Rhs rhs_expr wanteds
addGiven :: Avails -> Inst -> TcM Avails
-addGiven avails given = addAvailAndSCs AddSCs avails given (Given (instToId given))
+addGiven avails given = addAvailAndSCs AddSCs avails given (Given given)
-- Always add superclasses for 'givens'
--
-- No ASSERT( not (given `elemAvails` avails) ) because in an instance
-- decl for Ord t we can add both Ord t and Eq t as 'givens',
-- so the assert isn't true
-addRefinedGiven :: Refinement -> ([Inst], Avails) -> Inst -> TcM ([Inst], Avails)
-addRefinedGiven reft (refined_givens, avails) given
+addRefinedGiven :: Refinement -> Avails -> Inst -> TcM Avails
+addRefinedGiven reft avails given
| isDict given -- We sometimes have 'given' methods, but they
-- are always optional, so we can drop them
, let pred = dictPred given
= do { new_given <- newDictBndr (instLoc given) pred
; let rhs = L (instSpan given) $
HsWrap (WpCo co) (HsVar (instToId given))
- ; avails <- addAvailAndSCs AddSCs avails new_given (Rhs rhs [given])
- ; return (new_given:refined_givens, avails) }
+ ; addAvailAndSCs AddSCs avails new_given (Rhs rhs [given]) }
-- ToDo: the superclasses of the original given all exist in Avails
-- so we could really just cast them, but it's more awkward to do,
-- and hopefully the optimiser will spot the duplicated work
| otherwise
- = return (refined_givens, avails)
+ = return avails
\end{code}
Note [ImplicInst rigidity]
\begin{code}
addIrred :: WantSCs -> Avails -> Inst -> TcM Avails
addIrred want_scs avails irred = ASSERT2( not (irred `elemAvails` avails), ppr irred $$ ppr avails )
- addAvailAndSCs want_scs avails irred (IsIrred (instToId irred))
+ addAvailAndSCs want_scs avails irred IsIrred
addAvailAndSCs :: WantSCs -> Avails -> Inst -> AvailHow -> TcM Avails
addAvailAndSCs want_scs avails inst avail
where
is_loop pred = any (`tcEqType` mkPredTy pred) dep_tys
-- Note: this compares by *type*, not by Unique
- deps = findAllDeps (unitVarSet (instToId inst)) avail
+ deps = findAllDeps (unitVarSet (instToVar inst)) avail
dep_tys = map idType (varSetElems deps)
findAllDeps :: IdSet -> AvailHow -> IdSet
find_all :: IdSet -> Inst -> IdSet
find_all so_far kid
+ | isEqInst kid = so_far
| kid_id `elemVarSet` so_far = so_far
| Just avail <- findAvail avails kid = findAllDeps so_far' avail
| otherwise = so_far'
addSCs :: (TcPredType -> Bool) -> Avails -> Inst -> TcM Avails
-- Add all the superclasses of the Inst to Avails
- -- The first param says "dont do this because the original thing
+ -- The first param says "don't do this because the original thing
-- depends on this one, so you'd build a loop"
-- Invariant: the Inst is already in Avails.
where
(clas, tys) = getDictClassTys dict
(tyvars, sc_theta, sc_sels, _) = classBigSig clas
- sc_theta' = substTheta (zipTopTvSubst tyvars tys) sc_theta
+ sc_theta' = filter (not . isEqPred) $
+ substTheta (zipTopTvSubst tyvars tys) sc_theta
add_sc avails (sc_dict, sc_sel)
| is_loop (dictPred sc_dict) = return avails -- See Note [SUPERCLASS-LOOP 2]
; addSCs is_loop avails' sc_dict }
where
sc_sel_rhs = L (instSpan dict) (HsWrap co_fn (HsVar sc_sel))
- co_fn = WpApp (instToId dict) <.> mkWpTyApps tys
+ co_fn = WpApp (instToVar dict) <.> mkWpTyApps tys
is_given :: Inst -> Bool
is_given sc_dict = case findAvail avails sc_dict of
Just (Given _) -> True -- Given is cheaper than superclass selection
other -> False
+
+-- From the a set of insts obtain all equalities that (transitively) occur in
+-- superclass contexts of class constraints (aka the ancestor equalities).
+--
+ancestorEqualities :: [Inst] -> TcM [Inst]
+ancestorEqualities
+ = mapM mkWantedEqInst -- turn only equality predicates..
+ . filter isEqPred -- ..into wanted equality insts
+ . bagToList
+ . addAEsToBag emptyBag -- collect the superclass constraints..
+ . map dictPred -- ..of all predicates in a bag
+ . filter isClassDict
+ where
+ addAEsToBag :: Bag PredType -> [PredType] -> Bag PredType
+ addAEsToBag bag [] = bag
+ addAEsToBag bag (pred:preds)
+ | pred `elemBag` bag = addAEsToBag bag preds
+ | isEqPred pred = addAEsToBag bagWithPred preds
+ | isClassPred pred = addAEsToBag bagWithPred predsWithSCs
+ | otherwise = addAEsToBag bag preds
+ where
+ bagWithPred = bag `snocBag` pred
+ predsWithSCs = preds ++ substTheta (zipTopTvSubst tyvars tys) sc_theta
+ --
+ (tyvars, sc_theta, _, _) = classBigSig clas
+ (clas, tys) = getClassPredTys pred
\end{code}
+
%************************************************************************
%* *
\section{tcSimplifyTop: defaulting}
-- error message generation for the monomorphism restriction
tc_simplify_top doc interactive wanteds
= do { dflags <- getDOpts
- ; wanteds <- mapM zonkInst wanteds
+ ; wanteds <- zonkInsts wanteds
; mapM_ zonkTopTyVar (varSetElems (tyVarsOfInsts wanteds))
+ ; traceTc (text "tc_simplify_top 0: " <+> ppr wanteds)
; (irreds1, binds1) <- tryHardCheckLoop doc1 wanteds
+-- ; (irreds1, binds1) <- gentleInferLoop doc1 wanteds
+ ; traceTc (text "tc_simplify_top 1: " <+> ppr irreds1)
; (irreds2, binds2) <- approximateImplications doc2 (\d -> True) irreds1
+ ; traceTc (text "tc_simplify_top 2: " <+> ppr irreds2)
-- Use the defaulting rules to do extra unification
-- NB: irreds2 are already zonked
= return (insts, emptyBag)
| null defaultable_groups
- = do { traceTc (text "disambigutate" <+> vcat [ppr unaries, ppr bad_tvs, ppr defaultable_groups])
+ = do { traceTc (text "disambigutate, no defaultable groups" <+> vcat [ppr unaries, ppr insts, ppr bad_tvs, ppr defaultable_groups])
; return (insts, emptyBag) }
| otherwise
= do { -- Figure out what default types to use
default_tys <- getDefaultTys extended_defaulting ovl_strings
- ; traceTc (text "disambigutate" <+> vcat [ppr unaries, ppr bad_tvs, ppr defaultable_groups])
+ ; traceTc (text "disambiguate1" <+> vcat [ppr insts, ppr unaries, ppr bad_tvs, ppr defaultable_groups])
; mapM_ (disambigGroup default_tys) defaultable_groups
-- disambigGroup does unification, hence try again
disambigGroup default_tys dicts
= try_default default_tys
where
- (_,_,tyvar) = head dicts -- Should be non-empty
+ (_,_,tyvar) = ASSERT(not (null dicts)) head dicts -- Should be non-empty
classes = [c | (_,c,_) <- dicts]
try_default [] = return ()
-- After this we can't fail
; warnDefault dicts default_ty
- ; unifyType default_ty (mkTyVarTy tyvar) }
+ ; unifyType default_ty (mkTyVarTy tyvar)
+ ; return () -- TOMDO: do something with the coercion
+ }
-----------------------
-> TcM ThetaType -- Needed
-- Given instance (wanted) => C inst_ty
-- Simplify 'wanted' as much as possible
--- The inst_ty is needed only for the termination check
tcSimplifyDeriv orig tyvars theta
= do { (tvs, _, tenv) <- tcInstTyVars tyvars
; wanteds <- newDictBndrsO orig (substTheta tenv theta)
; (irreds, _) <- tryHardCheckLoop doc wanteds
- ; let (tv_dicts, others) = partition isTyVarDict irreds
+ ; let (tv_dicts, others) = partition ok irreds
; addNoInstanceErrs others
+ -- See Note [Exotic derived instance contexts] in TcMType
; let rev_env = zipTopTvSubst tvs (mkTyVarTys tyvars)
simpl_theta = substTheta rev_env (map dictPred tv_dicts)
; return simpl_theta }
where
doc = ptext SLIT("deriving classes for a data type")
-\end{code}
-
-Note [Exotic derived instance contexts]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-Consider
- data T a b c = MkT (Foo a b c) deriving( Eq )
- instance (C Int a, Eq b, Eq c) => Eq (Foo a b c)
-
-Notice that this instance (just) satisfies the Paterson termination
-conditions. Then we *could* derive an instance decl like this:
- instance (C Int a, Eq b, Eq c) => Eq (T a b c)
-
-even though there is no instance for (C Int a), because there just
-*might* be an instance for, say, (C Int Bool) at a site where we
-need the equality instance for T's.
-
-However, this seems pretty exotic, and it's quite tricky to allow
-this, and yet give sensible error messages in the (much more common)
-case where we really want that instance decl for C.
-
-So for now we simply require that the derived instance context
-should have only type-variable constraints.
-
-Here is another example:
- data Fix f = In (f (Fix f)) deriving( Eq )
-Here, if we are prepared to allow -fallow-undecidable-instances we
-could derive the instance
- instance Eq (f (Fix f)) => Eq (Fix f)
-but this is so delicate that I don't think it should happen inside
-'deriving'. If you want this, write it yourself!
-
-NB: if you want to lift this condition, make sure you still meet the
-termination conditions! If not, the deriving mechanism generates
-larger and larger constraints. Example:
- data Succ a = S a
- data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show
-
-Note the lack of a Show instance for Succ. First we'll generate
- instance (Show (Succ a), Show a) => Show (Seq a)
-and then
- instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a)
-and so on. Instead we want to complain of no instance for (Show (Succ a)).
+ ok dict | isDict dict = validDerivPred (dictPred dict)
+ | otherwise = False
+\end{code}
@tcSimplifyDefault@ just checks class-type constraints, essentially;
if null irreds then
returnM ()
else
- failM
+ traceTc (ptext SLIT("tcSimplifyDefault failing")) >> failM
where
doc = ptext SLIT("default declaration")
\end{code}
-- We want to report them together in error messages
groupErrs report_err []
- = returnM ()
+ = return ()
groupErrs report_err (inst:insts)
- = do_one (inst:friends) `thenM_`
- groupErrs report_err others
-
+ = do { do_one (inst:friends)
+ ; groupErrs report_err others }
where
-- (It may seem a bit crude to compare the error messages,
-- but it makes sure that we combine just what the user sees,
= groupErrs (report_no_instances tidy_env mb_what) insts
report_no_instances tidy_env mb_what insts
- = do { inst_envs <- tcGetInstEnvs
- ; let (implics, insts1) = partition isImplicInst insts
- (insts2, overlaps) = partitionWith (check_overlap inst_envs) insts1
- ; traceTc (text "reportNoInstnces" <+> vcat
- [ppr implics, ppr insts1, ppr insts2])
- ; mapM_ complain_implic implics
- ; mapM_ (\doc -> addErrTcM (tidy_env, doc)) overlaps
- ; groupErrs complain_no_inst insts2 }
+ = do { inst_envs <- tcGetInstEnvs
+ ; let (implics, insts1) = partition isImplicInst insts
+ (insts2, overlaps) = partitionWith (check_overlap inst_envs) insts1
+ (eqInsts, insts3) = partition isEqInst insts2
+ ; traceTc (text "reportNoInstances" <+> vcat
+ [ppr insts, ppr implics, ppr insts1, ppr insts2])
+ ; mapM_ complain_implic implics
+ ; mapM_ (\doc -> addErrTcM (tidy_env, doc)) overlaps
+ ; groupErrs complain_no_inst insts3
+ ; mapM_ (addErrTcM . mk_eq_err) eqInsts
+ }
where
complain_no_inst insts = addErrTcM (tidy_env, mk_no_inst_err insts)
| not (isClassDict wanted) = Left wanted
| otherwise
= case lookupInstEnv inst_envs clas tys of
- -- The case of exactly one match and no unifiers means
- -- a successful lookup. That can't happen here, becuase
- -- dicts only end up here if they didn't match in Inst.lookupInst
+ -- The case of exactly one match and no unifiers means a
+ -- successful lookup. That can't happen here, because dicts
+ -- only end up here if they didn't match in Inst.lookupInst
#ifdef DEBUG
([m],[]) -> pprPanic "reportNoInstance" (ppr wanted)
#endif
parens (vcat [ptext SLIT("The choice depends on the instantiation of") <+>
quotes (pprWithCommas ppr (varSetElems (tyVarsOfInst dict))),
ptext SLIT("To pick the first instance above, use -fallow-incoherent-instances"),
- ptext SLIT("when compiling the other instances")])]
+ ptext SLIT("when compiling the other instance declarations")])]
where
ispecs = [ispec | (ispec, _) <- matches]
+ mk_eq_err :: Inst -> (TidyEnv, SDoc)
+ mk_eq_err inst = misMatchMsg tidy_env (eqInstTys inst)
+
mk_no_inst_err insts
| null insts = empty
nest 2 (vcat docs),
monomorphism_fix dflags]
-isRuntimeUnk :: TcTyVar -> Bool
-isRuntimeUnk x | SkolemTv RuntimeUnkSkol <- tcTyVarDetails x = True
- | otherwise = False
-
monomorphism_fix :: DynFlags -> SDoc
monomorphism_fix dflags
= ptext SLIT("Probable fix:") <+> vcat
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