tcSimplifyCheck, tcSimplifyRestricted,
tcSimplifyToDicts, tcSimplifyIPs, tcSimplifyTop,
- tcSimplifyThetas, tcSimplifyCheckThetas,
+ tcSimplifyDeriv, tcSimplifyDefault,
bindInstsOfLocalFuns
) where
#include "HsVersions.h"
+import {-# SOURCE #-} TcUnify( unifyTauTy )
+
import HsSyn ( MonoBinds(..), HsExpr(..), andMonoBinds, andMonoBindList )
import TcHsSyn ( TcExpr, TcId,
TcMonoBinds, TcDictBinds
)
import TcMonad
-import Inst ( lookupInst, lookupSimpleInst, LookupInstResult(..),
- tyVarsOfInst, predsOfInsts, predsOfInst,
- isDict, isClassDict,
- isStdClassTyVarDict, isMethodFor,
- instToId, tyVarsOfInsts,
- ipNamesOfInsts, ipNamesOfInst,
+import Inst ( lookupInst, LookupInstResult(..),
+ tyVarsOfInst, predsOfInsts, predsOfInst, newDicts,
+ isDict, isClassDict, isLinearInst, linearInstType,
+ isStdClassTyVarDict, isMethodFor, isMethod,
+ instToId, tyVarsOfInsts, cloneDict,
+ ipNamesOfInsts, ipNamesOfInst, dictPred,
instBindingRequired, instCanBeGeneralised,
- newDictsFromOld,
+ newDictsFromOld, newMethodAtLoc,
getDictClassTys, isTyVarDict,
instLoc, pprInst, zonkInst, tidyInsts, tidyMoreInsts,
Inst, LIE, pprInsts, pprInstsInFull,
mkLIE, lieToList
)
-import TcEnv ( tcGetGlobalTyVars, tcGetInstEnv )
+import TcEnv ( tcGetGlobalTyVars, tcGetInstEnv, tcLookupGlobalId )
import InstEnv ( lookupInstEnv, classInstEnv, InstLookupResult(..) )
-
-import TcMType ( zonkTcTyVarsAndFV, tcInstTyVars, unifyTauTy )
-import TcType ( ThetaType, PredType, mkClassPred, isOverloadedTy,
- mkTyVarTy, tcGetTyVar, isTyVarClassPred,
- tyVarsOfPred, getClassPredTys_maybe, isClassPred, isIPPred,
- inheritablePred, predHasFDs )
-import Id ( idType )
+import TcMType ( zonkTcTyVarsAndFV, tcInstTyVars, checkAmbiguity )
+import TcType ( TcTyVar, TcTyVarSet, ThetaType, TyVarDetails(VanillaTv),
+ mkClassPred, isOverloadedTy, mkTyConApp,
+ mkTyVarTy, tcGetTyVar, isTyVarClassPred, mkTyVarTys,
+ tyVarsOfPred, isIPPred, isInheritablePred, predHasFDs )
+import Id ( idType, mkUserLocal )
+import Var ( TyVar )
+import Name ( getOccName, getSrcLoc )
import NameSet ( NameSet, mkNameSet, elemNameSet )
import Class ( classBigSig )
import FunDeps ( oclose, grow, improve, pprEquationDoc )
-import PrelInfo ( isNumericClass, isCreturnableClass, isCcallishClass )
+import PrelInfo ( isNumericClass, isCreturnableClass, isCcallishClass,
+ splitName, fstName, sndName )
import Subst ( mkTopTyVarSubst, substTheta, substTy )
-import TysWiredIn ( unitTy )
+import TysWiredIn ( unitTy, pairTyCon )
import VarSet
import FiniteMap
import Outputable
isFreeWhenInferring :: TyVarSet -> Inst -> Bool
isFreeWhenInferring qtvs inst
= isFreeWrtTyVars qtvs inst -- Constrains no quantified vars
- && all inheritablePred (predsOfInst inst) -- And no implicit parameter involved
+ && all isInheritablePred (predsOfInst inst) -- And no implicit parameter involved
-- (see "Notes on implicit parameters")
isFreeWhenChecking :: TyVarSet -- Quantified tyvars
-- tcSimplifyCheck is used when checking expression type signatures,
-- class decls, instance decls etc.
--- Note that we psss isFree (not isFreeAndInheritable) to tcSimplCheck
--- It's important that we can float out non-inheritable predicates
--- Example: (?x :: Int) is ok!
+--
+-- NB: tcSimplifyCheck does not consult the
+-- global type variables in the environment; so you don't
+-- need to worry about setting them before calling tcSimplifyCheck
tcSimplifyCheck doc qtvs givens wanted_lie
= tcSimplCheck doc get_qtvs
givens wanted_lie `thenTc` \ (qtvs', frees, binds) ->
| Free -- Return as free
+reduceMe :: Inst -> WhatToDo
+reduceMe inst = ReduceMe
+
data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses
-- of a predicate when adding it to the avails
\end{code}
\begin{code}
-type RedState = (Avails, -- What's available
- [Inst]) -- Insts for which try_me returned Free
-
type Avails = FiniteMap Inst Avail
data Avail
- = Irred -- Used for irreducible dictionaries,
+ = IsFree -- Used for free Insts
+ | Irred -- Used for irreducible dictionaries,
-- which are going to be lambda bound
- | BoundTo TcId -- Used for dictionaries for which we have a binding
+ | Given TcId -- Used for dictionaries for which we have a binding
-- e.g. those "given" in a signature
+ Bool -- True <=> actually consumed (splittable IPs only)
| NoRhs -- Used for Insts like (CCallable f)
-- where no witness is required.
TcExpr -- The RHS
[Inst] -- Insts free in the RHS; we need these too
-pprAvails avails = vcat [ppr inst <+> equals <+> pprAvail avail
+ | Linear -- Splittable Insts only.
+ Int -- The Int is always 2 or more; indicates how
+ -- many copies are required
+ Inst -- The splitter
+ Avail -- Where the "master copy" is
+
+ | LinRhss -- Splittable Insts only; this is used only internally
+ -- by extractResults, where a Linear
+ -- is turned into an LinRhss
+ [TcExpr] -- A supply of suitable RHSs
+
+pprAvails avails = vcat [sep [ppr inst, nest 2 (equals <+> pprAvail avail)]
| (inst,avail) <- fmToList avails ]
instance Outputable Avail where
ppr = pprAvail
-pprAvail NoRhs = text "<no rhs>"
-pprAvail Irred = text "Irred"
-pprAvail (BoundTo x) = text "Bound to" <+> ppr x
-pprAvail (Rhs rhs bs) = ppr rhs <+> braces (ppr bs)
+pprAvail NoRhs = text "<no rhs>"
+pprAvail IsFree = text "Free"
+pprAvail Irred = text "Irred"
+pprAvail (Given x b) = text "Given" <+> ppr x <+>
+ if b then text "(used)" else empty
+pprAvail (Rhs rhs bs) = text "Rhs" <+> ppr rhs <+> braces (ppr bs)
+pprAvail (Linear n i a) = text "Linear" <+> ppr n <+> braces (ppr i) <+> ppr a
+pprAvail (LinRhss rhss) = text "LinRhss" <+> ppr rhss
\end{code}
Extracting the bindings from a bunch of Avails.
The loop startes
\begin{code}
-bindsAndIrreds :: Avails
+extractResults :: Avails
-> [Inst] -- Wanted
- -> (TcDictBinds, -- Bindings
- [Inst]) -- Irreducible ones
+ -> NF_TcM (TcDictBinds, -- Bindings
+ [Inst], -- Irreducible ones
+ [Inst]) -- Free ones
-bindsAndIrreds avails wanteds
- = go avails EmptyMonoBinds [] wanteds
+extractResults avails wanteds
+ = go avails EmptyMonoBinds [] [] wanteds
where
- go avails binds irreds [] = (binds, irreds)
+ go avails binds irreds frees []
+ = returnNF_Tc (binds, irreds, frees)
- go avails binds irreds (w:ws)
+ go avails binds irreds frees (w:ws)
= case lookupFM avails w of
- Nothing -> -- Free guys come out here
- -- (If we didn't do addFree we could use this as the
- -- criterion for free-ness, and pick up the free ones here too)
- go avails binds irreds ws
+ Nothing -> pprTrace "Urk: extractResults" (ppr w) $
+ go avails binds irreds frees ws
- Just NoRhs -> go avails binds irreds ws
+ Just NoRhs -> go avails binds irreds frees ws
+ Just IsFree -> go (add_free avails w) binds irreds (w:frees) ws
+ Just Irred -> go (add_given avails w) binds (w:irreds) frees ws
- Just Irred -> go (addToFM avails w (BoundTo (instToId w))) binds (w:irreds) ws
-
- Just (BoundTo id) -> go avails new_binds irreds ws
+ Just (Given id _) -> go avails new_binds irreds frees ws
where
- -- For implicit parameters, all occurrences share the same
- -- Id, so there is no need for synonym bindings
- new_binds | new_id == id = binds
- | otherwise = addBind binds new_id (HsVar id)
- new_id = instToId w
+ new_binds | id == instToId w = binds
+ | otherwise = addBind binds w (HsVar id)
+ -- 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 (Rhs rhs ws') -> go avails' (addBind binds id rhs) irreds (ws' ++ ws)
+ Just (Rhs rhs ws') -> go (add_given avails w) new_binds irreds frees (ws' ++ ws)
where
- id = instToId w
- avails' = addToFM avails w (BoundTo id)
-
-addBind binds id rhs = binds `AndMonoBinds` VarMonoBind id rhs
+ new_binds = addBind binds w rhs
+
+ Just (Linear n split_inst avail) -- Transform Linear --> LinRhss
+ -> get_root irreds frees avail w `thenNF_Tc` \ (irreds', frees', root_id) ->
+ split n (instToId split_inst) root_id w `thenNF_Tc` \ (binds', rhss) ->
+ go (addToFM avails w (LinRhss rhss))
+ (binds `AndMonoBinds` binds')
+ irreds' frees' (split_inst : w : ws)
+
+ Just (LinRhss (rhs:rhss)) -- Consume one of the Rhss
+ -> go new_avails new_binds irreds frees ws
+ where
+ new_binds = addBind binds w rhs
+ new_avails = addToFM avails w (LinRhss rhss)
+
+ get_root irreds frees (Given id _) w = returnNF_Tc (irreds, frees, id)
+ get_root irreds frees Irred w = cloneDict w `thenNF_Tc` \ w' ->
+ returnNF_Tc (w':irreds, frees, instToId w')
+ get_root irreds frees IsFree w = cloneDict w `thenNF_Tc` \ w' ->
+ returnNF_Tc (irreds, w':frees, instToId w')
+
+ add_given avails w
+ | instBindingRequired w = addToFM avails w (Given (instToId w) True)
+ | otherwise = addToFM avails w NoRhs
+ -- NB: make sure that CCallable/CReturnable use NoRhs rather
+ -- than Given, else we end up with bogus bindings.
+
+ add_free avails w | isMethod w = avails
+ | otherwise = add_given avails w
+ -- NB: Hack alert!
+ -- Do *not* replace Free by Given if it's a method.
+ -- The following situation shows why this is bad:
+ -- truncate :: forall a. RealFrac a => forall b. Integral b => a -> b
+ -- From an application (truncate f i) we get
+ -- t1 = truncate at f
+ -- t2 = t1 at i
+ -- If we have also have a second occurrence of truncate, we get
+ -- t3 = truncate at f
+ -- t4 = t3 at i
+ -- When simplifying with i,f free, we might still notice that
+ -- t1=t3; but alas, the binding for t2 (which mentions t1)
+ -- will continue to float out!
+ -- (split n i a) returns: n rhss
+ -- auxiliary bindings
+ -- 1 or 0 insts to add to irreds
+
+
+split :: Int -> TcId -> TcId -> Inst
+ -> NF_TcM (TcDictBinds, [TcExpr])
+-- (split n split_id root_id wanted) returns
+-- * a list of 'n' expressions, all of which witness 'avail'
+-- * a bunch of auxiliary bindings to support these expressions
+-- * one or zero insts needed to witness the whole lot
+-- (maybe be zero if the initial Inst is a Given)
+--
+-- NB: 'wanted' is just a template
+
+split n split_id root_id wanted
+ = go n
+ where
+ ty = linearInstType wanted
+ pair_ty = mkTyConApp pairTyCon [ty,ty]
+ id = instToId wanted
+ occ = getOccName id
+ loc = getSrcLoc id
+
+ go 1 = returnNF_Tc (EmptyMonoBinds, [HsVar root_id])
+
+ go n = go ((n+1) `div` 2) `thenNF_Tc` \ (binds1, rhss) ->
+ expand n rhss `thenNF_Tc` \ (binds2, rhss') ->
+ returnNF_Tc (binds1 `AndMonoBinds` binds2, rhss')
+
+ -- (expand n rhss)
+ -- Given ((n+1)/2) rhss, make n rhss, using auxiliary bindings
+ -- e.g. expand 3 [rhs1, rhs2]
+ -- = ( { x = split rhs1 },
+ -- [fst x, snd x, rhs2] )
+ expand n rhss
+ | n `rem` 2 == 0 = go rhss -- n is even
+ | otherwise = go (tail rhss) `thenNF_Tc` \ (binds', rhss') ->
+ returnNF_Tc (binds', head rhss : rhss')
+ where
+ go rhss = mapAndUnzipNF_Tc do_one rhss `thenNF_Tc` \ (binds', rhss') ->
+ returnNF_Tc (andMonoBindList binds', concat rhss')
+
+ do_one rhs = tcGetUnique `thenNF_Tc` \ uniq ->
+ tcLookupGlobalId fstName `thenNF_Tc` \ fst_id ->
+ tcLookupGlobalId sndName `thenNF_Tc` \ snd_id ->
+ let
+ x = mkUserLocal occ uniq pair_ty loc
+ in
+ returnNF_Tc (VarMonoBind x (mk_app split_id rhs),
+ [mk_fs_app fst_id ty x, mk_fs_app snd_id ty x])
+
+mk_fs_app id ty var = HsVar id `TyApp` [ty,ty] `HsApp` HsVar var
+
+mk_app id rhs = HsApp (HsVar id) rhs
+
+addBind binds inst rhs = binds `AndMonoBinds` VarMonoBind (instToId inst) rhs
\end{code}
])) `thenNF_Tc_`
-- Build the Avail mapping from "givens"
- foldlNF_Tc addGiven (emptyFM, []) givens `thenNF_Tc` \ init_state ->
+ foldlNF_Tc addGiven emptyFM givens `thenNF_Tc` \ init_state ->
-- Do the real work
- reduceList (0,[]) try_me wanteds init_state `thenNF_Tc` \ state@(avails, frees) ->
+ reduceList (0,[]) try_me wanteds init_state `thenNF_Tc` \ avails ->
-- Do improvement, using everything in avails
-- In particular, avails includes all superclasses of everything
tcImprove avails `thenTc` \ no_improvement ->
+ extractResults avails wanteds `thenNF_Tc` \ (binds, irreds, frees) ->
+
traceTc (text "reduceContext end" <+> (vcat [
text "----------------------",
doc,
text "no_improvement =" <+> ppr no_improvement,
text "----------------------"
])) `thenNF_Tc_`
- let
- (binds, irreds) = bindsAndIrreds avails wanteds
- in
- returnTc (no_improvement, frees, binds, irreds)
+
+ returnTc (no_improvement, frees, binds, irreds)
tcImprove avails
= tcGetInstEnv `thenTc` \ inst_env ->
returnTc False
where
unify ((qtvs, t1, t2), doc)
- = tcAddErrCtxt doc $
- tcInstTyVars (varSetElems qtvs) `thenNF_Tc` \ (_, _, tenv) ->
+ = tcAddErrCtxt doc $
+ tcInstTyVars VanillaTv (varSetElems qtvs) `thenNF_Tc` \ (_, _, tenv) ->
unifyTauTy (substTy tenv t1) (substTy tenv t2)
\end{code}
-- along with its depth
-> (Inst -> WhatToDo)
-> [Inst]
- -> RedState
- -> TcM RedState
+ -> Avails
+ -> TcM Avails
\end{code}
@reduce@ is passed
Free return this in "frees"
wanteds: The list of insts to reduce
- state: An accumulating parameter of type RedState
+ state: An accumulating parameter of type Avails
that contains the state of the algorithm
- It returns a RedState.
+ It returns a Avails.
The (n,stack) pair is just used for error reporting.
n is always the depth of the stack.
-- Base case: we're done!
reduce stack try_me wanted state
-- It's the same as an existing inst, or a superclass thereof
- | isAvailable state wanted
- = returnTc state
+ | Just avail <- isAvailable state wanted
+ = if isLinearInst wanted then
+ addLinearAvailable state avail wanted `thenNF_Tc` \ (state', wanteds') ->
+ reduceList stack try_me wanteds' state'
+ else
+ returnTc state -- No op for non-linear things
| otherwise
= case try_me wanted of {
\begin{code}
-isAvailable :: RedState -> Inst -> Bool
-isAvailable (avails, _) wanted = wanted `elemFM` avails
- -- NB: the Ord instance of Inst compares by the class/type info
+-------------------------
+isAvailable :: Avails -> Inst -> Maybe Avail
+isAvailable avails wanted = lookupFM avails wanted
+ -- NB 1: the Ord instance of Inst compares by the class/type info
-- *not* by unique. So
-- d1::C Int == d2::C Int
+addLinearAvailable :: Avails -> Avail -> Inst -> NF_TcM (Avails, [Inst])
+addLinearAvailable avails avail wanted
+ -- avails currently maps [wanted -> avail]
+ -- Extend avails to reflect a neeed for an extra copy of avail
+
+ | Just avail' <- split_avail avail
+ = returnNF_Tc (addToFM avails wanted avail', [])
+
+ | otherwise
+ = tcLookupGlobalId splitName `thenNF_Tc` \ split_id ->
+ newMethodAtLoc (instLoc wanted) split_id
+ [linearInstType wanted] `thenNF_Tc` \ (split_inst,_) ->
+ returnNF_Tc (addToFM avails wanted (Linear 2 split_inst avail), [split_inst])
+
+ where
+ split_avail :: Avail -> Maybe Avail
+ -- (Just av) if there's a modified version of avail that
+ -- we can use to replace avail in avails
+ -- Nothing if there isn't, so we need to create a Linear
+ split_avail (Linear n i a) = Just (Linear (n+1) i a)
+ split_avail (Given id used) | not used = Just (Given id True)
+ | otherwise = Nothing
+ split_avail Irred = Nothing
+ split_avail IsFree = Nothing
+ split_avail other = pprPanic "addLinearAvailable" (ppr avail $$ ppr wanted $$ ppr avails)
+
-------------------------
-addFree :: RedState -> Inst -> NF_TcM RedState
+addFree :: Avails -> Inst -> NF_TcM Avails
-- When an Inst is tossed upstairs as 'free' we nevertheless add it
-- to avails, so that any other equal Insts will be commoned up right
-- here rather than also being tossed upstairs. This is really just
-- an optimisation, and perhaps it is more trouble that it is worth,
-- as the following comments show!
--
- -- NB1: do *not* add superclasses. If we have
+ -- NB: do *not* add superclasses. If we have
-- df::Floating a
-- dn::Num a
-- but a is not bound here, then we *don't* want to derive
-- dn from df here lest we lose sharing.
--
- -- NB2: do *not* add the Inst to avails at all if it's a method.
- -- The following situation shows why this is bad:
- -- truncate :: forall a. RealFrac a => forall b. Integral b => a -> b
- -- From an application (truncate f i) we get
- -- t1 = truncate at f
- -- t2 = t1 at i
- -- If we have also have a second occurrence of truncate, we get
- -- t3 = truncate at f
- -- t4 = t3 at i
- -- When simplifying with i,f free, we might still notice that
- -- t1=t3; but alas, the binding for t2 (which mentions t1)
- -- will continue to float out!
- -- Solution: never put methods in avail till they are captured
- -- in which case addFree isn't used
- --
- -- NB3: make sure that CCallable/CReturnable use NoRhs rather
- -- than BoundTo, else we end up with bogus bindings.
- -- c.f. instBindingRequired in addWanted
-addFree (avails, frees) free
- | isDict free = returnNF_Tc (addToFM avails free avail, free:frees)
- | otherwise = returnNF_Tc (avails, free:frees)
- where
- avail | instBindingRequired free = BoundTo (instToId free)
- | otherwise = NoRhs
-
-addWanted :: RedState -> Inst -> TcExpr -> [Inst] -> NF_TcM RedState
-addWanted state@(avails, frees) wanted rhs_expr wanteds
--- Do *not* add superclasses as well. Here's an example of why not
--- class Eq a => Foo a b
--- instance Eq a => Foo [a] a
--- If we are reducing
--- (Foo [t] t)
--- we'll first deduce that it holds (via the instance decl). We
--- must not then overwrite the Eq t constraint with a superclass selection!
--- ToDo: this isn't entirely unsatisfactory, because
--- we may also lose some entirely-legitimate sharing this way
-
- = ASSERT( not (isAvailable state wanted) )
- returnNF_Tc (addToFM avails wanted avail, frees)
+addFree avails free = returnNF_Tc (addToFM avails free IsFree)
+
+addWanted :: Avails -> Inst -> TcExpr -> [Inst] -> NF_TcM Avails
+addWanted avails wanted rhs_expr wanteds
+ = ASSERT2( not (wanted `elemFM` avails), ppr wanted $$ ppr avails )
+ addAvailAndSCs avails wanted avail
where
avail | instBindingRequired wanted = Rhs rhs_expr wanteds
| otherwise = ASSERT( null wanteds ) NoRhs
-addGiven :: RedState -> Inst -> NF_TcM RedState
-addGiven state given = addAvailAndSCs state given (BoundTo (instToId given))
-
-addIrred :: WantSCs -> RedState -> Inst -> NF_TcM RedState
-addIrred NoSCs (avails,frees) irred = returnNF_Tc (addToFM avails irred Irred, frees)
-addIrred AddSCs state irred = addAvailAndSCs state irred Irred
-
-addAvailAndSCs :: RedState -> Inst -> Avail -> NF_TcM RedState
-addAvailAndSCs (avails, frees) wanted avail
- = add_avail_and_scs avails wanted avail `thenNF_Tc` \ avails' ->
- returnNF_Tc (avails', frees)
-
----------------------
-add_avail_and_scs :: Avails -> Inst -> Avail -> NF_TcM Avails
-add_avail_and_scs avails wanted avail
- = add_scs (addToFM avails wanted avail) wanted
-
-add_scs :: Avails -> Inst -> NF_TcM Avails
+addGiven :: Avails -> Inst -> NF_TcM Avails
+addGiven state given = addAvailAndSCs state given (Given (instToId given) False)
+ -- No ASSERT( not (given `elemFM` avails) ) because in an instance
+ -- decl for Ord t we can add both Ord t and Eq t as 'givens',
+ -- so the assert isn't true
+
+addIrred :: WantSCs -> Avails -> Inst -> NF_TcM Avails
+addIrred NoSCs avails irred = returnNF_Tc (addToFM avails irred Irred)
+addIrred AddSCs avails irred = ASSERT2( not (irred `elemFM` avails), ppr irred $$ ppr avails )
+ addAvailAndSCs avails irred Irred
+
+addAvailAndSCs :: Avails -> Inst -> Avail -> NF_TcM Avails
+addAvailAndSCs avails inst avail
+ | not (isClassDict inst) = returnNF_Tc avails1
+ | otherwise = addSCs is_loop avails1 inst
+ where
+ avails1 = addToFM avails inst avail
+ is_loop inst = inst `elem` deps -- Note: this compares by *type*, not by Unique
+ deps = findAllDeps avails avail
+
+findAllDeps :: Avails -> Avail -> [Inst]
+-- Find all the Insts that this one depends on
+-- See Note [SUPERCLASS-LOOP]
+findAllDeps avails (Rhs _ kids) = kids ++ concat (map (find_all_deps_help avails) kids)
+findAllDeps avails other = []
+
+find_all_deps_help :: Avails -> Inst -> [Inst]
+find_all_deps_help avails inst
+ = case lookupFM avails inst of
+ Just avail -> findAllDeps avails avail
+ Nothing -> []
+
+addSCs :: (Inst -> Bool) -> Avails -> Inst -> NF_TcM Avails
-- Add all the superclasses of the Inst to Avails
+ -- The first param says "dont do this because the original thing
+ -- depends on this one, so you'd build a loop"
-- Invariant: the Inst is already in Avails.
-add_scs avails dict
- | not (isClassDict dict)
- = returnNF_Tc avails
-
- | otherwise -- It is a dictionary
+addSCs is_loop avails dict
= newDictsFromOld dict sc_theta' `thenNF_Tc` \ sc_dicts ->
foldlNF_Tc add_sc avails (zipEqual "add_scs" sc_dicts sc_sels)
where
add_sc avails (sc_dict, sc_sel) -- Add it, and its superclasses
= case lookupFM avails sc_dict of
- Just (BoundTo _) -> returnNF_Tc avails -- See Note [SUPER] below
- other -> add_avail_and_scs avails sc_dict avail
+ Just (Given _ _) -> returnNF_Tc avails -- Given is cheaper than
+ -- a superclass selection
+ Just other | is_loop sc_dict -> returnNF_Tc avails -- See Note [SUPERCLASS-LOOP]
+ | otherwise -> returnNF_Tc avails' -- SCs already added
+
+ Nothing -> addSCs is_loop avails' sc_dict
where
sc_sel_rhs = DictApp (TyApp (HsVar sc_sel) tys) [instToId dict]
avail = Rhs sc_sel_rhs [dict]
+ avails' = addToFM avails sc_dict avail
\end{code}
-Note [SUPER]. We have to be careful here. If we are *given* d1:Ord a,
+Note [SUPERCLASS-LOOP]: Checking for loops
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+We have to be careful here. If we are *given* d1:Ord a,
and want to deduce (d2:C [a]) where
class Ord a => C a where
Then we'll use the instance decl to deduce C [a] and then add the
superclasses of C [a] to avails. But we must not overwrite the binding
for d1:Ord a (which is given) with a superclass selection or we'll just
-build a loop! Hence looking for BoundTo. Crudely, BoundTo is cheaper
-than a selection.
+build a loop!
+
+Here's another example
+ class Eq b => Foo a b
+ instance Eq a => Foo [a] a
+If we are reducing
+ (Foo [t] t)
+
+we'll first deduce that it holds (via the instance decl). We must not
+then overwrite the Eq t constraint with a superclass selection!
+
+At first I had a gross hack, whereby I simply did not add superclass constraints
+in addWanted, though I did for addGiven and addIrred. This was sub-optimal,
+becuase it lost legitimate superclass sharing, and it still didn't do the job:
+I found a very obscure program (now tcrun021) in which improvement meant the
+simplifier got two bites a the cherry... so something seemed to be an Irred
+first time, but reducible next time.
+
+Now we implement the Right Solution, which is to check for loops directly
+when adding superclasses. It's a bit like the occurs check in unification.
+
%************************************************************************
%************************************************************************
-If a dictionary constrains a type variable which is
- * not mentioned in the environment
- * and not mentioned in the type of the expression
-then it is ambiguous. No further information will arise to instantiate
-the type variable; nor will it be generalised and turned into an extra
-parameter to a function.
-
-It is an error for this to occur, except that Haskell provided for
-certain rules to be applied in the special case of numeric types.
-Specifically, if
- * at least one of its classes is a numeric class, and
- * all of its classes are numeric or standard
-then the type variable can be defaulted to the first type in the
-default-type list which is an instance of all the offending classes.
-
-So here is the function which does the work. It takes the ambiguous
-dictionaries and either resolves them (producing bindings) or
-complains. It works by splitting the dictionary list by type
-variable, and using @disambigOne@ to do the real business.
-
@tcSimplifyTop@ is called once per module to simplify all the constant
and ambiguous Insts.
\begin{code}
tcSimplifyTop :: LIE -> TcM TcDictBinds
tcSimplifyTop wanted_lie
- = simpleReduceLoop (text "tcSimplTop") try_me wanteds `thenTc` \ (frees, binds, irreds) ->
+ = simpleReduceLoop (text "tcSimplTop") reduceMe wanteds `thenTc` \ (frees, binds, irreds) ->
ASSERT( null frees )
let
-- Collect together all the bad guys
bad_guys = non_stds ++ concat std_bads
in
- -- Disambiguate the ones that look feasible
- mapTc disambigGroup std_oks `thenTc` \ binds_ambig ->
- -- And complain about the ones that don't
+ ifErrsTc (returnTc []) (
+ -- Don't check for ambiguous things
+ -- if there has been an error; errors often
+ -- give rise to spurious ambiguous Insts
+
+
+ -- And complain about the ones that don't fall under
+ -- the Haskell rules for disambiguation
-- This group includes both non-existent instances
-- e.g. Num (IO a) and Eq (Int -> Int)
-- and ambiguous dictionaries
-- e.g. Num a
- addTopAmbigErrs bad_guys `thenNF_Tc_`
+ addTopAmbigErrs bad_guys `thenNF_Tc_`
+
+ -- Disambiguate the ones that look feasible
+ mapTc disambigGroup std_oks
+ ) `thenTc` \ binds_ambig ->
+
returnTc (binds `andMonoBinds` andMonoBindList binds_ambig)
where
wanteds = lieToList wanted_lie
- try_me inst = ReduceMe
d1 `cmp_by_tyvar` d2 = get_tv d1 `compare` get_tv d2
(clas, [ty]) -> clas
\end{code}
+If a dictionary constrains a type variable which is
+ * not mentioned in the environment
+ * and not mentioned in the type of the expression
+then it is ambiguous. No further information will arise to instantiate
+the type variable; nor will it be generalised and turned into an extra
+parameter to a function.
+
+It is an error for this to occur, except that Haskell provided for
+certain rules to be applied in the special case of numeric types.
+Specifically, if
+ * at least one of its classes is a numeric class, and
+ * all of its classes are numeric or standard
+then the type variable can be defaulted to the first type in the
+default-type list which is an instance of all the offending classes.
+
+So here is the function which does the work. It takes the ambiguous
+dictionaries and either resolves them (producing bindings) or
+complains. It works by splitting the dictionary list by type
+variable, and using @disambigOne@ to do the real business.
+
@disambigOne@ assumes that its arguments dictionaries constrain all
the same type variable.
try_default (default_ty : default_tys)
= tryTc_ (try_default default_tys) $ -- If default_ty fails, we try
-- default_tys instead
- tcSimplifyCheckThetas [] theta `thenTc` \ _ ->
+ tcSimplifyDefault theta `thenTc` \ _ ->
returnTc default_ty
where
theta = [mkClassPred clas [default_ty] | clas <- classes]
-- Bind the type variable and reduce the context, for real this time
unifyTauTy chosen_default_ty (mkTyVarTy tyvar) `thenTc_`
simpleReduceLoop (text "disambig" <+> ppr dicts)
- try_me dicts `thenTc` \ (frees, binds, ambigs) ->
+ reduceMe dicts `thenTc` \ (frees, binds, ambigs) ->
WARN( not (null frees && null ambigs), ppr frees $$ ppr ambigs )
warnDefault dicts chosen_default_ty `thenTc_`
returnTc binds
returnTc EmptyMonoBinds
where
- try_me inst = ReduceMe -- This reduce should not fail
tyvar = get_tv (head dicts) -- Should be non-empty
classes = map get_clas dicts
\end{code}
instance declarations.
\begin{code}
-tcSimplifyThetas :: ThetaType -- Wanted
- -> TcM ThetaType -- Needed
-
-tcSimplifyThetas wanteds
- = doptsTc Opt_GlasgowExts `thenNF_Tc` \ glaExts ->
- reduceSimple [] wanteds `thenNF_Tc` \ irreds ->
+tcSimplifyDeriv :: [TyVar]
+ -> ThetaType -- Wanted
+ -> TcM ThetaType -- Needed
+
+tcSimplifyDeriv tyvars theta
+ = tcInstTyVars VanillaTv tyvars `thenNF_Tc` \ (tvs, _, tenv) ->
+ -- The main loop may do unification, and that may crash if
+ -- it doesn't see a TcTyVar, so we have to instantiate. Sigh
+ -- ToDo: what if two of them do get unified?
+ newDicts DataDeclOrigin (substTheta tenv theta) `thenNF_Tc` \ wanteds ->
+ simpleReduceLoop doc reduceMe wanteds `thenTc` \ (frees, _, irreds) ->
+ ASSERT( null frees ) -- reduceMe never returns Free
+
+ doptsTc Opt_AllowUndecidableInstances `thenNF_Tc` \ undecidable_ok ->
let
- -- For multi-param Haskell, check that the returned dictionaries
- -- don't have any of the form (C Int Bool) for which
- -- we expect an instance here
- -- For Haskell 98, check that all the constraints are of the form C a,
- -- where a is a type variable
- bad_guys | glaExts = [pred | pred <- irreds,
- isEmptyVarSet (tyVarsOfPred pred)]
- | otherwise = [pred | pred <- irreds,
- not (isTyVarClassPred pred)]
+ tv_set = mkVarSet tvs
+ simpl_theta = map dictPred irreds -- reduceMe squashes all non-dicts
+
+ check_pred pred
+ | isEmptyVarSet pred_tyvars -- Things like (Eq T) should be rejected
+ = addErrTc (noInstErr pred)
+
+ | not undecidable_ok && not (isTyVarClassPred pred)
+ -- Check that the returned dictionaries are all of form (C a b)
+ -- (where a, b are type variables).
+ -- We allow this if we had -fallow-undecidable-instances,
+ -- but note that risks non-termination in the 'deriving' context-inference
+ -- fixpoint loop. It is useful for situations like
+ -- data Min h a = E | M a (h a)
+ -- which gives the instance decl
+ -- instance (Eq a, Eq (h a)) => Eq (Min h a)
+ = addErrTc (noInstErr pred)
+
+ | not (pred_tyvars `subVarSet` tv_set)
+ -- Check for a bizarre corner case, when the derived instance decl should
+ -- have form instance C a b => D (T a) where ...
+ -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
+ -- of problems; in particular, it's hard to compare solutions for
+ -- equality when finding the fixpoint. So I just rule it out for now.
+ = addErrTc (badDerivedPred pred)
+
+ | otherwise
+ = returnNF_Tc ()
+ where
+ pred_tyvars = tyVarsOfPred pred
+
+ rev_env = mkTopTyVarSubst tvs (mkTyVarTys tyvars)
+ -- This reverse-mapping is a Royal Pain,
+ -- but the result should mention TyVars not TcTyVars
in
- if null bad_guys then
- returnTc irreds
- else
- mapNF_Tc addNoInstErr bad_guys `thenNF_Tc_`
- failTc
+
+ mapNF_Tc check_pred simpl_theta `thenNF_Tc_`
+ checkAmbiguity tvs simpl_theta tv_set `thenTc_`
+ returnTc (substTheta rev_env simpl_theta)
+ where
+ doc = ptext SLIT("deriving classes for a data type")
\end{code}
-@tcSimplifyCheckThetas@ just checks class-type constraints, essentially;
+@tcSimplifyDefault@ just checks class-type constraints, essentially;
used with \tr{default} declarations. We are only interested in
whether it worked or not.
\begin{code}
-tcSimplifyCheckThetas :: ThetaType -- Given
- -> ThetaType -- Wanted
- -> TcM ()
-
-tcSimplifyCheckThetas givens wanteds
- = reduceSimple givens wanteds `thenNF_Tc` \ irreds ->
+tcSimplifyDefault :: ThetaType -- Wanted; has no type variables in it
+ -> TcM ()
+
+tcSimplifyDefault theta
+ = newDicts DataDeclOrigin theta `thenNF_Tc` \ wanteds ->
+ simpleReduceLoop doc reduceMe wanteds `thenTc` \ (frees, _, irreds) ->
+ ASSERT( null frees ) -- try_me never returns Free
+ mapNF_Tc (addErrTc . noInstErr) irreds `thenNF_Tc_`
if null irreds then
- returnTc ()
+ returnTc ()
else
- mapNF_Tc addNoInstErr irreds `thenNF_Tc_`
- failTc
-\end{code}
-
-
-\begin{code}
-type AvailsSimple = FiniteMap PredType Bool
- -- True => irreducible
- -- False => given, or can be derived from a given or from an irreducible
-
-reduceSimple :: ThetaType -- Given
- -> ThetaType -- Wanted
- -> NF_TcM ThetaType -- Irreducible
-
-reduceSimple givens wanteds
- = reduce_simple (0,[]) givens_fm wanteds `thenNF_Tc` \ givens_fm' ->
- returnNF_Tc [pred | (pred,True) <- fmToList givens_fm']
+ failTc
where
- givens_fm = foldl addNonIrred emptyFM givens
-
-reduce_simple :: (Int,ThetaType) -- Stack
- -> AvailsSimple
- -> ThetaType
- -> NF_TcM AvailsSimple
-
-reduce_simple (n,stack) avails wanteds
- = go avails wanteds
- where
- go avails [] = returnNF_Tc avails
- go avails (w:ws) = reduce_simple_help (n+1,w:stack) avails w `thenNF_Tc` \ avails' ->
- go avails' ws
-
-reduce_simple_help stack givens wanted
- | wanted `elemFM` givens
- = returnNF_Tc givens
-
- | Just (clas, tys) <- getClassPredTys_maybe wanted
- = lookupSimpleInst clas tys `thenNF_Tc` \ maybe_theta ->
- case maybe_theta of
- Nothing -> returnNF_Tc (addSimpleIrred givens wanted)
- Just theta -> reduce_simple stack (addNonIrred givens wanted) theta
-
- | otherwise
- = returnNF_Tc (addSimpleIrred givens wanted)
-
-addSimpleIrred :: AvailsSimple -> PredType -> AvailsSimple
-addSimpleIrred givens pred
- = addSCs (addToFM givens pred True) pred
-
-addNonIrred :: AvailsSimple -> PredType -> AvailsSimple
-addNonIrred givens pred
- = addSCs (addToFM givens pred False) pred
-
-addSCs givens pred
- | not (isClassPred pred) = givens
- | otherwise = foldl add givens sc_theta
- where
- Just (clas,tys) = getClassPredTys_maybe pred
- (tyvars, sc_theta_tmpl, _, _) = classBigSig clas
- sc_theta = substTheta (mkTopTyVarSubst tyvars tys) sc_theta_tmpl
-
- add givens ct
- = case lookupFM givens ct of
- Nothing -> -- Add it and its superclasses
- addSCs (addToFM givens ct False) ct
-
- Just True -> -- Set its flag to False; superclasses already done
- addToFM givens ct False
-
- Just False -> -- Already done
- givens
-
+ doc = ptext SLIT("default declaration")
\end{code}
pprInstsInFull tidy_dicts]
complainCheck doc givens irreds
- = mapNF_Tc zonkInst given_dicts `thenNF_Tc` \ givens' ->
+ = mapNF_Tc zonkInst given_dicts_and_ips `thenNF_Tc` \ givens' ->
mapNF_Tc (addNoInstanceErrs doc givens') (groupInsts irreds) `thenNF_Tc_`
returnNF_Tc ()
where
- given_dicts = filter isDict givens
+ given_dicts_and_ips = filter (not . isMethod) givens
-- Filter out methods, which are only added to
-- the given set as an optimisation
addNoInstanceErrs what_doc givens dicts
- = tcGetInstEnv `thenNF_Tc` \ inst_env ->
+ = getDOptsTc `thenNF_Tc` \ dflags ->
+ tcGetInstEnv `thenNF_Tc` \ inst_env ->
let
(tidy_env1, tidy_givens) = tidyInsts givens
(tidy_env2, tidy_dicts) = tidyMoreInsts tidy_env1 dicts
ambig_overlap = any ambig_overlap1 dicts
ambig_overlap1 dict
| isClassDict dict
- = case lookupInstEnv inst_env clas tys of
+ = case lookupInstEnv dflags inst_env clas tys of
NoMatch ambig -> ambig
other -> False
| otherwise = False
addInstErrTcM (instLoc (head dicts)) (tidy_env2, doc)
-- Used for the ...Thetas variants; all top level
-addNoInstErr pred
- = addErrTc (ptext SLIT("No instance for") <+> quotes (ppr pred))
+noInstErr pred = ptext SLIT("No instance for") <+> quotes (ppr pred)
+
+badDerivedPred pred
+ = vcat [ptext SLIT("Can't derive instances where the instance context mentions"),
+ ptext SLIT("type variables that are not data type parameters"),
+ nest 2 (ptext SLIT("Offending constraint:") <+> ppr pred)]
reduceDepthErr n stack
= vcat [ptext SLIT("Context reduction stack overflow; size =") <+> int n,
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