Commit 143f4381 authored by chak@cse.unsw.edu.au.'s avatar chak@cse.unsw.edu.au.
Browse files

Adding TcGadt.lhs

- Seperate patch as diff doesn't capture file adds
- Includes the patch:
  Wed Jul 26 08:23:35 EDT 2006  simonpj@microsoft.com
  Add a missing mkSymCoercion in TcGadt.uVar
parent 55923428
%
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
%************************************************************************
%* *
Type refinement for GADTs
%* *
%************************************************************************
\begin{code}
module TcGadt (
Refinement, emptyRefinement, gadtRefine,
refineType, refineResType,
dataConCanMatch,
tcUnifyTys, BindFlag(..)
) where
import HsSyn ( ExprCoFn(..), idCoercion, isIdCoercion )
import Coercion ( Coercion, mkSymCoercion, mkTransCoercion, mkUnsafeCoercion,
mkLeftCoercion, mkRightCoercion,
splitCoercionKind, decomposeCo )
import TcType ( TvSubst(..), TvSubstEnv, substTy, mkTvSubst,
substTyVar, zipTopTvSubst, typeKind,
eqKind, isSubKind, repSplitAppTy_maybe,
tcView
)
import Type ( Type, tyVarsOfType, tyVarsOfTypes )
import TypeRep ( Type(..), PredType(..) )
import DataCon ( DataCon, dataConUnivTyVars, dataConEqSpec )
import Var ( CoVar, TyVar, tyVarKind )
import VarEnv
import VarSet
import ErrUtils ( Message )
import Maybes ( MaybeErr(..), isJust )
import Control.Monad ( foldM )
import Outputable
#include "HsVersions.h"
\end{code}
%************************************************************************
%* *
What a refinement is
%* *
%************************************************************************
\begin{code}
data Refinement = Reft InScopeSet TvSubstEnv CoSubstEnv
type CoSubstEnv = TvSubstEnv -- Maps type variables to *coercions*
-- INVARIANT: in the refinement (tsub, csub)
-- forall a. (csub(a) :: a:=:tsub(a))
instance Outputable Refinement where
ppr (Reft in_scope tv_env co_env)
= ptext SLIT("Refinment") <+>
braces (ppr tv_env $$ ppr co_env)
emptyRefinement :: Refinement
emptyRefinement = Reft emptyInScopeSet emptyVarEnv emptyVarEnv
refineType :: Refinement -> Type -> (ExprCoFn, Type)
-- Apply the refinement to the type.
-- If (refineType r ty) = (co, ty')
-- Then co :: ty:=:ty'
refineType (Reft in_scope tv_env co_env) ty
| not (isEmptyVarEnv tv_env), -- Common case
any (`elemVarEnv` tv_env) (varSetElems (tyVarsOfType ty))
= (ExprCoFn (substTy co_subst ty), substTy tv_subst ty)
| otherwise
= (idCoercion, ty) -- The type doesn't mention any refined type variables
where
tv_subst = mkTvSubst in_scope tv_env
co_subst = mkTvSubst in_scope co_env
refineResType :: Refinement -> Type -> (ExprCoFn, Type)
-- Like refineType, but returns the 'sym' coercion
-- If (refineResType r ty) = (co, ty')
-- Then co :: ty':=:ty
refineResType reft ty
= case refineType reft ty of
(ExprCoFn co, ty1) -> (ExprCoFn (mkSymCoercion co), ty1)
(id_co, ty1) -> ASSERT( isIdCoercion id_co )
(idCoercion, ty1)
\end{code}
%************************************************************************
%* *
Generating a type refinement
%* *
%************************************************************************
\begin{code}
gadtRefine :: Refinement
-> [TyVar] -- Bind these by preference
-> [CoVar]
-> MaybeErr Message Refinement
\end{code}
(gadtRefine cvs) takes a list of coercion variables, and returns a
list of coercions, obtained by unifying the types equated by the
incoming coercions. The returned coercions all have kinds of form
(a:=:ty), where a is a rigid type variable.
Example:
gadtRefine [c :: (a,Int):=:(Bool,b)]
= [ right (left c) :: a:=:Bool,
sym (right c) :: b:=:Int ]
That is, given evidence 'c' that (a,Int)=(Bool,b), it returns derived
evidence in easy-to-use form. In particular, given any e::ty, we know
that:
e `cast` ty[right (left c)/a, sym (right c)/b]
:: ty [Bool/a, Int/b]
Two refinements:
- It can fail, if the coercion is unsatisfiable.
- It's biased, by being given a set of type variables to bind
when there is a choice. Example:
MkT :: forall a. a -> T [a]
f :: forall b. T [b] -> b
f x = let v = case x of { MkT y -> y }
in ...
Here we want to bind [a->b], not the other way round, because
in this example the return type is wobbly, and we want the
program to typecheck
-- E.g. (a, Bool, right (left c))
-- INVARIANT: in the triple (tv, ty, co), we have (co :: tv:=:ty)
-- The result is idempotent: the
\begin{code}
gadtRefine (Reft in_scope tv_env1 co_env1)
ex_tvs co_vars
-- Precondition: fvs( co_vars ) # tv_env1
-- That is, the kinds of the co_vars are a
-- fixed point of the incoming refinement
= initUM (tryToBind tv_set) $
do { -- Run the unifier, starting with an empty env
; env2 <- foldM do_one emptyInternalReft co_vars
-- Find the fixed point of the resulting
-- non-idempotent substitution
; let tv_env2 = tv_env1 `plusVarEnv` mapVarEnv snd env2
co_env2 = co_env1 `plusVarEnv` mapVarEnv fst env2
; return (Reft in_scope' (fixTvSubstEnv in_scope' tv_env2)
(fixTvSubstEnv in_scope' co_env2)) }
where
tv_set = mkVarSet ex_tvs
in_scope' = foldr extend in_scope co_vars
extend co_var in_scope
= extendInScopeSetSet (extendInScopeSet in_scope co_var)
(tyVarsOfType (tyVarKind co_var))
do_one reft co_var = unify reft (TyVarTy co_var) ty1 ty2
where
(ty1,ty2) = splitCoercionKind (tyVarKind co_var)
\end{code}
%************************************************************************
%* *
Unification
%* *
%************************************************************************
\begin{code}
tcUnifyTys :: (TyVar -> BindFlag)
-> [Type] -> [Type]
-> Maybe TvSubst -- A regular one-shot substitution
-- The two types may have common type variables, and indeed do so in the
-- second call to tcUnifyTys in FunDeps.checkClsFD
--
-- We implement tcUnifyTys using the evidence-generating 'unify' function
-- in this module, even though we don't need to generate any evidence.
-- This is simply to avoid replicating all all the code for unify
tcUnifyTys bind_fn tys1 tys2
= maybeErrToMaybe $ initUM bind_fn $
do { reft <- unifyList emptyInternalReft cos tys1 tys2
-- Find the fixed point of the resulting non-idempotent substitution
; let in_scope = mkInScopeSet (tvs1 `unionVarSet` tvs2)
tv_env = fixTvSubstEnv in_scope (mapVarEnv snd reft)
; return (mkTvSubst in_scope tv_env) }
where
tvs1 = tyVarsOfTypes tys1
tvs2 = tyVarsOfTypes tys2
cos = zipWith mkUnsafeCoercion tys1 tys2
----------------------------
fixTvSubstEnv :: InScopeSet -> TvSubstEnv -> TvSubstEnv
-- Find the fixed point of a TvSubstEnv
-- (assuming it has no loops!)
fixTvSubstEnv in_scope env
= fixpt
where
fixpt = mapVarEnv (substTy (mkTvSubst in_scope fixpt)) env
----------------------------
dataConCanMatch :: DataCon -> [Type] -> Bool
-- Returns True iff the data con can match a scrutinee of type (T tys)
-- where T is the type constructor for the data con
--
-- Instantiate the equations and try to unify them
dataConCanMatch con tys
= isJust (tcUnifyTys (\tv -> BindMe)
(map (substTyVar subst . fst) eq_spec)
(map snd eq_spec))
where
dc_tvs = dataConUnivTyVars con
eq_spec = dataConEqSpec con
subst = zipTopTvSubst dc_tvs tys
----------------------------
tryToBind :: TyVarSet -> TyVar -> BindFlag
tryToBind tv_set tv | tv `elemVarSet` tv_set = BindMe
| otherwise = AvoidMe
\end{code}
%************************************************************************
%* *
The workhorse
%* *
%************************************************************************
\begin{code}
type InternalReft = TyVarEnv (Coercion, Type)
-- INVARIANT: a->(co,ty) then co :: (a:=:ty)
-- Not necessarily idemopotent
emptyInternalReft :: InternalReft
emptyInternalReft = emptyVarEnv
unify :: InternalReft -- An existing substitution to extend
-> Coercion -- Witness of their equality
-> Type -> Type -- Types to be unified, and witness of their equality
-> UM InternalReft -- Just the extended substitution,
-- Nothing if unification failed
-- We do not require the incoming substitution to be idempotent,
-- nor guarantee that the outgoing one is. That's fixed up by
-- the wrappers.
-- PRE-CONDITION: in the call (unify r co ty1 ty2), we know that
-- co :: (ty1:=:ty2)
-- Respects newtypes, PredTypes
unify subst co ty1 ty2 = -- pprTrace "unify" (ppr subst <+> pprParendType ty1 <+> pprParendType ty2) $
unify_ subst co ty1 ty2
-- in unify_, any NewTcApps/Preds should be taken at face value
unify_ subst co (TyVarTy tv1) ty2 = uVar False subst co tv1 ty2
unify_ subst co ty1 (TyVarTy tv2) = uVar True subst co tv2 ty1
unify_ subst co ty1 ty2 | Just ty1' <- tcView ty1 = unify subst co ty1' ty2
unify_ subst co ty1 ty2 | Just ty2' <- tcView ty2 = unify subst co ty1 ty2'
unify_ subst co (PredTy p1) (PredTy p2) = unify_pred subst co p1 p2
unify_ subst co t1@(TyConApp tyc1 tys1) t2@(TyConApp tyc2 tys2)
| tyc1 == tyc2 = unify_tys subst co tys1 tys2
unify_ subst co (FunTy ty1a ty1b) (FunTy ty2a ty2b)
= do { let [co1,co2] = decomposeCo 2 co
; subst' <- unify subst co1 ty1a ty2a
; unify subst' co2 ty1b ty2b }
-- 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
unify_ subst co (AppTy ty1a ty1b) ty2
| Just (ty2a, ty2b) <- repSplitAppTy_maybe ty2
= do { subst' <- unify subst (mkLeftCoercion co) ty1a ty2a
; unify subst' (mkRightCoercion co) ty1b ty2b }
unify_ subst co ty1 (AppTy ty2a ty2b)
| Just (ty1a, ty1b) <- repSplitAppTy_maybe ty1
= do { subst' <- unify subst (mkLeftCoercion co) ty1a ty2a
; unify subst' (mkRightCoercion co) ty1b ty2b }
unify_ subst co ty1 ty2 = failWith (misMatch ty1 ty2)
-- ForAlls??
------------------------------
unify_pred subst co (ClassP c1 tys1) (ClassP c2 tys2)
| c1 == c2 = unify_tys subst co tys1 tys2
unify_pred subst co (IParam n1 t1) (IParam n2 t2)
| n1 == n2 = unify subst co t1 t2
unify_pred subst co p1 p2 = failWith (misMatch (PredTy p1) (PredTy p2))
------------------------------
unify_tys :: InternalReft -> Coercion -> [Type] -> [Type] -> UM InternalReft
unify_tys subst co xs ys
= unifyList subst (decomposeCo (length xs) co) xs ys
unifyList :: InternalReft -> [Coercion] -> [Type] -> [Type] -> UM InternalReft
unifyList subst orig_cos orig_xs orig_ys
= go subst orig_cos orig_xs orig_ys
where
go subst _ [] [] = return subst
go subst (co:cos) (x:xs) (y:ys) = do { subst' <- unify subst co x y
; go subst' cos xs ys }
go subst _ _ _ = failWith (lengthMisMatch orig_xs orig_ys)
---------------------------------
uVar :: Bool -- Swapped
-> InternalReft -- An existing substitution to extend
-> Coercion
-> TyVar -- Type variable to be unified
-> Type -- with this type
-> UM InternalReft
-- PRE-CONDITION: in the call (uVar swap r co tv1 ty), we know that
-- if swap=False co :: (tv1:=:ty)
-- if swap=True co :: (ty:=:tv1)
uVar swap subst co tv1 ty
= -- Check to see whether tv1 is refined by the substitution
case (lookupVarEnv subst tv1) of
-- Yes, call back into unify'
Just (co',ty') -- co' :: (tv1:=:ty')
| swap -- co :: (ty:=:tv1)
-> unify subst (mkTransCoercion co co') ty ty'
| otherwise -- co :: (tv1:=:ty)
-> unify subst (mkTransCoercion (mkSymCoercion co') co) ty' ty
-- No, continue
Nothing -> uUnrefined subst (if swap then mkSymCoercion co else co)
tv1 ty ty
uUnrefined :: InternalReft -- An existing substitution to extend
-> Coercion
-> TyVar -- Type variable to be unified
-> Type -- with this type
-> Type -- (de-noted version)
-> UM InternalReft
-- We know that tv1 isn't refined
-- PRE-CONDITION: in the call (uUnrefined r co tv ty ty'), we know that
-- co :: tv:=:ty
uUnrefined subst co tv1 ty2 ty2'
| Just ty2'' <- tcView ty2'
= uUnrefined subst co tv1 ty2 ty2'' -- Unwrap synonyms
-- This is essential, in case we have
-- type Foo a = a
-- and then unify a :=: Foo a
uUnrefined subst co tv1 ty2 (TyVarTy tv2)
| tv1 == tv2 -- Same type variable
= return subst
-- Check to see whether tv2 is refined
| Just (co',ty') <- lookupVarEnv subst tv2
= uUnrefined subst (mkTransCoercion co co') tv1 ty' ty'
-- So both are unrefined; next, see if the kinds force the direction
| eqKind k1 k2 -- Can update either; so check the bind-flags
= do { b1 <- tvBindFlag tv1
; b2 <- tvBindFlag tv2
; case (b1,b2) of
(BindMe, _) -> bind tv1 ty2
(AvoidMe, BindMe) -> bind tv2 ty1
(AvoidMe, _) -> bind tv1 ty2
(WildCard, WildCard) -> return subst
(WildCard, Skolem) -> return subst
(WildCard, _) -> bind tv2 ty1
(Skolem, WildCard) -> return subst
(Skolem, Skolem) -> failWith (misMatch ty1 ty2)
(Skolem, _) -> bind tv2 ty1
}
| k1 `isSubKind` k2 = bindTv subst co tv2 ty1 -- Must update tv2
| k2 `isSubKind` k1 = bindTv subst co tv1 ty2 -- Must update tv1
| otherwise = failWith (kindMisMatch tv1 ty2)
where
ty1 = TyVarTy tv1
k1 = tyVarKind tv1
k2 = tyVarKind tv2
bind tv ty = return (extendVarEnv subst tv (co,ty))
uUnrefined subst co tv1 ty2 ty2' -- ty2 is not a type variable
| tv1 `elemVarSet` substTvSet subst (tyVarsOfType ty2')
= failWith (occursCheck tv1 ty2) -- Occurs check
| not (k2 `isSubKind` k1)
= failWith (kindMisMatch tv1 ty2) -- Kind check
| otherwise
= bindTv subst co tv1 ty2 -- Bind tyvar to the synonym if poss
where
k1 = tyVarKind tv1
k2 = typeKind ty2'
substTvSet :: InternalReft -> TyVarSet -> TyVarSet
-- Apply the non-idempotent substitution to a set of type variables,
-- remembering that the substitution isn't necessarily idempotent
substTvSet subst tvs
= foldVarSet (unionVarSet . get) emptyVarSet tvs
where
get tv = case lookupVarEnv subst tv of
Nothing -> unitVarSet tv
Just (_,ty) -> substTvSet subst (tyVarsOfType ty)
bindTv subst co tv ty -- ty is not a type variable
= do { b <- tvBindFlag tv
; case b of
Skolem -> failWith (misMatch (TyVarTy tv) ty)
WildCard -> return subst
other -> return (extendVarEnv subst tv (co,ty))
}
\end{code}
%************************************************************************
%* *
Unification monad
%* *
%************************************************************************
\begin{code}
data BindFlag
= BindMe -- A regular type variable
| AvoidMe -- Like BindMe but, given the choice, avoid binding it
| Skolem -- This type variable is a skolem constant
-- Don't bind it; it only matches itself
| WildCard -- This type variable matches anything,
-- and does not affect the substitution
newtype UM a = UM { unUM :: (TyVar -> BindFlag)
-> MaybeErr Message a }
instance Monad UM where
return a = UM (\tvs -> Succeeded a)
fail s = UM (\tvs -> Failed (text s))
m >>= k = UM (\tvs -> case unUM m tvs of
Failed err -> Failed err
Succeeded v -> unUM (k v) tvs)
initUM :: (TyVar -> BindFlag) -> UM a -> MaybeErr Message a
initUM badtvs um = unUM um badtvs
tvBindFlag :: TyVar -> UM BindFlag
tvBindFlag tv = UM (\tv_fn -> Succeeded (tv_fn tv))
failWith :: Message -> UM a
failWith msg = UM (\tv_fn -> Failed msg)
maybeErrToMaybe :: MaybeErr fail succ -> Maybe succ
maybeErrToMaybe (Succeeded a) = Just a
maybeErrToMaybe (Failed m) = Nothing
\end{code}
%************************************************************************
%* *
Error reporting
We go to a lot more trouble to tidy the types
in TcUnify. Maybe we'll end up having to do that
here too, but I'll leave it for now.
%* *
%************************************************************************
\begin{code}
misMatch t1 t2
= ptext SLIT("Can't match types") <+> quotes (ppr t1) <+>
ptext SLIT("and") <+> quotes (ppr t2)
lengthMisMatch tys1 tys2
= sep [ptext SLIT("Can't match unequal length lists"),
nest 2 (ppr tys1), nest 2 (ppr tys2) ]
kindMisMatch tv1 t2
= vcat [ptext SLIT("Can't match kinds") <+> quotes (ppr (tyVarKind tv1)) <+>
ptext SLIT("and") <+> quotes (ppr (typeKind t2)),
ptext SLIT("when matching") <+> quotes (ppr tv1) <+>
ptext SLIT("with") <+> quotes (ppr t2)]
occursCheck tv ty
= hang (ptext SLIT("Can't construct the infinite type"))
2 (ppr tv <+> equals <+> ppr ty)
\end{code}
\ No newline at end of file
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