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% (c) The University of Glasgow 2006
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-- the "tc" prefix indicates that matching always
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-- respects newtypes (rather than looking through them)
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tcMatchTy, tcMatchTys, tcMatchTyX,
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ruleMatchTyX, tcMatchPreds, MatchEnv(..),
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-- Side-effect free unification
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tcUnifyTys, BindFlag(..)
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#include "HsVersions.h"
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%************************************************************************
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%************************************************************************
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Matching is much tricker than you might think.
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1. The substitution we generate binds the *template type variables*
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which are given to us explicitly.
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2. We want to match in the presence of foralls;
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e.g (forall a. t1) ~ (forall b. t2)
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That is what the RnEnv2 is for; it does the alpha-renaming
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that makes it as if a and b were the same variable.
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Initialising the RnEnv2, so that it can generate a fresh
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binder when necessary, entails knowing the free variables of
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3. We must be careful not to bind a template type variable to a
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locally bound variable. E.g.
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(forall a. x) ~ (forall b. b)
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where x is the template type variable. Then we do not want to
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bind x to a/b! This is a kind of occurs check.
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The necessary locals accumulate in the RnEnv2.
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= ME { me_tmpls :: VarSet -- Template tyvars
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, me_env :: RnEnv2 -- Renaming envt for nested foralls
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} -- In-scope set includes template tyvars
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tcMatchTy :: TyVarSet -- Template tyvars
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-> Maybe TvSubst -- One-shot; in principle the template
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-- variables could be free in the target
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tcMatchTy tmpls ty1 ty2
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= case match menv emptyTvSubstEnv ty1 ty2 of
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Just subst_env -> Just (TvSubst in_scope subst_env)
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menv = ME { me_tmpls = tmpls, me_env = mkRnEnv2 in_scope }
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in_scope = mkInScopeSet (tmpls `unionVarSet` tyVarsOfType ty2)
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-- We're assuming that all the interesting
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-- tyvars in tys1 are in tmpls
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tcMatchTys :: TyVarSet -- Template tyvars
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-> Maybe TvSubst -- One-shot; in principle the template
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-- variables could be free in the target
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tcMatchTys tmpls tys1 tys2
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= case match_tys menv emptyTvSubstEnv tys1 tys2 of
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Just subst_env -> Just (TvSubst in_scope subst_env)
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menv = ME { me_tmpls = tmpls, me_env = mkRnEnv2 in_scope }
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in_scope = mkInScopeSet (tmpls `unionVarSet` tyVarsOfTypes tys2)
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-- We're assuming that all the interesting
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-- tyvars in tys1 are in tmpls
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-- This is similar, but extends a substitution
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tcMatchTyX :: TyVarSet -- Template tyvars
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-> TvSubst -- Substitution to extend
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tcMatchTyX tmpls (TvSubst in_scope subst_env) ty1 ty2
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= case match menv subst_env ty1 ty2 of
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Just subst_env -> Just (TvSubst in_scope subst_env)
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menv = ME {me_tmpls = tmpls, me_env = mkRnEnv2 in_scope}
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:: [TyVar] -- Bind these
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-> [PredType] -> [PredType]
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tcMatchPreds tmpls ps1 ps2
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= match_list (match_pred menv) emptyTvSubstEnv ps1 ps2
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menv = ME { me_tmpls = mkVarSet tmpls, me_env = mkRnEnv2 in_scope_tyvars }
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in_scope_tyvars = mkInScopeSet (tyVarsOfTheta ps1 `unionVarSet` tyVarsOfTheta ps2)
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-- This one is called from the expression matcher, which already has a MatchEnv in hand
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ruleMatchTyX :: MatchEnv
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-> TvSubstEnv -- Substitution to extend
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ruleMatchTyX menv subst ty1 ty2 = match menv subst ty1 ty2 -- Rename for export
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Now the internals of matching
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match :: MatchEnv -- For the most part this is pushed downwards
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-> TvSubstEnv -- Substitution so far:
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-- Domain is subset of template tyvars
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-- Free vars of range is subset of
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-- in-scope set of the RnEnv2
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-> Type -> Type -- Template and target respectively
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-- This matcher works on core types; that is, it ignores PredTypes
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-- Watch out if newtypes become transparent agin!
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-- this matcher must respect newtypes
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match menv subst ty1 ty2 | Just ty1' <- coreView ty1 = match menv subst ty1' ty2
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| Just ty2' <- coreView ty2 = match menv subst ty1 ty2'
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match menv subst (TyVarTy tv1) ty2
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| Just ty1' <- lookupVarEnv subst tv1' -- tv1' is already bound
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= if tcEqTypeX (nukeRnEnvL rn_env) ty1' ty2
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-- ty1 has no locally-bound variables, hence nukeRnEnvL
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-- Note tcEqType...we are doing source-type matching here
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else Nothing -- ty2 doesn't match
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| tv1' `elemVarSet` me_tmpls menv
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= if any (inRnEnvR rn_env) (varSetElems (tyVarsOfType ty2))
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then Nothing -- Occurs check
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else do { subst1 <- match_kind menv subst tv1 ty2
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-- Note [Matching kinds]
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; return (extendVarEnv subst1 tv1' ty2) }
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| otherwise -- tv1 is not a template tyvar
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TyVarTy tv2 | tv1' == rnOccR rn_env tv2 -> Just subst
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tv1' = rnOccL rn_env tv1
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match menv subst (ForAllTy tv1 ty1) (ForAllTy tv2 ty2)
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= match menv' subst ty1 ty2
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where -- Use the magic of rnBndr2 to go under the binders
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menv' = menv { me_env = rnBndr2 (me_env menv) tv1 tv2 }
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match menv subst (PredTy p1) (PredTy p2)
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= match_pred menv subst p1 p2
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match menv subst (TyConApp tc1 tys1) (TyConApp tc2 tys2)
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| tc1 == tc2 = match_tys menv subst tys1 tys2
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match menv subst (FunTy ty1a ty1b) (FunTy ty2a ty2b)
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= do { subst' <- match menv subst ty1a ty2a
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; match menv subst' ty1b ty2b }
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match menv subst (AppTy ty1a ty1b) ty2
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| Just (ty2a, ty2b) <- repSplitAppTy_maybe ty2
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-- 'repSplit' used because the tcView stuff is done above
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= do { subst' <- match menv subst ty1a ty2a
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; match menv subst' ty1b ty2b }
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match_kind :: MatchEnv -> TvSubstEnv -> TyVar -> Type -> Maybe TvSubstEnv
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-- Match the kind of the template tyvar with the kind of Type
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-- Note [Matching kinds]
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match_kind menv subst tv ty
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| isCoVar tv = do { let (ty1,ty2) = coVarKind tv
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(ty3,ty4) = coercionKind ty
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; subst1 <- match menv subst ty1 ty3
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; match menv subst1 ty2 ty4 }
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| otherwise = if typeKind ty `isSubKind` tyVarKind tv
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-- Note [Matching kinds]
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-- ~~~~~~~~~~~~~~~~~~~~~
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-- For ordinary type variables, we don't want (m a) to match (n b)
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-- if say (a::*) and (b::*->*). This is just a yes/no issue.
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-- For coercion kinds matters are more complicated. If we have a
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-- coercion template variable co::a~[b], where a,b are presumably also
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-- template type variables, then we must match co's kind against the
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-- kind of the actual argument, so as to give bindings to a,b.
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-- In fact I have no example in mind that *requires* this kind-matching
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-- to instantiate template type variables, but it seems like the right
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-- thing to do. C.f. Note [Matching variable types] in Rules.lhs
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match_tys :: MatchEnv -> TvSubstEnv -> [Type] -> [Type] -> Maybe TvSubstEnv
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match_tys menv subst tys1 tys2 = match_list (match menv) subst tys1 tys2
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match_list :: (TvSubstEnv -> a -> a -> Maybe TvSubstEnv)
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-> TvSubstEnv -> [a] -> [a] -> Maybe TvSubstEnv
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match_list _ subst [] [] = Just subst
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match_list fn subst (ty1:tys1) (ty2:tys2) = do { subst' <- fn subst ty1 ty2
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; match_list fn subst' tys1 tys2 }
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match_list _ _ _ _ = Nothing
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match_pred :: MatchEnv -> TvSubstEnv -> PredType -> PredType -> Maybe TvSubstEnv
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match_pred menv subst (ClassP c1 tys1) (ClassP c2 tys2)
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| c1 == c2 = match_tys menv subst tys1 tys2
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match_pred menv subst (IParam n1 t1) (IParam n2 t2)
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| n1 == n2 = match menv subst t1 t2
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match_pred _ _ _ _ = Nothing
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%************************************************************************
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%************************************************************************
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Note [Pruning dead case alternatives]
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Consider data T a where
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type instance F Bool = Int
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Now consider case x of { T1 -> e1; T2 -> e2 }
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The question before the house is this: if I know something about the type
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of x, can I prune away the T1 alternative?
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Suppose x::T Char. It's impossible to construct a (T Char) using T1,
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Answer = YES (clearly)
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Suppose x::T (F a), where 'a' is in scope. Then 'a' might be instantiated
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to 'Bool', in which case x::T Int, so
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ANSWER = NO (clearly)
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Suppose x::T X. Then *in Haskell* it's impossible to construct a (non-bottom)
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value of type (T X) using T1. But *in FC* it's quite possible. The newtype
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So (T CoX) :: T X ~ T Int; hence (T1 `cast` sym (T CoX)) is a non-bottom value
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of type (T X) constructed with T1. Hence
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ANSWER = NO (surprisingly)
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Furthermore, this can even happen; see Trac #1251. GHC's newtype-deriving
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mechanism uses a cast, just as above, to move from one dictionary to another,
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in effect giving the programmer access to CoX.
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Finally, suppose x::T Y. Then *even in FC* we can't construct a
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non-bottom value of type (T Y) using T1. That's because we can get
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from Y to Char, but not to Int.
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Here's a related question. data Eq a b where EQ :: Eq a a
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case x of { EQ -> ... }
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Suppose x::Eq Int Char. Is the alternative dead? Clearly yes.
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What about x::Eq Int a, in a context where we have evidence that a~Char.
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Then again the alternative is dead.
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We are really doing a test for unsatisfiability of the type
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constraints implied by the match. And that is clearly, in general, a
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However, since we are simply dropping dead code, a conservative test
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suffices. There is a continuum of tests, ranging from easy to hard, that
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drop more and more dead code.
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For now we implement a very simple test: type variables match
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anything, type functions (incl newtypes) match anything, and only
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distinct data types fail to match. We can elaborate later.
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dataConCannotMatch :: [Type] -> DataCon -> Bool
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-- Returns True iff the data con *definitely cannot* match a
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-- scrutinee of type (T tys)
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-- where T is the type constructor for the data con
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dataConCannotMatch tys con
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| null eq_spec = False -- Common
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| all isTyVarTy tys = False -- Also common
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= cant_match_s (map (substTyVar subst . fst) eq_spec)
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dc_tvs = dataConUnivTyVars con
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eq_spec = dataConEqSpec con
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subst = zipTopTvSubst dc_tvs tys
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cant_match_s :: [Type] -> [Type] -> Bool
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cant_match_s tys1 tys2 = ASSERT( equalLength tys1 tys2 )
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or (zipWith cant_match tys1 tys2)
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cant_match :: Type -> Type -> Bool
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| Just t1' <- coreView t1 = cant_match t1' t2
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| Just t2' <- coreView t2 = cant_match t1 t2'
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cant_match (FunTy a1 r1) (FunTy a2 r2)
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= cant_match a1 a2 || cant_match r1 r2
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cant_match (TyConApp tc1 tys1) (TyConApp tc2 tys2)
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| isDataTyCon tc1 && isDataTyCon tc2
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= tc1 /= tc2 || cant_match_s tys1 tys2
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cant_match (FunTy {}) (TyConApp tc _) = isDataTyCon tc
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cant_match (TyConApp tc _) (FunTy {}) = isDataTyCon tc
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-- tc can't be FunTyCon by invariant
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cant_match (AppTy f1 a1) ty2
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| Just (f2, a2) <- repSplitAppTy_maybe ty2
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= cant_match f1 f2 || cant_match a1 a2
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cant_match ty1 (AppTy f2 a2)
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| Just (f1, a1) <- repSplitAppTy_maybe ty1
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= cant_match f1 f2 || cant_match a1 a2
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cant_match _ _ = False -- Safe!
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-- Things we could add;
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-- look through newtypes
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-- take account of tyvar bindings (EQ example above)
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%************************************************************************
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%************************************************************************
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tcUnifyTys :: (TyVar -> BindFlag)
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-> Maybe TvSubst -- A regular one-shot (idempotent) substitution
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-- The two types may have common type variables, and indeed do so in the
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-- second call to tcUnifyTys in FunDeps.checkClsFD
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tcUnifyTys bind_fn tys1 tys2
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= maybeErrToMaybe $ initUM bind_fn $
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do { subst <- unifyList emptyTvSubstEnv tys1 tys2
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-- Find the fixed point of the resulting non-idempotent substitution
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; let in_scope = mkInScopeSet (tvs1 `unionVarSet` tvs2)
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tv_env = fixTvSubstEnv in_scope subst
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; return (mkTvSubst in_scope tv_env) }
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tvs1 = tyVarsOfTypes tys1
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tvs2 = tyVarsOfTypes tys2
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----------------------------
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-- XXX Can we do this more nicely, by exploiting laziness?
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-- Or avoid needing it in the first place?
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fixTvSubstEnv :: InScopeSet -> TvSubstEnv -> TvSubstEnv
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fixTvSubstEnv in_scope env = f env
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f e = let e' = mapUFM (substTy (mkTvSubst in_scope e)) e
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in if and $ eltsUFM $ intersectUFM_C tcEqType e e'
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%************************************************************************
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%************************************************************************
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unify :: TvSubstEnv -- An existing substitution to extend
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-> Type -> Type -- Types to be unified, and witness of their equality
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-> UM TvSubstEnv -- Just the extended substitution,
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-- Nothing if unification failed
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-- We do not require the incoming substitution to be idempotent,
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-- nor guarantee that the outgoing one is. That's fixed up by
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-- Respects newtypes, PredTypes
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-- in unify, any NewTcApps/Preds should be taken at face value
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unify subst (TyVarTy tv1) ty2 = uVar subst tv1 ty2
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unify subst ty1 (TyVarTy tv2) = uVar subst tv2 ty1
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unify subst ty1 ty2 | Just ty1' <- tcView ty1 = unify subst ty1' ty2
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unify subst ty1 ty2 | Just ty2' <- tcView ty2 = unify subst ty1 ty2'
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unify subst (PredTy p1) (PredTy p2) = unify_pred subst p1 p2
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unify subst (TyConApp tyc1 tys1) (TyConApp tyc2 tys2)
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| tyc1 == tyc2 = unify_tys subst tys1 tys2
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unify subst (FunTy ty1a ty1b) (FunTy ty2a ty2b)
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= do { subst' <- unify subst ty1a ty2a
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; unify subst' ty1b ty2b }
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-- Applications need a bit of care!
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-- They can match FunTy and TyConApp, so use splitAppTy_maybe
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-- NB: we've already dealt with type variables and Notes,
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-- so if one type is an App the other one jolly well better be too
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unify subst (AppTy ty1a ty1b) ty2
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| Just (ty2a, ty2b) <- repSplitAppTy_maybe ty2
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= do { subst' <- unify subst ty1a ty2a
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; unify subst' ty1b ty2b }
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unify subst ty1 (AppTy ty2a ty2b)
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| Just (ty1a, ty1b) <- repSplitAppTy_maybe ty1
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= do { subst' <- unify subst ty1a ty2a
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; unify subst' ty1b ty2b }
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unify _ ty1 ty2 = failWith (misMatch ty1 ty2)
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------------------------------
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unify_pred :: TvSubstEnv -> PredType -> PredType -> UM TvSubstEnv
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unify_pred subst (ClassP c1 tys1) (ClassP c2 tys2)
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| c1 == c2 = unify_tys subst tys1 tys2
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unify_pred subst (IParam n1 t1) (IParam n2 t2)
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| n1 == n2 = unify subst t1 t2
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unify_pred _ p1 p2 = failWith (misMatch (PredTy p1) (PredTy p2))
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------------------------------
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unify_tys :: TvSubstEnv -> [Type] -> [Type] -> UM TvSubstEnv
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unify_tys subst xs ys = unifyList subst xs ys
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unifyList :: TvSubstEnv -> [Type] -> [Type] -> UM TvSubstEnv
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unifyList subst orig_xs orig_ys
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= go subst orig_xs orig_ys
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go subst [] [] = return subst
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go subst (x:xs) (y:ys) = do { subst' <- unify subst x y
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go _ _ _ = failWith (lengthMisMatch orig_xs orig_ys)
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---------------------------------
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uVar :: TvSubstEnv -- An existing substitution to extend
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-> TyVar -- Type variable to be unified
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-> Type -- with this type
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-- PRE-CONDITION: in the call (uVar swap r tv1 ty), we know that
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-- if swap=False (tv1~ty)
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-- if swap=True (ty~tv1)
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= -- Check to see whether tv1 is refined by the substitution
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case (lookupVarEnv subst tv1) of
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Just ty' -> unify subst ty' ty -- Yes, call back into unify'
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Nothing -> uUnrefined subst -- No, continue
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uUnrefined :: TvSubstEnv -- An existing substitution to extend
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-> TyVar -- Type variable to be unified
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-> Type -- with this type
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-> Type -- (version w/ expanded synonyms)
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-- We know that tv1 isn't refined
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uUnrefined subst tv1 ty2 ty2'
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| Just ty2'' <- tcView ty2'
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= uUnrefined subst tv1 ty2 ty2'' -- Unwrap synonyms
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-- This is essential, in case we have
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-- and then unify a ~ Foo a
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uUnrefined subst tv1 ty2 (TyVarTy tv2)
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| tv1 == tv2 -- Same type variable
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-- Check to see whether tv2 is refined
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| Just ty' <- lookupVarEnv subst tv2
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= uUnrefined subst tv1 ty' ty'
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-- So both are unrefined; next, see if the kinds force the direction
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| eqKind k1 k2 -- Can update either; so check the bind-flags
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= do { b1 <- tvBindFlag tv1
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; b2 <- tvBindFlag tv2
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(BindMe, _) -> bind tv1 ty2
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(Skolem, Skolem) -> failWith (misMatch ty1 ty2)
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(Skolem, _) -> bind tv2 ty1
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| k1 `isSubKind` k2 = bindTv subst tv2 ty1 -- Must update tv2
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| k2 `isSubKind` k1 = bindTv subst tv1 ty2 -- Must update tv1
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| otherwise = failWith (kindMisMatch tv1 ty2)
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bind tv ty = return $ extendVarEnv subst tv ty
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uUnrefined subst tv1 ty2 ty2' -- ty2 is not a type variable
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| tv1 `elemVarSet` substTvSet subst (tyVarsOfType ty2')
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= failWith (occursCheck tv1 ty2) -- Occurs check
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| not (k2 `isSubKind` k1)
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= failWith (kindMisMatch tv1 ty2) -- Kind check
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= bindTv subst tv1 ty2 -- Bind tyvar to the synonym if poss
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substTvSet :: TvSubstEnv -> TyVarSet -> TyVarSet
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-- Apply the non-idempotent substitution to a set of type variables,
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-- remembering that the substitution isn't necessarily idempotent
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= foldVarSet (unionVarSet . get) emptyVarSet tvs
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get tv = case lookupVarEnv subst tv of
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Nothing -> unitVarSet tv
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Just ty -> substTvSet subst (tyVarsOfType ty)
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bindTv :: TvSubstEnv -> TyVar -> Type -> UM TvSubstEnv
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bindTv subst tv ty -- ty is not a type variable
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= do { b <- tvBindFlag tv
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Skolem -> failWith (misMatch (TyVarTy tv) ty)
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BindMe -> return $ extendVarEnv subst tv ty
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%************************************************************************
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%************************************************************************
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= BindMe -- A regular type variable
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| Skolem -- This type variable is a skolem constant
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-- Don't bind it; it only matches itself
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%************************************************************************
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%************************************************************************
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newtype UM a = UM { unUM :: (TyVar -> BindFlag)
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-> MaybeErr Message a }
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instance Monad UM where
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return a = UM (\_tvs -> Succeeded a)
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fail s = UM (\_tvs -> Failed (text s))
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m >>= k = UM (\tvs -> case unUM m tvs of
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Failed err -> Failed err
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Succeeded v -> unUM (k v) tvs)
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initUM :: (TyVar -> BindFlag) -> UM a -> MaybeErr Message a
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initUM badtvs um = unUM um badtvs
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tvBindFlag :: TyVar -> UM BindFlag
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tvBindFlag tv = UM (\tv_fn -> Succeeded (tv_fn tv))
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failWith :: Message -> UM a
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failWith msg = UM (\_tv_fn -> Failed msg)
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maybeErrToMaybe :: MaybeErr fail succ -> Maybe succ
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maybeErrToMaybe (Succeeded a) = Just a
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maybeErrToMaybe (Failed _) = Nothing
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%************************************************************************
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We go to a lot more trouble to tidy the types
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in TcUnify. Maybe we'll end up having to do that
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here too, but I'll leave it for now.
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%************************************************************************
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misMatch :: Type -> Type -> SDoc
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= ptext (sLit "Can't match types") <+> quotes (ppr t1) <+>
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ptext (sLit "and") <+> quotes (ppr t2)
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lengthMisMatch :: [Type] -> [Type] -> SDoc
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lengthMisMatch tys1 tys2
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= sep [ptext (sLit "Can't match unequal length lists"),
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nest 2 (ppr tys1), nest 2 (ppr tys2) ]
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kindMisMatch :: TyVar -> Type -> SDoc
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= vcat [ptext (sLit "Can't match kinds") <+> quotes (ppr (tyVarKind tv1)) <+>
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ptext (sLit "and") <+> quotes (ppr (typeKind t2)),
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ptext (sLit "when matching") <+> quotes (ppr tv1) <+>
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ptext (sLit "with") <+> quotes (ppr t2)]
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occursCheck :: TyVar -> Type -> SDoc
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= hang (ptext (sLit "Can't construct the infinite type"))
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2 (ppr tv <+> equals <+> ppr ty)