Commit a008eadf authored by Simon Peyton Jones's avatar Simon Peyton Jones
Browse files

Take type-function arity into account

...when computing the size of a call on the RHS of a type
instance declaration.

This came up in Trac #11581.  The change is in
   TcType.tcTyFamInsts
which now trims the type arguments in a call.  See the
comments with that function definition.
parent 4d031cf9
......@@ -693,13 +693,23 @@ isSigMaybe _ = Nothing
-}
-- | Finds outermost type-family applications occuring in a type,
-- after expanding synonyms. In the list (F, tys) that is returned
-- we guarantee that tys matches F's arity. For example, given
-- type family F a :: * -> * (arity 1)
-- calling tcTyFamInsts on (Maybe (F Int Bool) will return
-- (F, [Int]), not (F, [Int,Bool])
--
-- This is important for its use in deciding termination of type
-- instances (see Trac #11581). E.g.
-- type instance G [Int] = ...(F Int <big type>)...
-- we don't need to take <big type> into account when asking if
-- the calls on the RHS are smaller than the LHS
tcTyFamInsts :: Type -> [(TyCon, [Type])]
tcTyFamInsts ty
| Just exp_ty <- coreView ty = tcTyFamInsts exp_ty
tcTyFamInsts (TyVarTy _) = []
tcTyFamInsts (TyConApp tc tys)
tcTyFamInsts (TyConApp tc tys)
| isTypeFamilyTyCon tc = [(tc, take (tyConArity tc) tys)]
| otherwise = concat (map tcTyFamInsts tys)
tcTyFamInsts (LitTy {}) = []
tcTyFamInsts (ForAllTy bndr ty) = tcTyFamInsts (binderType bndr)
......
{-# LANGUAGE TypeFamilies #-}
module T11581 where
type family F a :: * -> *
type family G a
type instance G [a] = F a (Int,Bool)
......@@ -274,3 +274,4 @@ test('T11408', normal, compile, [''])
test('T11361', normal, compile, ['-dunique-increment=-1'])
# -dunique-increment=-1 doesn't work inside the file
test('T11361a', normal, compile_fail, [''])
test('T11581', normal, compile, [''])
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