Joachim
Aha! I think my comment on Phab is redundant. You are simply avoiding name capture, which is obviously right; you are not restricting the applicability of the transformation.
Needless to say, a Note would help in due course.
S
| -----Original Message-----
| From: ghc-commits [mailto:ghc-commits-bounces@haskell.org] On Behalf
| Of git@git.haskell.org
| Sent: 02 August 2017 04:07
| To: ghc-commits@haskell.org
| Subject: [commit: ghc] wip/T14068: Avoid name capture in setJoinResTy
| (6a50466)
|
| Repository : ssh://git@git.haskell.org/ghc
|
| On branch : wip/T14068
| Link :
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| >---------------------------------------------------------------
|
| commit 6a5046684f93f1870663119b447fff6baeb7a7c4
| Author: Joachim Breitner
| Date: Tue Aug 1 22:48:26 2017 -0400
|
| Avoid name capture in setJoinResTy
|
|
| >---------------------------------------------------------------
|
| 6a5046684f93f1870663119b447fff6baeb7a7c4
| compiler/types/Type.hs | 22 ++++++++++++++++------
| 1 file changed, 16 insertions(+), 6 deletions(-)
|
| diff --git a/compiler/types/Type.hs b/compiler/types/Type.hs index
| 50a35b0..c69d4ff 100644
| --- a/compiler/types/Type.hs
| +++ b/compiler/types/Type.hs
| @@ -2445,11 +2445,21 @@ setJoinResTy :: Int -- Number of binders to
| skip
| -> Type -- New type
| -- INVARIANT: If any of the first n binders are foralls, those tyvars
| cannot
| -- appear in the original result type. See isValidJoinPointType.
| +--
| +-- When we set the return type under a forall, avoid capture!
| setJoinResTy orig_ar new_res_ty orig_ty
| - = go orig_ar orig_ty
| + = go init_subst orig_ar orig_ty
| where
| - go 0 _ = new_res_ty
| - go n ty | Just (arg_bndr, res_ty) <- splitPiTy_maybe ty
| - = mkPiTy arg_bndr (go (n-1) res_ty)
| - | otherwise
| - = pprPanic "setJoinResTy" (ppr orig_ar <+> ppr orig_ty)
| + init_subst :: TCvSubst
| + init_subst = mkEmptyTCvSubst (mkInScopeSet (tyCoVarsOfType
| + new_res_ty))
| +
| + go _ 0 _ = new_res_ty
| + go subst n ty
| + | Just (t, ty') <- splitForAllTy_maybe ty
| + , let (subst', t') = substTyVarBndr subst t
| + = mkForAllTy t' Inferred (go subst' (n-1) ty')
| + | Just (arg_ty, ty') <- splitFunTy_maybe ty
| + , let arg_ty' = substTy subst arg_ty
| + = mkFunTy arg_ty' (go subst (n-1) ty')
| + | otherwise
| + = pprPanic "setJoinResTy" (ppr orig_ar <+> ppr orig_ty)
|
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