#16315: Pattern synonym implicit existential quantification -------------------------------------+------------------------------------- Reporter: int-index | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): I know what is happening here. There are two different culprits in the codebase: the renamer and the typechecker. The typechecker problem is much easier to understand. When typechecking the body of a pattern synonym, we bring certain type variables into scope. [https://gitlab.haskell.org/ghc/ghc/blob/4af0a2d609651c4bae45d84d2bf7ce9fe8cc... Here] is the code that does it: {{{#!hs tcExtendTyVarEnv univ_tvs $ tcExtendKindEnvList [(getName (binderVar ex_tv), APromotionErr PatSynExPE) | ex_tv <- extra_ex] $ }}} We bring the universal (required) type variables into scope, of course. We also bring the "existential" (provided) type variables into scope so that if we find any occurrences of them in the pattern synonym body, we can throw the relevant `APromotionErr` error. Why did I write "existential" in scare quotes? Because the only existential type variables that are being brought into scope are the //implicitly quantified// ones (`extra_ex`). The remaining explicitly quantified ones (which live in `ex_bndrs`) are never brought into scope. Fortunately, fixing that is as simple as replacing `extra_ex` with `ex_bndrs` in the code above. However, that change alone won't be enough. There is another problem in that GHC treats the occurrence of `k` in the pattern synonym type signature as being completely different from the occurrence of `k` in the body. Why? It's ultimately due to the renamer. In particular, because of the `forall`-or-nothing rule, the only explicitly quantified type variables that can ever brought into scope over the body must appear in a `forall` at the very front of the type signature. Since `k` is quantified by a nested `forall`, it is never brought into scope over the body during renaming, so the renamer gives a different unique to the occurrence of `k` in the body. (This is why GHC doesn't throw an error about `k` when typechecking the body, since the `k` that would throw an `APromotionErr` is actually different from the `k` in the body.) I believe that we should be able to tweak things so that the renamer brings in the explicitly quantified universals //and// existentials from a pattern synonym type signature. Experimentally, this change was sufficient to accomplish that: {{{#!diff diff --git a/compiler/hsSyn/HsTypes.hs b/compiler/hsSyn/HsTypes.hs index 5eeca6eac2..fe6849b955 100644 --- a/compiler/hsSyn/HsTypes.hs +++ b/compiler/hsSyn/HsTypes.hs @@ -51,7 +51,7 @@ module HsTypes ( mkEmptyImplicitBndrs, mkEmptyWildCardBndrs, mkHsQTvs, hsQTvExplicit, emptyLHsQTvs, isEmptyLHsQTvs, isHsKindedTyVar, hsTvbAllKinded, isLHsForAllTy, - hsScopedTvs, hsWcScopedTvs, dropWildCards, + hsScopedTvs, hsPatSynScopedTvs, hsWcScopedTvs, dropWildCards, hsTyVarName, hsAllLTyVarNames, hsLTyVarLocNames, hsLTyVarName, hsLTyVarLocName, hsExplicitLTyVarNames, splitLHsInstDeclTy, getLHsInstDeclHead, getLHsInstDeclClass_maybe, @@ -959,6 +959,20 @@ hsScopedTvs sig_ty | otherwise = [] +hsPatSynScopedTvs :: LHsSigType GhcRn -> [Name] +hsPatSynScopedTvs sig_ty + | HsIB { hsib_ext = vars + , hsib_body = sig_ty2 } <- sig_ty + , L _ (HsForAllTy { hst_bndrs = tvs, hst_body = body }) <- sig_ty2 + = vars ++ map hsLTyVarName (tvs ++ body_tvs body) + | otherwise + = [] + where + body_tvs :: LHsType GhcRn -> [LHsTyVarBndr GhcRn] + body_tvs (L _ (HsForAllTy { hst_bndrs = tvs })) = tvs + body_tvs (L _ (HsQualTy { hst_body = body })) = body_tvs body + body_tvs _ = [] + {- Note [Scoping of named wildcards] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider diff --git a/compiler/rename/RnBinds.hs b/compiler/rename/RnBinds.hs index ade67b7a49..fc5ffddfb0 100644 --- a/compiler/rename/RnBinds.hs +++ b/compiler/rename/RnBinds.hs @@ -606,7 +606,7 @@ mkScopedTvFn sigs = \n -> lookupNameEnv env n `orElse` [] get_scoped_tvs (L _ (TypeSig _ names sig_ty)) = Just (names, hsWcScopedTvs sig_ty) get_scoped_tvs (L _ (PatSynSig _ names sig_ty)) - = Just (names, hsScopedTvs sig_ty) + = Just (names, hsPatSynScopedTvs sig_ty) get_scoped_tvs _ = Nothing -- Process the fixity declarations, making a FastString -> (Located Fixity) map }}} Does this sound like the right approach? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16315#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler