[GHC] #16315: Pattern synonym implicit existential quantification
#16315: Pattern synonym implicit existential quantification -------------------------------------+------------------------------------- Reporter: int-index | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This code typechecks when `k` is brought into scope explicitly, but not implicitly (code example courtesy of RyanGlScott): {{{#!hs {-# LANGUAGE ExistentialQuantification #-} {-# LANGUAGE PatternSynonyms #-} {-# LANGUAGE PolyKinds #-} {-# LANGUAGE ScopedTypeVariables #-} module Bug where import Data.Kind import Data.Proxy data T = forall k (a :: k). MkT (Proxy a) -- Uncomment `k` and it typechecks pattern P :: forall. () => forall {-k-} (a :: k). Proxy a -> T pattern P x = MkT (x :: Proxy (a :: k)) }}} I discovered this because I was implementing https://github.com/ghc- proposals/ghc-proposals/blob/master/proposals/0024-no-kind-vars.rst and had to explicitly quantify some definitions in the test suite. Then the test case for #14498 stopped producing the error that it should. It seems indicative of an issue in typechecking pattern synonyms: I would expect equal treatment for implicit and explicit type/kind variables. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16315 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#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
#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 goldfire): From the `Note [Pattern synonym existentials do not scope]`, it sounds like we aim to ''reject'' this code, not accept it. Do you find differently? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16315#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#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): Yes, I wrote comment:1 in mind with the goal of rejecting the original program. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16315#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
We also bring the "existential" (provided) type variables into scope so
#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): In case it wasn't clear: Replying to [comment:1 RyanGlScott]: that if we find any occurrences of them in the pattern synonym body, //we can throw the relevant `APromotionErr` error//. (emphasis added) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16315#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#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 goldfire): OK. Now I understand what you're doing better. Here is how I see it: 1. The renamer really shouldn't bring any of the existentials into scope, as existentials in a pattern synonym type don't scope over the (entire) pattern synonym body. 2. But the renamer can't tell existential implicitly bound variables from universal ones. 3. So, the renamer brings into scope only the universals and all the implicitly bound ones. This brings more variables into scope than it should. 4. The pattern synonym body then is renamed with some extra variables in scope, so more uniques match between the body and the type than morally should. In addition, less implicit quantification happens in the body than morally should. 5. The type-checker must deal with this unfolding disaster by duplicating the renamer's strange behavior: it brings into scope implicitly-bound existentials, just so that weird out-of-scope errors don't happen in the body. But it arranges to error if these are encountered, regardless. 6. The program in this ticket observes that the difference in treatment between implicitly bound and explicitly bound is awkward. 7. Your proposed solution is just to bring all existentials into scope in both the renamer and the type-checker, so that treatment is uniform. This is done even though existentials really shouldn't scope over the body. 8. This ticket is all about uniformity. The behavior in both the explicit case and the implicit case is, by itself, correct. But it's awkward for GHC's behavior to depend on the user's choice of implicit vs explicit binding. Is that a fair assessment? If so, I advocate: do nothing. This problem goes away when type variables and kind variables are treated identically, because if the user writes a `forall`, they will have to bind the kind variables explicitly anyway. If the user does not write a `forall`, then no scoping of type variables happens, so there's no problem here. And, the patch as written actually makes GHC's behavior worse, in that it's further from the ideal of "existentials do not scope". The only reason to do this patch is that we cannot achieve the ideal, and this patch does indeed (and correctly, as far as I can tell) make the behavior more uniform. So I do think the best way to fix this problem is to ignore it and wait for it to go away. :) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16315#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#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): Replying to [comment:5 goldfire]:
If so, I advocate: do nothing. This problem goes away when type variables and kind variables are treated identically, because if the user writes a `forall`, they will have to bind the kind variables explicitly anyway. If the user does not write a `forall`, then no scoping of type variables happens, so there's no problem here.
You lost me here. The problem is that if you write out the kind variable in the original program explicitly: {{{#!hs pattern P :: forall. () => forall k (a :: k). Proxy a -> T pattern P x = MkT (x :: Proxy (a :: k)) }}} Then GHC //accepts// it, contrary to the claims of `Note [Pattern synonym existentials do not scope]`. So no, this problem won't go away when type and kind variables are treated identically. (In fact, int-index had to specifically mark this test case as `expect_broken` in his [https://gitlab.haskell.org/ghc/ghc/merge_requests/361 patch] to implement the type-kind-variable merger.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16315#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#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 goldfire): You have a red herring in your soup. The following snippet is also accepted: {{{#!hs pattern P :: forall. () => forall k (a :: k). Proxy a -> T pattern P x = MkT (x :: Proxy (b :: j)) }}} There's no scoping going on here, just the fact that these type variables are unconstrained. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16315#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#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): OK. If I understand you correctly, then the original program with `k` uncommented should be accepted. (I think you were hinting at this all along, but it wasn't obvious to me until comment:7.) If that's the case, then it sounds like the proper course of action is to: * Proceed with https://gitlab.haskell.org/ghc/ghc/merge_requests/374, as it will get rid of the awkwardness brought about by implicitly quantified variables being brought into scope. * Moreover, !374 can revert most of #14998: * There is no longer any need for `PatSynExPE`, since implicitly quantified existentials are no longer brought into scope. * `Note [Pattern synonym existentials do not scope]` can go away. * The `T14498` test case can be removed. (Or, alternatively, we could explicitly quantify `k` and move it to `patsyn/should_compile`.) Does this sound right? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16315#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#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 goldfire):
Does this sound right?
Yes, precisely. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16315#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#16315: Pattern synonym implicit existential quantification -------------------------------------+------------------------------------- Reporter: int-index | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 Resolution: fixed | 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: | -------------------------------------+------------------------------------- Changes (by int-index): * status: new => closed * resolution: => fixed -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16315#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#16315: Pattern synonym implicit existential quantification -------------------------------------+------------------------------------- Reporter: int-index | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 Resolution: fixed | 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: | https://gitlab.haskell.org/ghc/ghc/merge_requests/361 -------------------------------------+------------------------------------- Changes (by int-index): * differential: => https://gitlab.haskell.org/ghc/ghc/merge_requests/361 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16315#comment:11 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#16315: Pattern synonym implicit existential quantification -------------------------------------+------------------------------------- Reporter: int-index | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: 8.10.1 Component: Compiler | Version: 8.6.3 Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: | patsyn/should_compile/T14498 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | https://gitlab.haskell.org/ghc/ghc/merge_requests/361 -------------------------------------+------------------------------------- Changes (by RyanGlScott): * testcase: => patsyn/should_compile/T14498 * milestone: => 8.10.1 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16315#comment:12 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#16315: Pattern synonym implicit existential quantification
-------------------------------------+-------------------------------------
Reporter: int-index | Owner: (none)
Type: bug | Status: closed
Priority: normal | Milestone: 8.10.1
Component: Compiler | Version: 8.6.3
Resolution: fixed | Keywords:
Operating System: Unknown/Multiple | Architecture:
| Unknown/Multiple
Type of failure: None/Unknown | Test Case:
| patsyn/should_compile/T14498
Blocked By: | Blocking:
Related Tickets: | Differential Rev(s):
Wiki Page: | https://gitlab.haskell.org/ghc/ghc/merge_requests/361
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