#12820: Regression around pattern synonyms and higher-rank types -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): What makes you say it's correct? Consier {{{ f :: String -> Int f ( (\_ -> id) -> (x :: forall a. a->a) ) = 3 }}} This too is rejected with {{{ T12820.hs:10:20: error: * Couldn't match expected type `a0 -> a0' with actual type `forall a. a -> a' * When checking that the pattern signature: forall a. a -> a fits the type of its context: a0 -> a0 In the pattern: x :: forall a. a -> a In the pattern: (\ _ -> id) -> (x :: forall a. a -> a) }}} Suppose we are inferring the type of `f -> p`. W could either * Infer the type of `f`, and use that to check the type of pattern `p`, or * Infer the type of `p`, and use that to check the type of `f` (actually it only fixes teh result type of `f` but still. We use the first alternative, but perhpas the second makes more sense. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12820#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#12820: Regression around pattern synonyms and higher-rank types -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Hmm. Works ok if you put the forall after the arrow {{{ pattern P x <- (((\ _ -> id) :: forall b. b -> forall c. c -> c) -> x) }}} Reason: we infer the type of `f`, and then try to decompose and arrow, instantiating if necessary. But we don't re-generalise. I suppose we could but this is a pretty exotic corner and it's not keeping me awake at night. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12820#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#12820: Regression around pattern synonyms and higher-rank types -------------------------------------+------------------------------------- Reporter: goldfire | Owner: goldfire Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): This regression was introduced somewhere between commit 45bfd1a65978ee282d8d2cc1ddb7e3e5f4cd4717 (Refactor typechecking of pattern bindings) and 9417e57983dbcf7f500cca16163b11d1abb699e6 (Refactor occurrence-check logic). (I'm 90% sure it's 45bfd1a65978ee282d8d2cc1ddb7e3e5f4cd4717, but I can't say with certainty because that commit was in the middle of a slew of commits which didn't compile correctly until 9417e57983dbcf7f500cca16163b11d1abb699e6.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12820#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler