#10873: Bad error message for incorrect pattern synonym signature -------------------------------------+------------------------------------- Reporter: mpickering | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.0.1 Component: Compiler (Type | Version: 7.10.2 checker) | Resolution: | Keywords: newcomer Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): -------------------------------------+------------------------------------- Comment (by mpickering): Another note: there is another bad error message when there is an arity mismatch. Seems like some contexts need to be sprinkled in relevant places. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10873#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#10873: Bad error message for incorrect pattern synonym signature -------------------------------------+------------------------------------- Reporter: mpickering | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.0.1 Component: Compiler (Type | Version: 7.10.2 checker) | Resolution: | Keywords: newcomer 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 mpickering): The error should now be triggered by switching around the constraints. {{{ {-# LANGUAGE PatternSynonyms #-} pattern Pat :: () => Show a => a -> Maybe a pattern Pat a = Just a }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10873#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#10873: Bad error message for incorrect pattern synonym signature -------------------------------------+------------------------------------- Reporter: mpickering | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.0.1 Component: Compiler (Type | Version: 7.10.2 checker) | Resolution: | Keywords: newcomer Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by mpickering: Old description:

Consider the following program

{{{ {-# LANGUAGE PatternSynonyms #-}

pattern Pat :: Show a => () => a -> Maybe a pattern Pat a = Just a }}}

GHC complains that

{{{ test.hs:4:9: No instance for (Show a) arising from a pattern }}}

I think this is quite difficult to understand. The problem is that matching on `Just a` doesn't provide the show constraint (it provides no constraints). A better error message here would explain this fact and maybe a short explanation of the difference between prov/req in a pattern synonym signature.

New description: Consider the following program {{{ {-# LANGUAGE PatternSynonyms #-} pattern Pat :: () => Show a => a -> Maybe a pattern Pat a = Just a }}} GHC complains that {{{ test.hs:4:9: No instance for (Show a) arising from a pattern }}} I think this is quite difficult to understand. The problem is that matching on `Just a` doesn't provide the show constraint (it provides no constraints). A better error message here would explain this fact and maybe a short explanation of the difference between prov/req in a pattern synonym signature. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10873#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#10873: Bad error message for incorrect pattern synonym signature -------------------------------------+------------------------------------- Reporter: mpickering | Owner: rdragon Type: bug | Status: new Priority: normal | Milestone: 8.0.1 Component: Compiler (Type | Version: 7.10.2 checker) | Keywords: Resolution: | PatternSynonyms 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 simonpj): * keywords: newcomer => PatternSynonyms -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10873#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#10873: Bad error message for incorrect pattern synonym signature -------------------------------------+------------------------------------- Reporter: mpickering | Owner: rdragon Type: bug | Status: new Priority: normal | Milestone: 8.0.1 Component: Compiler (Type | Version: 7.10.2 checker) | Keywords: Resolution: | PatternSynonyms Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D1866 Wiki Page: | -------------------------------------+------------------------------------- Comment (by Ben Gamari ): In [changeset:"116528c8429257a0ae855251fd266547bb23d01d/ghc" 116528c/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="116528c8429257a0ae855251fd266547bb23d01d" Improve pattern synonym error messages (add `PatSynOrigin`) Adds a new data constructor `PatSynOrigin Bool Name` to the `CtOrigin` data type. This allows for better error messages when the origin of a wanted constraint is a pattern synonym declaration. Fixes T10873. Reviewers: mpickering, simonpj, austin, thomie, bgamari Reviewed By: simonpj, thomie, bgamari Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D1866 GHC Trac Issues: #10873 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10873#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler