[GHC] #12006: Can't infer constraint of pattern synonyms
#12006: Can't infer constraint of pattern synonyms -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Keywords: TypeSynonyms | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Can't infer constraint of: {{{#!hs -- pattern Real' :: Num a => a -> Complex a pattern Real' r <- r :+ _ where Real' r = r :+ 0 }}} {{{ teR4.hs:12:25: error: … • No instance for (Num t) arising from the literal ‘0’ Possible fix: add (Num t) to the context of the type signature for: Main.$bReal' :: t -> Complex t • In the second argument of ‘(:+)’, namely ‘0’ In the expression: r :+ 0 In an equation for ‘$bReal'’: $bReal' r = r :+ 0 Compilation failed. }}} Should it be able to? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12006 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#12006: Can't infer constraint of pattern synonyms -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Resolution: | Keywords: | 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 mpickering): * keywords: TypeSynonyms => PatternSynonyms Comment: The problem here is that the type signature of the builder is fixed by the type of the matcher. By fixed, I mean literally fixed, we first typecheck the matcher with either the signature or infer the type and then we create an `Id` for the builder with precisely the same type. So the problem here is that the inferred type for the matcher doesn't require the `Num` constraint and so the builder type is `t -> Complex t` rather than `Num t => t -> Complex t` as you desire. From what I could work out from reading the source, the way it is like this is for mainly because of implementation problems. There is some more confused discussion on this ticket -https://ghc.haskell.org/trac/ghc/ticket/8581 - it is something I hope to look at again one day. Thank you for all these reports, they are very useful, I am interested in any pattern synonyms tickets so feel free to add me to cc to make sure that I see them. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12006#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#12006: Can't infer constraint of pattern synonyms -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Resolution: | Keywords: | 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: | -------------------------------------+------------------------------------- Comment (by Iceland_jack): Sure thing, I was starting to worry I was spamming with tickets :) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12006#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#12006: Can't infer constraint of pattern synonyms -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Resolution: | Keywords: | 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: | -------------------------------------+------------------------------------- Comment (by simonpj): Yes, I think in principle it should be possible to infer a type that works for both builder and matcher. I'm not quite sure how hard that would be; probably not terrible. Of course you can always work around the difficulty by providing a signature. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12006#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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