[GHC] #14380: Compile error for PatternSynonyms together with OverloadedLists

21 Oct 2017

      #14380: Compile error for PatternSynonyms together with OverloadedLists
-------------------------------------+-------------------------------------
           Reporter:  lehins         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.3
           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:
-------------------------------------+-------------------------------------
 {{{#!hs
 {-# LANGUAGE OverloadedLists #-}
 {-# LANGUAGE PatternSynonyms #-}

 data Foo = Foo [Int]

 pattern Bar :: Foo
 pattern Bar = Foo []
 }}}

 This code results in a cryptic compilation error:

 {{{
 $ ghc main.hs
 [1 of 1] Compiling Main             ( main.hs, main.o )

 main.hs:7:19: error:
     • Couldn't match expected type ‘[a0] -> [Int]’
                   with actual type ‘[GHC.Exts.Item Int]’
     • This rebindable syntax expects a function with two arguments,
       but its type ‘Int -> [GHC.Exts.Item Int]’ has only one
       In the first argument of ‘Foo’, namely ‘[]’
       In the expression: Foo []
   |
 7 | pattern Bar = Foo []
   |                   ^^
 }}}

 As soon as `OverloadedLists` is removed, this error goes away. In itself
 the problem isn't too critical, but very unexpected.

 I got above error with GHC 8.0 and 8.2, so decided to check it with
 current version as well:
 {{{
 $ ghc --version
 The Glorious Glasgow Haskell Compilation System, version 8.3.20171020
 }}}

 Looking a bit more into `PatternSynonyms` extension I was able to work
 around this by declaring an explicit bidirectional pattern, which,
 surprisingly, solved the problem:

 {{{#!hs
 pattern Bar :: Foo
 pattern Bar <- Foo [] where
   Bar = Foo []
 }}}

 If fixing this issue is too much trouble than it is worth, than at least a
 mention about it in documentation will be helpful.

 Out of curiosity I also tried it with `String`, which did not trigger this
 error and compiled just fine:
 {{{#!hs
 {-# LANGUAGE OverloadedLists #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE PatternSynonyms #-}

 data Foo = Foo [Char]

 pattern Bar :: Foo
 pattern Bar = Foo ""
 }}}

 PS. Even if this issue is closed, searching the internet for the error
 message will now at least return some info. :)

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14380
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