[GHC] #15645: TypeChecking of Monad patterns incorrect with RebindableSyntax and OverloadedStrings

14 Sep 2018

      #15645: TypeChecking of Monad patterns incorrect with RebindableSyntax and
OverloadedStrings
-------------------------------------+-------------------------------------
           Reporter:  NeilMitchell   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.3
           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:
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 Using GHC 8.4.3, I'd expect the following to work:

 {{{#!hs
 {-# LANGUAGE RebindableSyntax, OverloadedStrings #-}

 module Fail where

 import Prelude hiding (fail)

 foo x = do
     Just y <- x
     return y

 newtype Text = Text String

 fail :: Text -> a
 fail (Text x) = error x

 fromString :: String -> Text
 fromString = Text
 }}}

 But it fails with:

 {{{
 Fail.hs:8:5-15: error:
     * Couldn't match expected type `[Char]' with actual type `Text'
     * In a stmt of a 'do' block: Just y <- x
       In the expression:
         do Just y <- x
            return y
       In an equation for `foo':
           foo x
             = do Just y <- x
                  return y
   |
 8 |     Just y <- x
   |     ^^^^^^^^^^^
 }}}

 Given the enabled extensions, I'd expect {{{foo}}} to desugar as:

 {{{#!hs
 foo x = x >>= \v -> case v of
     Just y -> return y
     _ -> fail (fromString "pattern match error")
 }}}

 But looking at TcMatches.tcMonadFailOp it checks the fail operation (which
 is literally {{{fail}}}) takes an argument of type tyString (e.g.
 {{{[Char]}}}). One way around that would be to make the "fail-op" being
 passed around be {{{fail . fromString}}} if the appropriate extensions are
 enabled.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15645
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