[GHC] #12126: Bad error messages for SPECIALIZE pragmas

27 May 2016

      #12126: Bad error messages for SPECIALIZE pragmas
-------------------------------------+-------------------------------------
           Reporter:  antalsz        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:  Specialize,    |  Operating System:  Unknown/Multiple
  ErrorMessages                      |
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 When playing with SPECIALIZE pragmas, I've encountered one large problem
 with the error messages, and two minor ones.

 1. Given a function with a `Num` constraint, applying `SPECIALIZE` at a
 non-function type triggers defaulting incorrectly; the code

 {{{#!hs
 f1 :: Num a => a -> a
 f1 x = x + x
 {-# SPECIALIZE f1 :: Bool #-}
 }}}

 produces the error

 {{{
 …/Test1.hs:5:1: error:
     • Couldn't match expected type ‘Integer -> Integer’
                   with actual type ‘Bool’
     • In the SPECIALISE pragma {-# SPECIALIZE f1 :: Bool #-}
 }}}

 (This is in the attached file `Test1.hs`.)

 2. Given a function, applying `SPECIALIZE` at a well-formed type without
 the looked-for instance produces an error with some extra text (the first
 line ends with "a SPECIALISE pragma for ‘f2’", even though this is
 repeated on the next line); the code

 {{{#!hs
 f2 :: Num a => a -> a
 f2 x = x + x
 {-# SPECIALIZE f2 :: Bool -> Bool #-}
 }}}

 produces the error

 {{{
 …/Test2.hs:5:1: error:
     • No instance for (Num Bool) a SPECIALISE pragma for ‘f2’
     • In the SPECIALISE pragma {-# SPECIALIZE f2 :: Bool -> Bool #-}
 }}}

 (This is in the attached file `Test2.hs`.)

 3. This is minor, but: both of these error messages talk about a
 `SPECIALISE` pragma when I wrote a `SPECIALIZE` pragma.  Even more so,
 case is not preserved in the quoted pragma; if I provide a

 {{{#!hs
 {-# specialize … #-}
 }}}

 pragma, the error message ends with

 {{{
 • In the SPECIALISE pragma {-# SPECIALIZE … #-}`.
 }}}

 even though that isn't what I wrote.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12126
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