Re: [GHC] #11541: Type errors sometimes has a lot of irrelevant information

12 Feb 2016

      #11541: Type errors sometimes has a lot of irrelevant information
-------------------------------------+-------------------------------------
        Reporter:  augustss          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.8.3
      Resolution:                    |             Keywords:
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 j.waldmann):

 I have seen this behvaiour a few times.

 Here is a simple self-contained test case:
 {{{
 g :: Ord k => k -> v -> ()
 g k v = ()

 f x y =
   let m = min x y
   in  g m foo
 }}}
 8-rc2 outputs this:
 {{{
 ghci  C.hs
 GHCi, version 8.0.0.20160204: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( C.hs, interpreted )

 C.hs:5:11: error:
     • No instance for (Ord t) arising from a use of ‘min’
       Possible fix:
         add (Ord t) to the context of
           the inferred type of f :: t -> t -> ()
     • In the expression: min x y
       In an equation for ‘m’: m = min x y
       In the expression: let m = min x y in g m foo

 C.hs:6:11: error: Variable not in scope: foo
 }}}

 The relevant error message (not in scope) comes last, the message before
 is bogus.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11541#comment:5
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler