[GHC] #15413: Poor error message when trying to import non-existent class methods

18 Jul 2018

      #15413: Poor error message when trying to import non-existent class methods
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           Reporter:                 |             Owner:  (none)
  quasicomputational                 |
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.2
           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:
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 {{{
 $ cat A.hs
 module A where

 class C a where
   foo :: a -> Int
   bar :: a -> Int
 $ cat B.hs
 module B where

 import A (C (foo, bar, baz))

 instance C Int where
   foo = id
   bar = id
 $ ghci B.hs
 GHCi, version 8.4.2: http://www.haskell.org/ghc/  :? for help
 [1 of 2] Compiling A                ( A.hs, interpreted )
 [2 of 2] Compiling B                ( B.hs, interpreted )

 B.hs:3:11: error: Module ‘A’ does not export ‘C(foo, bar, baz)’
   |
 3 | import A (C (foo, bar, baz))
   |           ^^^^^^^^^^^^^^^^^
 Failed, one module loaded.
 }}}

 If you're importing a dozen or so methods, not mentioning the specific
 method that couldn't be imported gets quite annoying.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15413
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