[GHC] #14273: Typed holes are oblivious to type class constraints

25 Sep 2017

      #14273: Typed holes are oblivious to type class constraints
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.3
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Poor/confusing
  Unknown/Multiple                   |  error message
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 This example is taken from Chris Allen's blog post
 [http://bitemyapp.com//posts/2017-09-23-please-stop-using-typed-holes.html
 here]:

 {{{#!hs
 pleaseShow :: Show a => Bool -> a -> Maybe String
 pleaseShow False _ = Nothing
 pleaseShow True a = Just (show _a)
 }}}

 On a recent GHC HEAD build, compiling this outputs:

 {{{
 Bug.hs:3:32: error:
     • Found hole: _a :: a0
       Where: ‘a0’ is an ambiguous type variable
       Or perhaps ‘_a’ is mis-spelled, or not in scope
     • In the first argument of ‘show’, namely ‘_a’
       In the first argument of ‘Just’, namely ‘(show _a)’
       In the expression: Just (show _a)
     • Relevant bindings include
         a :: a (bound at Bug.hs:3:17)
         pleaseShow :: Bool -> a -> Maybe String (bound at Bug.hs:2:1)
       Valid substitutions include
         (++) :: forall a. [a] -> [a] -> [a]
           (imported from ‘Prelude’ at Bug.hs:1:1
            (and originally defined in ‘GHC.Base’))
         fail :: forall (m :: * -> *). Monad m => forall a. String -> m a
           (imported from ‘Prelude’ at Bug.hs:1:1
            (and originally defined in ‘GHC.Base’))
         return :: forall (m :: * -> *). Monad m => forall a. a -> m a
           (imported from ‘Prelude’ at Bug.hs:1:1
            (and originally defined in ‘GHC.Base’))
         errorWithoutStackTrace :: forall (a :: TYPE r). [Char] -> a
           (imported from ‘Prelude’ at Bug.hs:1:1
            (and originally defined in ‘GHC.Err’))
         seq :: forall a b. a -> b -> b
           (imported from ‘Prelude’ at Bug.hs:1:1
            (and originally defined in ‘GHC.Prim’))
         (<>) :: forall a. Semigroup a => a -> a -> a
           (imported from ‘Prelude’ at Bug.hs:1:1
            (and originally defined in ‘GHC.Base’))
         (Some substitutions suppressed; use -fmax-valid-substitutions=N or
 -fno-max-valid-substitutions)
   |
 3 | pleaseShow True a = Just (show _a)
   |                                ^^
 }}}

 There are a couple very unsavory things about this error:

 1. GHC makes no attempt to inform me that `a0` is a `Show` instance! This
 is the primary gripe in the blog post, and it's worth emphasizing, since
 without the `Show` constraint, `a0` just looks like any other random type
 variable. Speaking of which...
 2. The list of valid substitutions is incorrect! It suggests several
 things which have function types, such as `(++)` and `fail`, but `(->)`
 does not have a `Show` instance! This list ought to be pruned based on the
 current type class constraints in scope.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14273
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