[GHC] #15893: View Patterns affect typechecking in an unpredictable manner

[GHC]

#15893: View Patterns affect typechecking in an unpredictable manner
-------------------------------------+-------------------------------------
           Reporter:  theindigamer   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  ⊥
          Component:  Compiler       |           Version:  8.4.3
           Keywords:  ViewPatterns   |  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:
-------------------------------------+-------------------------------------
 Copying the text from my [https://stackoverflow.com/questions/53294823
 /viewpatterns-affects-typechecking-in-an-unpredictable-manner
 StackOverflow question].


 {{{
 {-# LANGUAGE ViewPatterns #-}

 import Data.Text (Text)
 import qualified Data.Text as T
 import qualified Data.Vector.Unboxed as UV
 import qualified Data.Vector.Generic as V

 bar :: Int -> UV.Vector Char -> (Text, Text)
 bar i v = (t_pre, t_post)
   where
     f = T.pack . V.toList
     (f -> t_pre, f -> t_post) = V.splitAt i v
 }}}

 This gives an unexpected type error (tested with GHC 8.4.3), concerning
 ambiguous type variables and `f` being out of scope.

 {{{
     • Ambiguous type variable ‘v0’ arising from a use of ‘V.toList’
       prevents the constraint ‘(V.Vector v0 Char)’ from being solved.
       Relevant bindings include
     f :: v0 Char -> Text (bound at Weird.hs:11:5)
       Probable fix: use a type annotation to specify what ‘v0’ should be.
       These potential instances exist:
     instance V.Vector UV.Vector Char
       -- Defined in ‘Data.Vector.Unboxed.Base’
     ...plus one instance involving out-of-scope types
       instance primitive-0.6.3.0:Data.Primitive.Types.Prim a =>
                V.Vector Data.Vector.Primitive.Vector a
         -- Defined in ‘Data.Vector.Primitive’
     • In the second argument of ‘(.)’, namely ‘V.toList’
       In the expression: T.pack . V.toList
       In an equation for ‘f’: f = T.pack . V.toList
    |
 11 |     f = T.pack . V.toList
    |                  ^^^^^^^^

 Weird.hs:13:6: error:
     Variable not in scope: f :: UV.Vector Char -> t
    |
 13 |     (f -> t_pre, f -> t_post) = V.splitAt i v
    |      ^

 Weird.hs:13:18: error:
     Variable not in scope: f :: UV.Vector Char -> t1
    |
 13 |     (f -> t_pre, f -> t_post) = V.splitAt i v
    |                  ^
 }}}

 Some of the other answers on SO have done some digging (e.g. enabling
 FlexibleContexts / NoMonomorphismRestriction doesn't solve the issue). I
 haven't yet figured out how to minimize the example further to remove the
 dependency on Vector...

 Is this a compiler bug or a documentation bug?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15893>
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