[GHC] #11186: Give strong preference to type variable names in scope when reporting hole contexts

[GHC]

#11186: Give strong preference to type variable names in scope when reporting hole
contexts
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        Reporter:  dfeuer            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.10.2
  checker)                           |
      Resolution:                    |             Keywords:  typed-holes
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dfeuer):

 Replying to [comment:1 simonpj]:
 > Can you give a concrete example?

 I finally found one, attempting to figure out how to write something like
 `reverse` for type-aligned lists. In this case, I use a pattern signature
 to name an existentially quantified type, but GHC's message doesn't seem
 to respect that.

 {{{#!hs
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE ScopedTypeVariables #-}

 module TAList where
 import Control.Category
 import Prelude hiding ((.), id)

 data TAList (cat :: k -> k -> *) (x :: k) (z :: k) where
    Nil :: TAList cat x x
    (:<) :: cat b c -> TAList cat a b -> TAList cat a c
 infixr 5 :<

 newtype Op cat x y = Op {op :: cat y x}

 reverseOntoTA :: TAList cat b d -> TAList (Op cat) d a -> TAList (Op cat)
 d a
 reverseOntoTA Nil ys = ys
 reverseOntoTA ((x :: cat c d) :< (xs :: TAList cat b c)) ys = _
 }}}

 I named the unknown type variable `c`, but GHC says:

 {{{
     Relevant bindings include
       ys :: TAList (Op cat) d a (bound at TAList.hs:32:58)
       xs :: TAList cat b b1 (bound at TAList.hs:32:35)
       x :: cat b1 d (bound at TAList.hs:32:17)
       reverseOntoTA :: TAList cat b d
                        -> TAList (Op cat) d a -> TAList (Op cat) d a
 }}}

 That is, it's decided to name that variable `b1` instead.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11186#comment:2>
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