Re: [GHC] #14419: Check kinds for ambiguity

27 Feb 2019


      #14419: Check kinds for ambiguity
-------------------------------------+-------------------------------------
        Reporter:  goldfire          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  TypeInType,
                                     |  VisibleDependentQuantification
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by RyanGlScott):

 * keywords:  TypeInType => TypeInType, VisibleDependentQuantification


Comment:

 Here is how to trigger the same issue using visible dependent
 quantification:

 {{{#!hs
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE TypeFamilies #-}
 module Bug where

 import Data.Kind

 type family F a
 data Ex (x :: forall a -> F a -> Type)
 }}}
 {{{
 GHCi, version 8.9.20190227: https://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:10:1: error:
     • Couldn't match type ‘F a’ with ‘F a0’
       Expected type: F a -> *
         Actual type: F a0 -> *
       NB: ‘F’ is a non-injective type family
       The type variable ‘a0’ is ambiguous
     • In the ambiguity check for the kind annotation on the type variable
 ‘x’
       To defer the ambiguity check to use sites, enable
 AllowAmbiguousTypes
       In the data type declaration for ‘Ex’
    |
 10 | data Ex (x :: forall a -> F a -> Type)
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14419#comment:14
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