Re: [GHC] #16074: Hopelessly confusing error involving runtime-reps

19 Dec 2018


      #16074: Hopelessly confusing error involving runtime-reps
-------------------------------------+-------------------------------------
        Reporter:  simonpj           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.3
      Resolution:                    |             Keywords:
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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Description changed by simonpj:

Old description:
...
Consider
{{{
{-# LANGUAGE GADTs, TypeOperators, PolyKinds #-}
import GHC.Types
data a :~: b where Refl :: a :~: a
foo :: TYPE a :~: TYPE b
foo = Refl
}}}
We get
{{{
    • Couldn't match type ‘'LiftedRep’ with ‘'LiftedRep’
      ‘a’ is a rigid type variable bound by
        the type signature for:
          foo :: * :~: *
        at Repr.hs:7:1-24
      ‘b’ is a rigid type variable bound by
        the type signature for:
          foo :: * :~: *
        at Repr.hs:7:1-24
}}}
That's a ridiculous message.  But if you asdd `-fprint-explicit-runtime-
reps` we get
{{{
    • Couldn't match type ‘a’ with ‘b’
      ‘a’ is a rigid type variable bound by
        the type signature for:
          foo :: forall (a :: RuntimeRep) (b :: RuntimeRep).
                 TYPE a :~: TYPE b
        at T16050.hs:9:1-24
      ‘b’ is a rigid type variable bound by
        the type signature for:
          foo :: forall (a :: RuntimeRep) (b :: RuntimeRep).
                 TYPE a :~: TYPE b
        at T16050.hs:9:1-24
      Expected type: TYPE a :~: TYPE b
        Actual type: TYPE a :~: TYPE a
    • In the expression: Refl
      In an equation for ‘foo’: foo = Refl
    • Relevant bindings include
        foo :: TYPE a :~: TYPE b (bound at T16050.hs:10:1)
}}}
which is the right error.
New description:

 Consider
 {{{
 {-# LANGUAGE GADTs, TypeOperators, PolyKinds #-}

 import GHC.Types

 data a :~: b where Refl :: a :~: a

 foo :: TYPE a :~: TYPE b
 foo = Refl
 }}}
 We get
 {{{
     • Couldn't match type ‘'LiftedRep’ with ‘'LiftedRep’
       ‘a’ is a rigid type variable bound by
         the type signature for:
           foo :: * :~: *
         at Repr.hs:7:1-24
       ‘b’ is a rigid type variable bound by
         the type signature for:
           foo :: * :~: *
         at Repr.hs:7:1-24
 }}}
 That's a ridiculous message.  But if you asdd `-fprint-explicit-runtime-
 reps` we get
 {{{
     • Couldn't match type ‘a’ with ‘b’
       ‘a’ is a rigid type variable bound by
         the type signature for:
           foo :: forall (a :: RuntimeRep) (b :: RuntimeRep).
                  TYPE a :~: TYPE b
         at T16050.hs:9:1-24
       ‘b’ is a rigid type variable bound by
         the type signature for:
           foo :: forall (a :: RuntimeRep) (b :: RuntimeRep).
                  TYPE a :~: TYPE b
         at T16050.hs:9:1-24
       Expected type: TYPE a :~: TYPE b
         Actual type: TYPE a :~: TYPE a
     • In the expression: Refl
       In an equation for ‘foo’: foo = Refl
     • Relevant bindings include
         foo :: TYPE a :~: TYPE b (bound at T16050.hs:10:1)
 }}}
 which is the right error.

 Reported in comment:5 of #16050

--

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16074#comment:2
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