[GHC] #13530: Horrible error message due to TypeInType

[GHC]

#13530: Horrible error message due to TypeInType
-------------------------------------+-------------------------------------
        Reporter:  simonpj           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Description changed by simonpj:

Old description:

> Consider this
> {{{
> {-# LANGUAGE MagicHash, UnboxedTuples #-}
>
> module Foo where
>
> import GHC.Exts
>
> g :: Int -> (# Int#, a #)
> g (I# y) = (# y, undefined #)
>
> f :: Int -> (# Int#, Int# #)
> f x = g x
> }}}
> With GHC 8 we get
> {{{
> Foo.hs:11:7: error:
>     • Couldn't match a lifted type with an unlifted type
>       Expected type: (# Int#, Int# #)
>         Actual type: (# Int#, Int# #)
> }}}
> What a terrible error message!!  It was much better in GHC 7.10:
> {{{
> Foo.hs:11:7:
>     Couldn't match kind ‘*’ with ‘#’
>     When matching types
>       a0 :: *
>       Int# :: #
>     Expected type: (# Int#, Int# #)
>       Actual type: (# Int#, a0 #)
> }}}
> What's going on?
>
> The constraint solver sees
> {{{
> [W] alpha::TYPE LiftedRep  ~  Int#::TYPE IntRep
> }}}
> So it homogenises the kinds, ''and unifies alpha'' (this did not happen
> in GHC 7.10), thus
> {{{
> alpha := Int# |> TYPE co
>
> [W] co :: LiftedRep ~ IntRep
> }}}
> Of course the new constraint fails.  But since we have unified alpha,
> when we print out the types are are unifying they both look like `(#
> Int#, Int# #)` (there's a suppressed cast in the second component).
>
> I'm not sure what to do here.

New description:

 Consider this
 {{{
 {-# LANGUAGE MagicHash, UnboxedTuples #-}

 module Foo where

 import GHC.Exts

 g :: Int -> (# Int#, a #)
 g (I# y) = (# y, undefined #)

 f :: Int -> (# Int#, Int# #)
 f x = g x
 }}}
 With GHC 8 we get
 {{{
 Foo.hs:11:7: error:
     • Couldn't match a lifted type with an unlifted type
       Expected type: (# Int#, Int# #)
         Actual type: (# Int#, Int# #)
 }}}
 What a terrible error message!!  It was much better in GHC 7.10:
 {{{
 Foo.hs:11:7:
     Couldn't match kind ‘*’ with ‘#’
     When matching types
       a0 :: *
       Int# :: #
     Expected type: (# Int#, Int# #)
       Actual type: (# Int#, a0 #)
 }}}
 What's going on?

 The constraint solver sees
 {{{
 [W] alpha::TYPE LiftedRep  ~  Int#::TYPE IntRep
 }}}
 So it homogenises the kinds, ''and unifies alpha'' (this did not happen in
 GHC 7.10), thus
 {{{
 alpha := Int# |> TYPE co

 [W] co :: LiftedRep ~ IntRep
 }}}
 Of course the new constraint fails.  But since we have unified alpha, when
 we print out the types are are unifying they both look like `(# Int#, Int#
 #)` (there's a suppressed cast in the second component).

 I'm not sure what to do here.

 (I tripped over this when debugging #13509.)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13530#comment:5>
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