[GHC] #10905: Incorrect number of parameters in "role" errors

[GHC]

#10905: Incorrect number of parameters in "role" errors
-------------------------------------+-------------------------------------
        Reporter:  crockeea          |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.2
      Resolution:                    |                Keywords:  roles
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
-------------------------------------+-------------------------------------

Old description:

> The following code produces the error:
>
> {{{
> import Data.Coerce
> import Data.Functor.Trans.Tagged
>
> toTT :: Tagged t [a] -> TaggedT t [] a
> toTT = coerce
> }}}
>

> {{{
> Couldn't match representation of type ‘[a]’
>                              with that of ‘Data.Functor.Identity.Identity
> [a]’
>     arising from trying to show that the representations of
>       ‘Tagged t [a]’ and
>       ‘TaggedT t [] a’ are the same
>     Relevant role signatures:
>       type role [] representational
>       type role Data.Functor.Identity.Identity representational
>       type role TaggedT nominal nominal phantom representational nominal
>     The data constructor ‘Data.Functor.Identity.Identity’
>       of newtype ‘Data.Functor.Identity.Identity’ is not in scope
>     Relevant bindings include
>       toTT :: Tagged t [a] -> TaggedT t [] a (bound at Main.hs:9:1)
>     In the expression: coerce
>     In an equation for ‘toTT’: toTT = coerce
> }}}
>
> First, kudos for correctly identifying the problem: I failed to import
> the `Identity` constructor.
> However, it seems that the report for the roles of the `TaggedT` type has
> too many parameters. It is defined in tagged-transformer
> [https://hackage.haskell.org/package/tagged-transformer-0.8/docs/Data-
> Functor-Trans-Tagged.html] as `newtype TaggedT s m b = TagT { untagT :: m
> b }`, but is listed with 5 roles.
>
> I did try to move the definition of `TaggedT/Tagged` into my module so I
> didn't have to import the library, but when I did the error didn't show
> up: TaggedT was listed with the expected roles `phantom representational
> nominal`.
>
> As a user, it wasn't clear to me why there are 5 roles listed for a type
> with 3 parameters, and I didn't know which roles I should be looking at
> to help me debug the `coerce` error.

New description:

 The following code produces the error:

 {{{
 import Data.Coerce
 import Data.Functor.Trans.Tagged

 toTT :: Tagged t [a] -> TaggedT t [] a
 toTT = coerce
 }}}


 {{{
 Couldn't match representation of type ‘[a]’
                              with that of ‘Data.Functor.Identity.Identity
 [a]’
     arising from trying to show that the representations of
       ‘Tagged t [a]’ and
       ‘TaggedT t [] a’ are the same
     Relevant role signatures:
       type role [] representational
       type role Data.Functor.Identity.Identity representational
       type role TaggedT nominal nominal phantom representational nominal
     The data constructor ‘Data.Functor.Identity.Identity’
       of newtype ‘Data.Functor.Identity.Identity’ is not in scope
     Relevant bindings include
       toTT :: Tagged t [a] -> TaggedT t [] a (bound at Main.hs:9:1)
     In the expression: coerce
     In an equation for ‘toTT’: toTT = coerce
 }}}

 First, kudos for correctly identifying the problem: I failed to import the
 `Identity` constructor.
 However, it seems that the report for the roles of the `TaggedT` type has
 too many parameters. It is defined in tagged-transformer
 [https://hackage.haskell.org/package/tagged-transformer-0.8/docs/Data-
 Functor-Trans-Tagged.html] as `newtype TaggedT s m b = TagT { untagT :: m
 b }`, but is listed with 5 roles.

 I did try to move the definition of `TaggedT/Tagged` into my module so I
 didn't have to import the library, but when I did the error didn't show
 up: TaggedT was listed with the expected roles `phantom representational
 nominal`.

 As a user, it wasn't clear to me why there are 5 roles listed for a type
 with 3 parameters, and I didn't know which roles I should be looking at to
 help me debug the `coerce` error. It's also not clear why using the
 library vs defining the types locally changed the behavior.

--

Comment (by crockeea):

 I agree with your test case, and it appears that 5 roles are printed with
 both `-fprint-explicit-kinds` and with `-fno-print-explicit-kinds`, which
 seems odd (or is that merely because of `PolyKinds`?).

 I suppose it makes sense to print the roles of the kinds if you have
 `fprint-explicit-kinds` enabled, but in my original test case (without
 `fprint-explicit-kinds` or `PolyKinds`, the kind roles should probably not
 be printed.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10905#comment:3>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler