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

21 Sep 2015

      #10905: Incorrect number of parameters in "role" errors
-------------------------------------+-------------------------------------
              Reporter:  crockeea    |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.2
              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:              |
-------------------------------------+-------------------------------------
 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.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10905
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler