#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