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Type synonyms are not expanded in the data type declaration return kind

    • Closed Issue created by thomasw @trac-thomasw

    I was playing around with the cool new -XTypeInType stuff when I encountered the following issue:

    {-# LANGUAGE TypeInType #-}
{-# LANGUAGE GADTs #-}
module TypeInTypeBug where

import qualified Data.Kind


-- This works, using Data.Kind.Type as return kind
--------------vvvvvvvvvvvvvv
data Works :: Data.Kind.Type where
  WorksConstr :: Works

type Set = Data.Kind.Type

-- This doesn't work, using a type synonym for Data.Kind.Type as return kind
---------------vvv
data Doesnt :: Set where
  DoesntConstr :: Doesnt
    TypeInTypeBug.hs:17:1: error:
    • Kind signature on data type declaration has non-* return kind Set
    • In the data declaration for ‘Doesnt’

    I suppose type synonyms should be expanded in the return kind of a data type declaration before checking it is *.

    I also think the error message is not totally correct, take the following valid data declaration:

    data Foo :: Bool -> Data.Kind.Type where
    Tru :: Foo True
    Fal :: Foo False

    The return kind of the data declaration is actually Bool -> Data.Kind.Type (or Bool -> *), which is not *. I assume the error message is talking about the return kind (*) of the return kind (Bool -> *)

    Trac metadata
    Trac field Value
    Version 7.11
    Type Bug
    TypeOfFailure OtherFailure
    Priority normal
    Resolution Unresolved
    Component Compiler (Type checker)
    Test case
    Differential revisions
    BlockedBy
    Related
    Blocking
    CC goldfire
    Operating system
    Architecture
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