Kinds of type synonym arguments

6 Dec 2015

      In this program:

    {-# LANGUAGE MagicHash, UnboxedTuples #-}

    module Main where

    import GHC.Prim
    import GHC.Types

    type Tuple a b = (# a, b #)

    main = do
      let -- x :: Tuple Int# Float#
          x :: (# Int#, Float# #)
          x = (# 1#, 0.0# #)

      return ()

If I use the first type declaration for 'x' I'm getting this error message:

    Expecting a lifted type, but ‘Int#’ is unlifted

Indeed, if I look at the kinds of arguments of 'Tuple':

    λ:7> :k Tuple
    Tuple :: * -> * -> #

It's star. I was wondering why is this not 'OpenKind'(or whatever the
super-kind of star and hash). Is there a problem with this? Is this a bug?
Or is this simply because type synonyms are implemented before OpenKinds?

Kinds of type synonym arguments

Ömer Sinan Ağacan