Promoted types accepted without language extensions

José Pedro Magalhães jpm at cs.uu.nl
Mon Nov 19 15:24:03 CET 2012


Hi Sean,

On Mon, Nov 19, 2012 at 2:18 PM, Sean Leather <leather at cs.uu.nl> wrote:

> We discovered that GHC and GHCi (7.4.1) accept promoted types without
> specifying language extensions (and even when specifying -XHaskell98).
>
> For example, promoted lists are accepted:
>
> Prelude> type T = [Int,Char]
> Prelude> :i T
> type T = (:) * Int ((:) * Char ([] *))
>   -- Defined at <interactive>:2:6
>

This is rejected in 7.6.1:

<interactive>:2:10:
    Illegal type: '[Int, Char] Perhaps you intended to use -XDataKinds


>
> Also, promoted datatypes:
>
> > data N = Z
> > type T = 'Z
>

This isn't rejected, but I guess it should, indeed.


Thanks,
Pedro


>
> I'm guessing you can't use these types anywhere (since they are not kind
> *), so they won't be very useful, but it seems like this should be
> disallowed.
>
> Regards,
> Sean
>
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