Alastair Reid writes:
> I strongly agree that we should definitely add the ability to
> declare types
> whose definition is provided externally. (i.e., provide the
> feature that empty datatype decls currently provide.)
>
> Before adding them, we need to agree on the semantics and
> syntax (in that order I think).
>
> The obvious semantics based on the syntax is:
>
> [[
> The declaration
>
> data T
>
> introduces a type T whose only value is bottom.
> ]]
>
> This semantics is obviously flawed though because it would
> suggest that any
> two values of type T are equal (and equal to bottom) and that
> optimizations
> based on that equality are valid. Using an unpointed type
> (i.e., the value
> is not bottom) or saying there are no values bottom or
> otherwise don't help.
>
> The correct semantics has to be something roughly like:
>
> [[
> The declaration
>
> data T
>
> declares a type T whose set of values are defined externally to the
> language. [optional sentence: There are no legal Haskell operations
> on values of type T.]
> ]]
If we were actually using values of type T in the way you suggest, I would agree. But
the way these empty datatypes are being used
is as dummy type arguments to Ptr, as in 'Ptr T'. So I don't think there's any need
to give special semantics to datatypes declared
with empty declarations.
(this is essentially what Marcin just said, but I'm being a bit more verbose).
Cheers,
Simon
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