Ashley Yakeley wrote:
>I'd like to be able to declare the kinds of new types and synonyms,
>because sometimes Haskell can't infer them. For instance:
>
> data CMap0 p q = MkCMap0;
>
>Without evidence, Haskell assumes that p and q have kind '*' (as per sec.
>4.6), and therefore CMap0 has kind '* -> * -> *'. Actually, I wanted p
>and q to both have kind '* -> *', giving CMap0 kind '(* -> *) -> (* -> *)
>-> *'.
...
>It's not currently possible to specify kinds, is it?
It is possible using a trick due to John Hughes. In
Proceedings of the 1999 Haskell Workshop,
he wrote in his article
Restricted Data Types in Haskell
this note:
3 There is one unpleasant hack in the figure: The constructor Unused
in the
data type definition for Set. It is there purely in order to force the
compiler
to assign the parameter cxt the correct kind: without it, cxt does not
appear
at all in the right hand side of the definition, and is therefore
assigned the
(incorrect) kind *. The application cxt a forces the correct kind * ->
* to be
assigned, and embedding it in the type cxt a -> () prevents the type
of the
context from interfering with the derivation of a Show instance.
The figure mentioned contains
data Set cxt a = Set [a] | Unused (cxt a -> ()) deriving Show
You can follow the example of John, writing
data CMap0 p q = MkCMap0 | Unused (p a -> ()) (q a -> ());
(I think I'm correctly applying the trick, but other
Proceedings-readers will correct me if I'm wrong.)
As John writes, this is a hack, but we have no
other choice.
Rijk-Jan van Haaften
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