[Haskell-cafe] Intersection types for Haskell?

Brian Hulley brianh at metamilk.com
Wed Jan 11 11:54:34 EST 2006

David Menendez wrote:
> Brian Hulley writes:
>> Also, as a second point, could functional dependencies in type
>> classes be written using a similar syntax eg instead of
>>     class Insert t c a | c a -> t where
>>         insert :: t -> c a -> c a
>> we could write:
>>     class Insert (h (c a)) c a where
>>         insert :: h (c a) -> c a -> c a
> <snip>
> Your example, in the syntax of associated type synonyms, would look
> something like this:
>     class Insert c a where
>         type T c a
>         insert :: T c a -> c a -> c a

Thanks for the link. I suppose h(c a) can't be used on its own because h is 
assumed to be a tycon rather than a general type function, but if this 
restriction were lifted, the syntax "h a" meaning "some type determined by a 
by the type function h" could be used both for intersection typing and 
multi-param type classes.

Regards, Brian. 

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