[Haskell-cafe] type class question

Jules Bean jules at jellybean.co.uk
Thu Dec 6 10:06:25 EST 2007

Peter Padawitz wrote:
> Yes, the recursive calls of compCommand are supposed to be calls of 
> compBlock.
> The intention of the program is a generic evaluator comp... of 
> Sigma-terms in arbitrary Sigma-algebras. The signature Sigma is given by 
> the first 4 types (and the corresponding functions in the class 
> declaration), the terms are the objects of the types, and the algebras 
> are the class instances.
> The problem with my implementation in terms of multiple-parameter 
> classes seems to be - I conclude this from Ryan's comment - that  the 
> intended dependency among the parameters is not reflected here. But what 
> are the alternatives? Roughly said, I need a construct that allows me 
> gather several type variables such that an instance is always an 
> instance of all of them.

well, given

class Java a b c d where ....

if it is true that everything is determined by choice of any one of 
them, you could write

class Java a b c d | a -> b, b -> c, c -> d, d -> a where...

otherwise, well you can express whatever dependency network you want...


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