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classgenericshaskelltype-constructor

Haskell construct a type contains class

I want to construct a haskell type:

    type SinglePP = (String,GLattice)

Where the GLattice is defined as:

    class GLattice l where
        join :: l->l->l
        ....

Is there a way for me to do that?

Solution 
  • type SinglePP a = (String, a)

    and then when you use SinglePP in a function, restrict a to be a GLattice

    someFunc :: GLattice a => SinglePP a -> ()
someFunc a = doMagic a

    If you like, you can use more type system-foo and go with existential types, which let you avoid the boilerplate after each function, but in exchange you must use a language extension and a data declaration with an explicit constructor. This means more pattern matching when you want to get at the a but less typing in the type declarations.

    However most of the types can be inferred.