instance - Specifying that a type can only exist in one class if it already exists in other classes in Haskell -


i working on problem involving unification, user-defined type subst a. problem reads:

"define type class unifiable specifies function unify::a -> -> maybe (subst a) must defined type inside class. type can in unifiable class if in eq , substitutable classes, , should represented in definition."

this how have defined substitutable , unifiable far: 

class substitutable subst :: subst -> ->  class unifiable unify :: -> -> maybe (subst a)

however, i'm not sure how go specifying type can in unifiable class if in eq , substitutable classes.

i'm not looking answer specific problem, in general, how go specifying type can in 1 class if in other classes?

thanks.

the class definition can contain constraints, function.

class (eq a, substitutable a) => unifiable     unify :: -> -> maybe (subst a)

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