[Haskell-cafe] Are associated types synonyms like type classes?

Brian Smith brianlsmith at gmail.com
Fri Sep 1 12:32:55 EDT 2006

I read the easy parts of the "Associated Types with Class" and "Associated
Type Synonyms" papers. An associated type synonym seems to kind of work
similarly to a restricted form of class. In what way are the two following
examples different?

    -- define a class with a type synonym, and a set of operations
    class A a where
        type B b
        foo :: a -> B b

    instance A Int where
        type B = Bool
        foo = (==0)

    -- define two classes, and an function that .
    class A a, B b where
        foo :: a -> b

     instance A Int, B Bool where
          foo = (==0)

Also, has anybody written a paper on the differences between typeclasses +
associated types and ML's module system + overloading?

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