[dllthomas]

As an easy to follow (though questionably idiomatic) example, consider the typeclass Default:

    class Default a where
        def :: a

and an unwise pair of instances:

    instance Default Int where
        def = 7

    instance Default String where
        def = "foo"

This means I can say:

    3 + def + length ("c" ++ def)

and get back 14. Note that which "def" depends on the type expected where it is used.

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It's not limited to values, either. Consider the "return" function in the Monad typeclass:

    return :: Monad m => a -> m a

which is a function that takes something and returns that something wrapped in a monad. Which monad? Whatever is expected at the call site.

    [1, 2, 3] ++ return 4

gives us [1, 2, 3, 4] because list is a monad and return for lists gives a single element list, whereas

    putStrLn "foo" >> return 4

gives us an IO action that, when executed, prints "foo" and yeilds a 4.

---

A super complex example is variadic functions like printf, with the type

    printf :: PrintfType r => String -> r

PrintfType can be a String or IO (), giving you something like C's sprintf or printf based on the call site (which is itself cool), but it can also be a function that takes an instance of PrintfArg and gives back some new PrintfType (in an interesting intersection with currying).