Re: Implict parameters and monomorphism

[Lennart Augustsson]

OK, so since noone liked my original example here's another one.
It involves no defaulting and no classes in the funny function definition.

-- Here's the type signature that makes a difference.
--fun :: a -> Char
fun x = const (fun x) (fun True)

fix f = let x = f x in x

class C a where
    m :: a -> String

instance C Char where
    m _ = "has signature"

instance C Bool where
    m _ = "no signature"

main = putStrLn (m (fix fun))


It is not at all surprising that you can write this.  Originally
type signatures only allowed you to put a signature that was 
more specific.
Polymorhic recursion on the other hand allows you to make the
type more general by putting a type signature on a definition.
Combining these you can make the signature be incomparable to
the deduced type.  Using the class system you can then dispatch
on the type and get different behaviour.

        -- Lennart

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