Koen writes:
Consider the following datatype declaration:
data Eq a => Foo a = Foo a
It introduces a new constructor function with the type:
Foo :: Eq a => a -> Foo a
So far so good. This is what we expect.
It also introduces a new pattern, which will result in the following
typing:
unFoo :: Eq a => Foo a -> a
unFoo (Foo a) = a
Why?? I require the context at _construction_ time, not at _destruction_
time! Actually, when I unpack (Foo a), I _know_ there must be a dictionary
(Eq a) somewhere! Because I required that at construction time!
This very issue is under discussion right now in the Standard Haskell
committee. See
http://www.cs.chalmers.se/~rjmh/Haskell/Messages/Display.cgi?id=302
John Hughes
