The most interesting example is fs ($ (1, "2")) Which I haven't been able to make typecheck. Here's some well-typed code:
fs2 f g = (f fst, g snd) ab f = f ('a', "b") test = fs2 ab ab -- test2 = fs ab
The question is, is it possible to write fs such that your examples
typecheck and test2 also typechecks?
I find this example interesting because it's the smallest example I've
seen of a well-typed program which would "just work" in Scheme or
Lisp, but which we can't assign a type to in Haskell.
-- ryan
On Sun, Jun 7, 2009 at 9:20 AM, Wouter Swierstra
The idea is that fs accepts a polymorphic function as its argument. What type signature can I specify for f in order to compile this code?
As you said yourself, you need to add a type signature to fs:
{-# LANGUAGE RankNTypes #-}
fs :: ((forall a . ((a, a) -> a)) -> t) -> (t, t) fs g = (g fst, g snd)
examples = (fs id, fs repeat, fs (\x -> [x]), fs ((,)id))
Hope this helps,
Wouter
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