thanks, to both of you! "data Fooable" is the solution, and also very neat. it took me a moment to learn the useful fact that a little explicit type information can be worse than none, in particular with incomplete contexts. but in the end it worked both without type signatures and with the right ones. cheers, matthias On Sat, Oct 07, 2006 at 12:25:07AM -0700, oleg@pobox.com wrote:
To: fis@wiwi.hu-berlin.de Cc: haskell-cafe@haskell.org From: oleg@pobox.com Date: Sat, 7 Oct 2006 00:25:07 -0700 (PDT) Subject: Trying to understand HList / hMapOut
I am using a heterogenous list as in [1] all elements of which are of a given class C. Since foo maps all class members to Int, hMapOut should be a straight-forward way to produce homogenous Int lists from heterogenous CLists:
test :: (CList l) => l -> [Int] test = hMapOut foo
Well, `foo' is a polymorphic function -- which is not, strictly speaking, a first-class object in Haskell. Indeed, one cannot store polymorphic functions in data structures, unless one wraps them in a `newtype' or provide the explicit signature in some other way. In other words, higher-rank types become necessary.
Fortunately, Haskell98 already has some rudimentary higher-ranked types (and multi-parameter type classes make them far more usable). So, even if Haskell had not had higher-ranked types, we could very easily get them from typeclasses, where they have been lurking all the time. In HList, the class Apply can be used to pry them out.
Here's the complete code that seems to solve the original problem. There is no need to define the class CList.
{-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-}
module Foo where import HListPrelude
data T = T Int
class C a where foo :: a -> Int instance C T where foo (T i) = i
data Fooable = Fooable instance C a => Apply Fooable a Int where apply _ x = foo x
test l = hMapOut Fooable l
testc = test (HCons (T 1) (HCons (T 2) HNil))
The inferred types are
*Foo> :t test test :: (HMapOut Fooable r e) => r -> [e] *Foo> :t testc testc :: [Int]
so no explicit signatures are needed.
-- Institute of Information Systems, Humboldt-Universitaet zu Berlin web: http://www.wiwi.hu-berlin.de/~fis/ e-mail: fis@wiwi.hu-berlin.de tel: +49 30 2093-5742 fax: +49 30 2093-5741 office: Spandauer Strasse 1, R.324, 10178 Berlin, Germany pgp: AD67 CF64 7BB4 3B9A 6F25 0996 4D73 F1FD 8D32 9BAA