On Thu, 25 Feb 2010 03:32:25 +0000, Jon Harrop
On Thursday 25 February 2010 00:31:59 Brent Yorgey wrote:
On Wed, Feb 24, 2010 at 11:53:50PM +0000, Jon Harrop wrote:
On Wednesday 24 February 2010 20:53:14 Edward Z. Yang wrote:
That leads to the following source:
http://haskell.org/ghc/docs/latest/html/libraries/base/src/Data-Function. html
but it uses a workaround rather than extending the type system. I'm looking for an equivalent of OCaml's -rectypes that enables recursive types in Haskell (e.g. in ghci)?
Haskell has recursive types, but they are iso-recursive rather than equi-recursive; the recursion must always be guarded by a data constructor. I am not sure what you mean by saying that Data.Function contains a "workaround".
Guarding the recursion with a constructor is the workaround I was referring to, like this:
# let fix f = (fun (`X x) -> f(x (`X x))) (`X(fun (`X x) y -> f(x (`X x)) y));; val fix : (('a -> 'b) -> 'a -> 'b) -> 'a -> 'b = <fun>
If the reason for avoiding a data constructor is performances, then Haskell "newtype"s will help. -- Nicolas Pouillard http://nicolaspouillard.fr