Hello,
There is a thread on comp.lang.functional that talks about why haskell
does not support recursive types:
http://groups.google.com/groups?q=ocaml+rectypes&hl=en&lr=lang_en&ie=UTF-8&oe=UTF-8&safe=off&selm=8giqpt%24oee%241%40rivesaltes.inria.fr&rnum=1
(searching for 'ocaml rectypes' on google groups turns up a number of
useful threads about the joys and dangers of allowing recursive types).
Jeremy Shaw.
At Wed, 3 Dec 2003 14:07:03 -0800,
Jeffrey A. Scofield
Greetings.
Say I have the following function, adapted from Pierce, Types and Programming languages:
f n () = (n, f (n + 1))
Now, this function fails to typecheck in ghc 6.0.1 because the subexpression f (n + 1) has the infinite type t = () -> (t1, t) And so I assume (perhaps wrongly?) that f is ill-typed in Haskell 98.
I have two questions:
1. How can I tell from the Haskell 98 Revised Report that this function isn't allowed? The discussions of typing in the Report generally defer to the ``standard Hindley-Milner analysis.'' Is this really enough to tell that the function isn't well-typed in Haskell? I concede that I haven't read the cited Hindley and Damas/Milner papers, but it seems like there might be more to say about what is and isn't allowed.
2. Is there by chance a compiler option for ghc that causes the restriction to be dropped?
I don't need this kind of function for anything practical (and I realize I can get an equivalent effect with an algebraic datatype). I'm just trying to figure out what typing restrictions (if any) are implicitly assumed in the Report. (Generally, this is the kind of stuff I don't know, as a relative newcomer to fp.)
Regards,
Jeff Scofield (PhD) Seattle