Say I have the following function, ... :
f n () = (n, f (n + 1)) ... 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.''
In your example, if we assume that "f" has type, say, "a->()->(a,b)", for some "a","b", then it is used, in its own definition, with type "a->()->b". In the Hindley-Damas/Milner type system, there is no "polymorphic recursion": a variable being defined may not be used polymorphically inside its own definition. In other words, for typing its own definition, a variable is assumed to have a simple (non-quantified) type. Even in the Milner-Mycroft type system, where we have polymorphic recursion (a variable may be used polymorphically in its own definition), this expression could not be typed, because (informally speaking) a polymorphic type may only be instantiated, and "a->()->b" is not (cannot be) an instance of "a->()->(a,b)", for any "a","b". Hope that helps. Cheers, Carlos
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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe