Richard Nathan Linger wrote:
'undefined' is a program that does not terminate.
On Wed, 24 Sep 2003, Norman Ramsey wrote:
Consider the following Haskell function:
asPair x = (fst x, snd x)
This function has type forall a b. (a, b) -> (a, b) and is almost equivalent to the identity function, except it can be used to make programs terminate that might otherwise fall into a black hole.
What is an example program that asPair rescues from nontermination?
'asPair undefined' is a program that terminates immediately (with a pair containing two nonterminating components). unfortunatelu in haskell you can tell the difference between those two:
test (x,y) = True
test undefind = undefined test (asPair undefined) = True
it may be nice to have tuples unlifted in haskell (and in general datatypes with a single constructor). this could be introduced in the language by either changing the semantics of 'data' (prolly not too nice) or by generalizing 'newtype' (and perhaps renaming it to 'record') to handle more than one type. the semantics of such thing could be just like now except that pattern matching on them is always lazy
(i.e.patterns have an implicit ~). then one could not tell the difference between `undefined' and (undefined,undefined). well i guess unless one used seq, but seq is also not very nice.
bye
iavor
-- ================================================== | Iavor S. Diatchki, Ph.D. student | | Department of Computer Science and Engineering | | School of OGI at OHSU | | http://www.cse.ogi.edu/~diatchki | ==================================================
_______________________________________________ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
