On Thu, 19 Oct 2006, Tomasz Zielonka
On Thu, Oct 19, 2006 at 01:37:16PM -0400, Cale Gibbard wrote:
In order to determine if [1..length xs] has an element at all, you have to evaluate length xs, which involves forcing the entire spine of xs, because integers can't be partially evaluated. Computing lengths of lists is a great way to introduce strictness.
Right, so if Ints were represented as a datatype with Succ and Zero constructors (so integers could be partially evaluated), then the version with length would behave nicely on large and infinite lists :-)
Using genericLength for unary, lazy natural numbers can be convenient for other tasks as well, for instance choosing the shorter of two lists in a simple and lazy way. See "Modular Lazy Search for Constraint Satisfaction Problems", Nordin and Tolmach, http://web.cecs.pdx.edu/~apt/, around page 25, for another (related) example. -- /NAD