At 12:03 PM +0100 7/17/03, Bayley, Alistair wrote:
I've just debugged a program that used a case expression, but where I was trying to match on constants rather than literals. Here's a contrived example:
module Main where one = 1 two = 2
test n = case n of one -> "one" two -> "two" _ -> "three"
main = putStrLn (test 2)
This initial version seems to me to be the "natural" way to write the case expression, but it doesn't work because the first alternative always succeeds.
The root of the problem is that a variable occurring in a pattern is always a new variable. A pattern variable provides a way to refer to the value to which the variable is bound when the pattern matches.
This is what I've turned it into to get it to work. It seems a bit clumsy; is there a better way to write this?
test n = case True of _ | n == one -> "one" | n == two -> "two" | otherwise -> "three"
As, Graham Klyne wrote at 2:52 PM +0100 7/17/03:
test n | n == one = "one" | n == two = "two" | otherwise = "three"
is a neater solution. Best, --Ham -- ------------------------------------------------------------------ Hamilton Richards, PhD Department of Computer Sciences Senior Lecturer The University of Texas at Austin 512-471-9525 1 University Station C0500 Taylor Hall 5.138 Austin, Texas 78712-1188 ham@cs.utexas.edu hrichrds@swbell.net ------------------------------------------------------------------