On Feb 17, 2007, at 21:32 , P. R. Stanley wrote:
Hi I understand the basic principle of recursion but have difficulty with the following: -- a recursive function -- for calculating the length of lists myLen [] = 0 myLen (x:xs) = 1 + myLen xs What's happening here?
This definition uses pattern matching. The first one matches an empty list; the second matches a list using constructor syntax (a list [a,b,c] in constructor syntax is a:b:c:[]) in order to extract the first element and the rest of the list into separate variables "x" and "xs", then recursively invokes itself on xs. The "x" being unused, that definition can also be rewritten as: myLen (_:xs) = 1 + myLen xs since _ can be used in a pattern match as a placeholder. -- brandon s. allbery [linux,solaris,freebsd,perl] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH