The definition of myLen says:
myLen [] = 0 The length for an empty list is zero
myLen (x:xs) = 1 + myLen xs The length of a list containing x and some other stuff (xs) is 1 + (the length of the other stuff).
So basically, if you've got a list [1,2,3], it will try to do this:
myLen (1:[2,3]) = 1 + myLen [2,3] The length of [1,2,3] is 1 + the length of [2,3]
myLen (2:[3]) = 1 + myLen [3] The length of [2,3] = 1 + the length of [3]
myLen (3:[]) = 1 + myLen [] This is the tricky part, now the other case of myLen is being called:
myLen [] = 0 Here you can see that it won't recurse anymore. Now there is going to be some replacement:
myLen (3:[]) can now be calculated, because "myLen []" is now known (0). Because we know this,
myLen (2:[3]) can now be calculated too, because we know that myLen [3] is 1. And so on.
I think the trick here is to see that "[1]" is exactly the same as "1: []". Once you grasp this, the rest will probably easy. Good luck! -chris On 17 Feb, 2007, at 19:32 , 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? Top marks for a comprehensive jargon-free explanation. Thanks in advance Paul
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