On 7/13/11, Roelof Wobben
Oke,
Then I made there a error. I want to check if xs is a empty list. So it must be if null xs then [] else xs
When you say safeTail (x:xs) = if null xs then [] else xs , what you're saying is, "if xs is [], return []. Otherwise, return xs." In other words, always return xs. That means you're defining safeTail as "safeTail (x:xs) = xs" which does give the tail, but is not safe. What happens when the whole list is []? (When you call "safeTail []") "safeTail (x:xs)" won't match it, because [] doesn't fit the pattern x:xs (an element added to the "front" of a string). In haskell, you can define a function multiple times, for different patterns. For example, I can say: f 0 = 0 f x = 10/x The patterns are tried "top-down" (first, the pattern "0" is matched against the input. If it doesn't match, haskell tries the next function definition). See if this helps you define a safeTail! Tom