Re: [Haskell-beginners] safetail problem

Oke, I understand it now. Now a new chapter list compreshions. I hope I get the hang of that. Roelof

Date: Wed, 13 Jul 2011 13:21:37 -0400 Subject: Re: [Haskell-beginners] safetail problem From: amindfv@gmail.com To: rwobben@hotmail.com CC: beginners@haskell.org

On 7/13/11, Roelof Wobben wrote:

...

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