[Haskell-beginners] safetail problem

[Roelof Wobben]

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 at gmail.com
> To: rwobben at hotmail.com
> CC: beginners at haskell.org
> 
> On 7/13/11, Roelof Wobben <rwobben at hotmail.com> 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
 		 	   		  
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