[Haskell-beginners] Appending to a list

[Adrian Neumann]

Yes, appending to the tail of a list is an expensive operation.
You may ask yourself why there is no pointer to the end of a list.  
That's because the end is in general not known (e.g. if the list is  
produced by a computation, or if it's infinite).
However if you know up front that your list will be finite and you  
don't rely on lazyness you may want to use a Data.Sequence instead

 > http://www.haskell.org/ghc/docs/latest/html/libraries/containers/ 
Data-Sequence.html

Cheers,
Adrian

Am 07.02.2009 um 21:29 schrieb Francesco Bochicchio:

> Hi all,
>
> in my haskell exercises I often build list by appending values at  
> the end.
> But I read somewhere that in haskell this is inefficient since  
> there is no
> 'pointer to the tail of the list'. I've also seen some example of  
> recursive
> functions which build the list tail-first and then in the base case  
> of the
> recursion returns the reverse of the accumulated results, counting  
> on the fact
> that in haskell 'reverse list'  just means 'consume the list from  
> the tail'.
>
> Just out of curiosity (I have no need to speed up my little  
> programs with which I try to teach myself
> some haskell): is this a consolidated pattern?   And are append  
> really expensive?
>
> Ciao
> -------
> FB
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