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?
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