hello i myself am not an experienced Haskell user, so please correct me if i am wrong. it is difficult in general to reason about the performance of lazy programs, so i don't think one can assume much. in particular i dont think 'any' and 'all' will perform in linear space. here is why i think so: take an example when 'any' is applied to some list (x:xs) and someone is actually interested in the result: any p (x:xs) 1. -> (or . map p) (x:xs) 2. -> or (map p (x:xs)) 3. -> foldr (||) False (map p (x:xs)) [at this stage we need to know what what kind of list is (map p (x:xs)) i.e. empty or a cons thing, so we need to do some evaluation] 4. -> foldr (||) False (p x : map p xs) 5. -> p x || (foldr (||) False (map p xs)) [at this stage we need to know what kind of thing is p x, i.e. True or False, so we need to evaluate p x] 6. -> if p x is True we are done (result True) 7. -> if p x is False the result is (foldr (||) False (map p xs)) and we go back to 3. note that p x has become garbage and so it doesnt really take up any space, so one really needs only enough space to process the list one element at a time. what causes problems is the need to create unnecessary cons cells (i.e. the evaluation after 3.). this is bad because it takes time. of course it only adds a constant so the complexity is the same but in practise programs run slower. this is where i would expect a good compiler to do some optimisation, i.e to remove the need for the intermediate list. i like the idea of programming at a higher level, as i believe it produces "better structured" programs. what i mean is that one manages to capture certain aspects of a program, which would be obscured if one always used explicit recursion. i think higher order functions like maps and folds really go a long way toward structuring functional programs. in a way this is simillar to using loops instead of explicit gotos in procedural programs. anyways these are my deep thought on the matter :) bye iavor On Tue, Jan 23, 2001 at 12:35:19PM -0600, Eric Shade wrote: ...
Then I get to 'any' and 'all', whose specification requires linear time and *linear* space when it should run in constant space. (By the way, I checked and GHC does *not* use the Prelude definitions but rather the obvious recursive ones, and most of its optimizations based on "good consumers/producers" use meta-linguistic rewrite rules. So without knowing the specific optimizations that a compiler provides, I think it's safe to assume that the Prelude 'any' and 'all' *will* require linear space.)
... -- +---------------------------------+---------------------------------------+ |Iavor S. Diatchki | email: diatchki@cse.ogi.edu | |Dept. of Computer Science | web: http://www.cse.ogi.edu/~diatchki | |Oregon Graduate Institute | tel: 5037481631 | +---------------------------------+---------------------------------------+