Eugene Crosser wrote:
Having read "Yet another Haskell tutorial" (note on p.20), doesn't foldl
have to read the complete list before it can start processing it
(beginning from the last element)? As opposed to foldr that can fetch
elements one by one as they are needed?
They're complementary.
If the result is of a type where partial evaluation is possible (say, a
list: between "not evaluated" and "fully evaluated", there are as many
intermediate stages of evaluation as there are elements in the list),
then foldr is the better choice, as it constructs the output list (or
whatever) lazily. (You also need to make sure that the fold parameter
function is lazy in the "rest of output" parameter.)
If the result is of a type that doesn't allow partial evaluation (an
integer, for example: there is no intermediate stage between "not
evaluated" and "fully evaluated"), or used in a context where laziness
is not a virtue, then it pays to avoid laziness in its evaluation: hence
foldl' is the better choice. (You also need to make sure that the fold
parameter function is strict in the accumulator parameter.)
In elementary (nth-language) Haskell, one is generally trying to learn
the stuff about Haskell that is *different* from conventional languages,
so in elementary tutorials the rule of thumb "foldr is better" works.
It's just one of the half-lies that people get told in elementary
courses that one needs to augment in later stages of learning :)
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