I would say a good practice with folds (and maybe in Haskell in general) is that either all be strict or all be lazy.
In the expression: foldXX f init list: Remember that foldr does: x `f` ( ... the accumulator ... ) and foldl: (... the accumulator ...) `f` x The accumulator has to match a non-strict argument of* f*, so that *f* will be able to return results *even if* the accumulator is not fully evaluated. More detailed: if *f* is lazy in its second argument, then use foldr. Everything is lazy, you build a very small thunk since nothing is evaluated. In the rare cases where* f *is (also) lazy in its first argument, you can use foldl. And of course, if *f* is strict in both its arguments, then use foldl'. Everything is then strict, you build no thunk. 2011/11/20 David Fox <dds...@gmail.com> > Does anyone have a quick way to decide which of the fold functions to > use in a given situation? There are times when I would like to find > out which to use in the quickest way possible, rather than reading a > long explanation of why each one behaves the way it does. > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >
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