11:13. I am re-reading Learn You a Haskell for Great Good!. I guess that's one reason to use foldl: sometimes you don't care about efficiency (in a particular context), and foldl is always available whereas foldl' must be coded if one wishes to be completely portable. Fo… Basic usage: >>> maybe False odd (Just 3) True >>> maybe False odd Nothing False Read an integer from a string using readMaybe. This topic has already been covered in the wiki: http://www.haskell.org/haskellwiki/Foldr_Foldl_Foldl%27. In Haskell recursion is the way to iterate. myFold : (f: elem -> acc -> acc) -> (init: acc) -> (xs: List elem) -> acc. Write foldl with foldr:-- file: ch04/Fold.hs myFoldl :: (a -> b -> a) -> a -> [b] -> a myFoldl f z xs = foldr step id xs z where step x g a = g (f a x) The above code confused me a lot, and some guy called dps rewrote it … Firstly, Real World Haskell, which I am reading, says to never use foldl and instead use foldl'.So I trust it. foldl' is not in the Haskell98 standard libraries, is it? Most of the time you should use foldr, as it’s more efficient. *, Computer Science, Haskell, tagged foldl, foldr, GHC, Haskell, heap profilling, optimisations, … So I trust it. Early Haskell did not have seq so could not write the strict one and my guess is that this is the reason we still have the lazy foldl to this day. Vim users are not invited! Why direction matters: foldr vs foldl. Click Here for Items Related To - Foldl In functional programming , fold (also termed reduce , accumulate , aggregate , compress , or inject ) refers to a family of higher-order functions that analyze a recursive data structure and through use of a given combining operation, recombine the results of recursively processing its constituent parts, building up a return value. Foldr vs Foldl – A small survey with the help of GHC. You'll understand it best on an example. Here are a few rules of thumb on which folds to use when. I guess that's one reason to use foldl: sometimes you don't care about efficiency (in a particular context), and foldl is always available whereas foldl' must be coded if one wishes to be completely portable. But I'm hazy on when to use foldr vs. foldl'. The difference is that foldl1 uses the first list element as the initial value of its accumulator, and isn’t defined for empty lists. The Haskell programming language community. Due to the thunking behavior of foldl, it is wise to avoid this function in real programs: even if it doesn’t fail outright, it will be unnecessarily inefficient. Sorry about the link to my own post, but the story it's sitting in (a pretty basic newbie question) has been downvoted quite a bit, and I think the foldl versus foldl' stuff comes up enough that maybe some other people would be interested in the thread. We apply (+3) to 2, that's 5 and we prepend (:) it to the accumulator, so the accumulator is now [5,6]. The implementation is similar to the max -function but with the opposite comparison. So how is it possible that we defined and used several functions that take more than one parameter so far? with the right-most element of the list, and, for completeness, here is a left fold expanded, which, for the sum example, would expand to, so, we can see that both foldr and foldl iterated the items of the list starting from the left, All the functions that accepted several parameters so far have been curried functions. It is also mentioned in the language report: http://www.haskell.org/onlinereport/haskell2010/haskellch20.html#x28-23100020.3. In Real World Haskell, Chapter 4. Press question mark to learn the rest of the keyboard shortcuts. The name foldl' I think comes as an essentially random decision. But I'm hazy on when to use foldr vs. foldl'.Though I can see the structure of how they work differently laid out in front of me, I'm too stupid to understand when "which is better." Every function in Haskell officially only takes one parameter. . It was used internally in the hugs library code with that name, but not exported. Related: foldl1, foldr, foldr1, scanl, scanl1, scanr, scanr1 The extraneous intermediate list structure can be eliminated with the continuation-passing style technique, foldr f z xs == foldl (\ k x-> k. f x) id xs z; similarly, foldl f z xs == foldr (\ x k-> k. flip f x) id xs z ( flip is only needed in languages like Haskell with its flipped order of arguments to the combining function of foldl unlike e.g., in Scheme where the same order of arguments is used for combining functions to … I'm a mathematician and a rather experienced programmer in various programming languages but only a beginner in Haskell, and every time I try to program something in Haskell, it sucks absolutely, not because the language sucks, but because it presents me with the illusion that I'm doing math and everything works the way it works in math, and I think about it with my "math mind" and not my programming mind, and of course in doing that I forget that it is obnoxiously lazy. Instead of comparing the two strings directly, we compare the all uppercase version. But apart from that, I think this is a good example of how lazy evaluation can hurt. foldr: Type: (a -> b -> b) -> b -> [a] -> b: Description: it takes the second argument and the last item of the list and applies the function, then it takes the penultimate item from the end and the result, and so on. This has been the definition since GHC 7.10, and in particular it was made possible by the call arity analysis introduced there. The second duality theorem states that foldr (#) u xs is equivalent to foldl ( ) u xs, if x # (y z) = (x # y) z and x # u = u x. The third duality theorem simply states: foldr op u xs = foldl (flip op) u (reverse xs) The higher-order scanl function foldl first applies the function to the left-most element, -- note the function application expression will be evaluated before the next iteration. Instead, import Data.List and use foldl’ Haskell Wiki compares foldr, foldl and foldl' and recommends using either foldr or foldl'. Haskell is a lazily evaluated language, which makes the discussion of folds a bit more interesting. In the real Haskell world, performance aside (and issues with let bindings and monomorphism aside now too), those two statements are equivalent. As Miran states in that same chapter, for right fold, ... the accumulator eats up the values from the right, The list is iterated from the left, but the first application of the function with the accumulator is with the right-most element, A simple implementation of right fold might look like, If we expand the foldr example from the book, we get, then, if we pop off the operations, the first addition is the initial accumlator value And I can recall my confusion from my initial reading over the example of foldr is... 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