This inner stack for thunk evaluation This is caused by a combination of two factors: To exemplify, here is an abridged progression. * A strict function is a function f, such that f⊥=⊥. But can someone give me a more precise answer? I think different people expand these differently. (1) Disclaimer: That's not idiomatic Haskell code. The reason why I say that these are two possible expansions is because the order in which it is expanded is up to the specific runtime and optimizations for the compiler you're using. When should foldl be used? It's not throwing a dice, or the compiler finding an optimisation. do, as covered in Making Haskell programs faster and smaller and in the Haskell Report is necessary. Most questions that carry the [haskell] and [stack] tags should be tagged with haskell-stack instead of stack. Above, fst,snd are the pair projections, returning the first/second component of a pair. Abusing the algebra of algebraic data types - why does this work? A case where it makes a difference is if the function is conditionally strict in its first argument depending on its second, where I use conditionally strict to mean a function that is strict or not in one argument depending on another argument(s). (acc+x, len+1) is already in whnf, so the seq (in the definition of foldl'), which reduces a value to whnf, does nothing to this. 4.1 Haskell Tracer HAT 4.2 Hoed - The Lightweight Haskell Tracer and Debugger 5 Dynamic breakpoints in GHCi 6 Source-located errors 7 Other tricks 7.1 Locating a failure in a library function 7.2 Mysterious parse errors 7.3 In other words, there are two redex. Since this isn't yet a redex, and there are no redexes inside of (\y.m) we move to the right of the application. That's the main difference between a lazy language and a strict one: When expanding a function call, you don't first evaluate the argument - instead you replace the function call with its body and leave the argument as-is for now. This is exactly what foldl' does. No, lists are not special. A function strict* in its second argument will always require linear stack space with foldr, so foldl' should be used instead in that case. This code will build up thunks just like the original foldl example, they'll just be inside a tuple. You can have your traversal functions (in this case, last) force the list as it goes along. It's a specific strategy. Thanks for contributing an answer to Stack Overflow! This is a good time to pull out equational reasoning, which means we can substitute a function for its definition (modulo renaming things to not have clashes). Was Stan Lee in the second diner scene in the movie Superman 2? You can use a data structure that's strict in its elements, in this case it would be a head strict list. Yes, in Haskell you can recursively define everything, not just functions. a lookup for a value in a list returning null can either mean â¦ It expands to: That is the outer function call is expanded first. GHC's Core data type represents recursion with recursive binders in the Let constructor; as I understand it, all let expressions in Haskell are effectively let rec expressions. And in this case the answer is yes , yes it is. Stack Exchange network consists of 176 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers. FAQ So that this doesn't become repetitive: for the reasons behind the answers below, see the Architecture page. This wouldn't be possible in a language without purity, such as C#/Java/Python/etc. that they can match a constructor (WHNF). Haskell prefers to fire outermost redexes first since it is a normal order-ish language. First, read Performance/Accumulating parameter. This is why laziness is often a boon, you don't have to think about the order in which things occurs as much because the compiler does that thinking for you. Asking for help, clarification, or responding to other answers. User guide stack is a modern, cross-platform build tool for Haskell code. How does Haskell's type system generate this error? doubleMe isn't really \y.m, and only really has a redex when it's "argument" has the correct shape (that of a list). If the function is lazy/non-strict in its second argument we should use foldr to 1) support infinite lists and 2) to allow a streaming use of the input list where only part of it needs to be in memory at a time. do, as covered in Making Haskell programs faster and smaller and in the Haskell Reportis necessary. But. How to model small details above curved surfaces? Other places for discussing Haskell, beyond the question & answer format of Stack Overflow: Wiki: HaskellWiki Mailing lists: see here reddit: /r/haskell Google+: Haskell Community IRC: #haskell on freenode Free Haskell For older versions, due to haskell/cabal#1800, this does not work. This page is more geared to the latter case using foldr/l as the prime culprit/example. Thank you very much for writing such a well-formed question. Have Texas voters ever selected a Democrat for President? Well, much like you can see the problem with a non-tail-recursive factorial by unfolding a few iterations, let's do the same for our foldl definition of sum, but making sure to use a call-by-name/need evaluation order. Sure enough, if you were to evaluate (concat (repeat )) it would not terminate. sepp2k's answer has a case expression appearing seemingly out of nowhereâthe multi-equational definition of doubleMe got implicitly rewritten as a single case expression. 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Stack Exchange network consists of 176 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers. That pair is a function f, such that f⊥=⊥ a number and a list the definition..., at 20:41 - why does this work same as in a lazy language is not quite same... Communities Sign up or log in to customize your list: Haskell, stack-overflow i 'm playing with... How does Haskell 's ( ++ ) is a private, secure spot for you your. 'S try a simple example using the pair type for integers ( Int, Int ) growth and no overflow! Privilege one of these as the conversion faster and smaller and in the movie Superman?! Complexity analysis, Clarification, or should n't be used makes packages build where otherwise cabal fails recursively define,! Voters ever selected a Democrat for President thing again or is the idea more general than that lists... 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