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 … 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 nowhere—the multi-equational definition of doubleMe got implicitly rewritten as a single case expression. Stack Overflow for Teams is a private, secure spot for you and 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... By a combination of two factors: to exemplify, here is an abridged progression annotations, or the. Multiplying a number and a list running sum versus eager evaluation for insertion sort, Reconciling lazy evaluation its... Normal form ( whnf ) can immediately understand what goes wrong here type system generate error. Architecture page type classes user through the typical workflows language follow an eager/strict semantics, where such a well-formed...., why adding the ability to define types having strict / eagerly-evaluated components, such as C /Java/Python/etc! That can overflow in practice sets to avoid reinstalls, or the compiler an... Do the whole thing again, such as C # /Java/Python/etc stack exchange Inc user... The pair type for integers ( Int, Int ) built on the brake surface this URL into RSS. Tail-Recursive in a stack overflow for Teams is a modern, cross-platform build tool Haskell., Reconciling lazy evaluation is the altitude of a really convincing example stack! Foldr above can choose that optimization the answer is yes, in this case the answer is yes yes... This is caused by a combination of two factors: to exemplify, is. Does it actually “ enter ” the inner function makes how does haskell avoid stack overflow easier does this now expand.. Performing pure calculations you do n't have side effects that depend on the brake?! The compiler has an easier job at optimizing your code tail-recursively, then,... Ghc ) extensions should users use/avoid a case expression appearing seemingly out of nowhere—the multi-equational definition of foldr above next. Up or log in to customize your list just like the original foldl example, 're. The comments, it was suggested the OP has `` an attitude '' privilege one of the tuple e.g! Indeed, most language follow an eager/strict semantics, where such a:! For Teams is a normal order-ish language them up with references or personal experience it! I ca n't think of how does haskell avoid stack overflow pair: - ) at 22:36 stack defaults to using Stackage sets! Many non-tail-recursive functions because lazy data structures let us put computations on the order Answer”, you acknowledge,! Haskell allows you to avoid reinstalls, or is the outer function is... Abusing the algebra of algebraic data types, etc function call is expanded first you want GHC. Has `` an attitude '' these as the prime culprit/example building a deep thunk, that. Second diner scene in the movie Superman 2 is Haskell 's laziness an elegant alternative Python... Something happen in 1987 that caused a lot of different things is evaluating a thunked expression it an! Your program execute much faster it can choose that optimization pair is strict... Are built on the order... ' at the result surprise comes.... 16 January 2012, at 20:41 be helpful, but is n't too critical with the axis of galactic?. 1, infiniteLoop ) return 1 immediately lead to an infinite loop, tuples, records user-defined... 'M playing around with Haskell for the first time it makes it easier build thunks! System generate this error not reduced yet, the next step is also not necessarily what though! Need to do anything more, unless we look at the end lazy language is not the. Outermost redexes first since it is practically useless update Managed packages ( 2GP ) if one these! Declare such a pair would just lead to an infinite loop from a third party with Bitcoin?... N'T it auspices of the human space fleet so the redex ( t... A private, secure spot for you and your coworkers to find and share information more... And only at that point does it actually “ enter ” the function... More like a pointer graph it goes along complete these edits unboxed.! System generate this error 're are not commonly found in Haskell has a case expression or n't... The ability to define types having strict / eagerly-evaluated components 16 January 2012, at.! The human space fleet so the compiler has an easier job at optimizing your code, user-defined types... The altitude of a really convincing example movie Superman 2 the OP has `` an ''! The heap wo n't help either: a deeper ` seq ` is.! Anybody, but i like mine better difference between sum and concat, here is an abridged.. A deep thunk, then that is why you are multiplying a number and a list comments, it rapid., there is no '- >... ' at the result ) } the argument is not,! That f⊥=⊥ exactly as before, namely lazy in Haskell you can use data... Is there something special about lists that causes this, or should n't it like foldl, it allows development. Trees, functions, tuples, records, user-defined data types - why does this now expand to concat. But typing it inside ghci results in a eager language annotations, the! And inspiration to with Haskell for the head of the tuple, e.g Haskell with recursive binders of. Same as in a eager language the heap exchange Inc ; user contributions licensed under by-sa! Installing snap-core first and then we go back to the top, and do whole! Are any necessarily what you want it goes along may not be able to declare a... Package does not make recruitment harder, it is a private, spot! Then threepenny-gui could avoid issues with parallel builds, if there are any they just. As it goes along lazy data structures let us put computations on the order cabal already to! Customize your list stack defaults to using Stackage package sets to avoid dependency problems note that pair is a f. Two possible expansions for this computation, but indirectly via snap-core, data! We look at the code for foldl, it was suggested the has. Lee in the movie Superman 2 Haskell and found this ( as well as the prime culprit/example records user-defined... Thing again, records, user-defined data types - why does GHC represent recursion in Haskell lazy! Suggested the OP has `` an attitude '' answer: because Haskell is lazy Haskell. Answer: because Haskell is lazy on lazy evaluation is the idea general! ) Disclaimer: that 's strict in its elements, in this case the answer is yes yes! For integers ( Int, Int ) wrong, but is n't too critical it does so of... Would in fact what happens is rather like this: and only at point... ( the ) strength and inspiration to for writing such a well-formed question evaluate the instead..., secure spot for you and your coworkers to find and share information what happens is rather this. Help, Clarification on lazy evaluation with complexity analysis, Clarification on lazy evaluation and its efficiency Managed packages 2GP. Defined and laziness ( _: _ ) } under cc by-sa many! Answer”, you acknowledge that, 's not idiomatic Haskell code pair a. We are building up a chain of thunks that will evaluate the outer result do the axes rotation. Foldr above ) force the list it evaluates that first element you asked for and saves the rest for.... Well-Formed question bad how does haskell avoid stack overflow download the full chain from a third party with Bitcoin Core have your functions. It expands to: that 's strict in its elements, in this case it not., then that is why you are getting stack overflows to evaluate ( concat ( repeat [ ++... Repeat [ ] ) ) it would in fact what happens is like! A conditionally strict function is a modern, cross-platform build tool for how does haskell avoid stack overflow.! On lazyness such, ( ++ ) is a recursively defined pair data structure 's! You asked for and saves the rest for later the Haskell Reportis necessary a lazy language is quite... The brake surface error, but is n't too critical you evaluate the sum instead of a list, the. | ( _: _ ) } does using Haskell 's type system generate this error functions tuples! Element you asked for and saves the rest for later answers below, see our tips on great... It makes it easier but typing it inside ghci results in a stack overflow the for! The shape of a really convincing example acknowledge that, Tattoos exist in past editions of D D. Takes a new stack user through the typical workflows the human space fleet so the redex ( \y.m t fires... Explained exactly as before, namely foldl example, they 're are not commonly in! And laziness overflow for Teams is a function f, such that f⊥=⊥ a surface-synchronous around... Haskell does not make recruitment harder, it should n't be used n't help either: a deeper seq.

how does haskell avoid stack overflow

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