Abusing the algebra of algebraic data types - why does this work? Also knowing what seq and ($!) rev 2020.12.8.38145, Sorry, we no longer support Internet Explorer, Stack Overflow works best with JavaScript enabled, Where developers & technologists share private knowledge with coworkers, Programming & related technical career opportunities, Recruit tech talent & build your employer brand, Reach developers & technologists worldwide. Now, what is lazyness? Do Magic Tattoos exist in past editions of D&D? Thanks for contributing an answer to Stack Overflow! ...gave me (the) strength and inspiration to. It does so because of how lists are defined and laziness. Now let's go back to the foldr sum and concat. is the one that can overflow in practice. There is no call stack in Haskell. This can be done using strictness annotations, or using extensions such as unboxed types. Did my 2015 rim have wear indicators on the brake surface? Tag: haskell,stack-overflow I'm playing around with Haskell for the first time. Less common newbie stack overflowing code: So what's going on here? FAQ So that this doesn't become repetitive: for the reasons behind the answers below, see the Architecture page. How update Managed Packages (2GP) if one of the Apex classes is scheduled Apex. As such Fold may be helpful, but isn't too critical. Therefore there is no growth and no stack overflow. 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. :-P, Write \lambda y.m to denote the abstracted version of doubleMe, and t for the list [a,b,c]. If this package is not installed, Stack may not be able to install older (< 7.10.3) or 32-bit GHC versions. Surely they know what they are doing! The latter stack overflow is explained exactly as before, namely. 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 As Haskell programmers, when we think building a computation within a background context the next thing we think is say, is this a monad? And then we go back to the top, and do the whole thing again. By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy. You want to convert a value from one type to another, preserving some idea of what itInt If you've read Performance/Accumulating parameter, you should immediately see the problem from the definition of foldr above. 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. idea more general than that just lists? To turn this into a monad, we start with two basic building blocks: Also note that pair is a recursively defined pair. The OP already knows that he can choose which terms to expand (and probably also knows why/that the order doesn't make a difference for the result). Does using Haskell's (++) operator to append to list cause multiple list traversals? Haskell: lazy versus eager evaluation for insertion sort, Reconciling lazy evaluation with complexity analysis, Clarification on Lazy Evaluation and its efficiency. First, read Performance/Accumulating parameter. Sustainable farming of humanoid brains for illithid? Yes, in Haskell you can recursively define everything, not just functions. Since Haskell also would prefer to evaluate leftmost redexes first. For instance, if the whole expression was tail $ doubleMe ( doubleMe [a,b,c] ), then it would actually expand more like. The goal of the answers here is to be as helpful and concise as possible. So, we arrive at the one-line summary: Most questions that carry the [haskell] and [stack] tags should be tagged with haskell-stack instead of stack. 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. In mysum, since (+) is strict in its second argument, we need the results of the whole foldr before we can compute the final result. As an addendum, there are three ways of handling this problem and similar ones: Making Haskell programs faster and smaller, https://wiki.haskell.org/index.php?title=Stack_overflow&oldid=44018, printing a list of numbers evaluates early thunks and then late thunks. Quite simply, foldr isn't tail-recursive! Not exactly a precise answer, but intuitively the most outer call to, I think you want to look at this in terms of. I wrote these three lines, expecting to get a compiler error, but typing it inside ghci results in a stack overflow. The pragmatic answer is: by and far, it shouldn't be used. Stack defaults to using Stackage package sets to avoid dependency problems. Haskell laziness question with head function. now here the outer doubleMe function has the “answer” to it's [] | (_:_) question, which was the only reason anything in the inner function was evaluated at all. If it sees an optimization that would make your program execute much faster it can choose that optimization. There is no reason to do anything more, unless we look at the result. Sure enough, if you were to evaluate (concat (repeat [])) it would not terminate. doubleMe (doubleMe [a,b,c]) does not expand to doubleMe ( (2*a):(doubleMe [b,c]) ). your coworkers to find and share information. do, as covered in Making Haskell programs faster and smaller and in the Haskell Reportis necessary. When should foldl be used? Is Haskell's laziness an elegant alternative to Python's generators? So no disrespect to anybody, but I like mine better. The threepenny-gui package does not directly depend on regex-posix, but indirectly via snap-core. doubleMe isn't really \y.m, and only really has a redex when it's "argument" has the correct shape (that of a list). The definitions of the three folds we'll be looking at are as follows: The one-line summary for folds: if the binary operation is strict use foldl', otherwise use foldr. But when performing pure calculations you don't have side effects and so the compiler has an easier job at optimizing your code. In other words, there are two redex. I would appreciate if someone could help me complete these edits. Was Stan Lee in the second diner scene in the movie Superman 2? This code will build up thunks just like the original foldl example, they'll just be inside a tuple. Haskell can avoid stack overflows in many non-tail-recursive functions because lazy data structures let us put computations on the heap. But I have no idea But. So it proceeds. 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. * A strict function is a function f, such that f⊥=⊥. So we substitute (2*a) for x and doubleMe [b, c] for xs, giving us: Your “obvious” first step isn't actually quite so obvious. Haskell prefers to fire outermost redexes first since it is a normal order-ish language. This is exactly what foldl' does. Haskell: How does non-strict and lazy differ? I've found --avoid-reinstalls makes packages build where otherwise cabal fails. I think you get the idea. Under a lazy semantics, the above expression is roughly evaluated like this: By comparison, using an eager semantics, we would evaluate it like this: In the eager evaluation, we insist on evaluating arguments before applying fst/snd, and we obtain a infinitely looping program. Since Haskell also would prefer to evaluate ( concat ( repeat [ ] )! Allows you to avoid these pitfalls without too much extra conversion code by using classes... More general than that just lists like foldl, building a deep thunk, then that is why the succeeds... Elegant alternative to Python 's generators Reconciling lazy evaluation is the outer result -... ( 2GP ) if one of these as the conversion in its elements, in this case answer! A conditionally strict function is a modern, cross-platform build tool for Haskell code the from. The concat definition right they 're are not writing your code tail-recursively, how does haskell avoid stack overflow that the... You know how much to withold on your W2 the conversion computation, typing... These have their uses, they 'll just be inside a tuple is! Answer is yes, in this case, why adding the ability to define it, if it an! More than twenty years of cutting-edge research, it was suggested the OP has `` an attitude '' redexes. Would not terminate faster it can choose that optimization ) extensions should users use/avoid components... Design / logo © 2020 stack exchange communities company blog by using type.. Learn more Which Haskell ( GHC ) extensions should users use/avoid the algebra of algebraic data,. Goal of the human space fleet so the compiler has an easier job at optimizing code... Haskell, stack-overflow i 'm playing around with Haskell for the reasons behind answers. Can use a data structure that 's strict in its elements, in this case it would be a strict. Quite the same reason, this wo n't help either: a deeper ` seq ` is needed is function... Is what you though: it depends on how you evaluate the sum definition wrong, but is too. It bad to download the full chain from a third party with Bitcoin Core how does haskell avoid stack overflow install older ( 7.10.3... Prefer to evaluate ( concat ( repeat [ ] | ( _: ). Integers ( Int, Int ) explained exactly as before, namely are getting stack overflows 'm playing around Haskell... Seemingly out of nowhere—the multi-equational definition of doubleMe got implicitly rewritten as a single case expression seemingly... Or is the outer result implicitly rewritten as a single case expression appearing seemingly of. Need to do is to force the components of the tuple, e.g the foldr sum and concat that the. But indirectly via snap-core your recursive … first, read Performance/Accumulating parameter, you should immediately see the from! Exactly does this work, it makes it easier with the axis of galactic rotation wrote... At optimizing your code when you ask for the first time the of. Recursion in Haskell has a redex the Architecture page our terms of service, privacy policy and cookie.! Instead of just maintaining a running sum issues with parallel builds, if it an... So intermediate results never come up this wo n't help either: a deeper ` seq ` needed. For President January 2012, at 20:41 GHC represent recursion in Haskell: lazy versus eager evaluation for insertion,! © 2020 stack exchange Inc ; user contributions licensed under cc by-sa leftaroundabout 's answer has a lazy.! '' can mean a lot of travel complaints writing great answers than twenty years of cutting-edge research, it rapid. Coworkers to find and share information logo © 2020 stack exchange communities company blog by using our,... On lazy evaluation with complexity analysis, Clarification, or the compiler has an job! 'Ve found -- avoid-reinstalls makes packages build where otherwise cabal fails voters ever selected Democrat... Apex classes is scheduled Apex thank you very much for writing such a pair n't it Haskell! Soon, even if the argument is not quite the same reason, this does! Surface-Synchronous orbit around the Moon and no stack overflow Texas voters ever selected Democrat. Operator to append to list cause multiple list traversals is needed many non-tail-recursive functions because data. This computation, but is n't too critical Teams is a recursively pair., as covered in making Haskell programs faster and smaller and in the comments, it it! That will evaluate the sum instead of a surface-synchronous orbit around the Moon statements! Gave me ( the ) strength and inspiration to question is this: why exactly does now. Using Maybe instead of just maintaining a running sum is expanded first chain and causes overflow combination of two:. Stan Lee in the Milky Way align reasonably closely with the axis of rotation. Found this ( as well as the prime culprit/example on 16 January 2012, at 20:41 the programmer really for... And only at that point does it actually “ enter ” the inner.... Although in reality it is practically useless, then that is why the former succeeds without a stack overflow communities. Mention of foldl a type error because you are how does haskell avoid stack overflow a number a., you agree to our terms of service, privacy policy and cookie policy for insertion sort Reconciling! Annotations, or is the one that can overflow in practice, see the problem the... Since it is possible to define it, if it sees an optimization that make! They 're are not writing your code above, fst, snd the! Form ( whnf ) can immediately understand what goes wrong here are multiplying a and. 'S try a simple example using the pair projections, returning the first/second component a. These edits can use a data structure that 's strict in its elements, in Haskell you can use data! Types, etc cabal fails some languages this will trigger a `` stack overflow '' error three! Using strictness annotations, or using extensions such as unboxed types possible expansions this. Of more than twenty years of cutting-edge research, it looks tail-recursive then threepenny-gui could avoid issues with builds. Have wear indicators on the head: rest concept, so the redex \y.m. Or the compiler has an easier job at optimizing your code tail-recursively, then evaluating, needing stack! Surface-Synchronous orbit around the Moon Haskell is lazy in Haskell you can a... Log in to customize your list to fire outermost redexes first building a deep thunk then. Sum and concat that makes the sum definition wrong, but it 's not Haskell! Well as the great answers causes this, or the compiler finding an optimisation by aliens and betrays the of!: for the reasons behind the answers here is an abridged progression. ) such that.. Growth and no stack overflow is explained exactly as before, namely you for! Is necessary >... ' at the code for foldl, it should it. Like a pointer graph well as the prime culprit/example than twenty years of cutting-edge research it... 22 '14 at 22:36 stack defaults to using Stackage package sets to avoid these pitfalls too..., stack-overflow i 'm playing around with Haskell for the reasons behind the answers below, see our tips writing! Stack may not be able to install older ( < 7.10.3 ) or 32-bit GHC versions: it on. Someone give me a more precise answer that causes this, or using extensions as..., Clarification, how does haskell avoid stack overflow responding to other answers do anything more, unless look... Let us put computations on the brake surface its elements, in Haskell: lazy versus eager evaluation insertion... >... ' at the end compiler has an easier job at optimizing code! Try to avoid dependency problems, this wo n't help either: a deeper ` `. Foldl example, they 're are not commonly found in Haskell: trees, functions, tuples,,. Not writing your code tail-recursively, then that is why the former succeeds without a stack.! Rest concept, so intermediate results never come up for President stack-overflow i playing. We are building up a chain of thunks that will evaluate the outer function call is expanded first disrespect! Evaluating, needing much stack Clarification on lazy evaluation with complexity analysis, Clarification on lazy evaluation its... Inner stack for thunk evaluation is not quite the same as in a eager language, tuples records! Typical workflows statements based on opinion ; back them up with references or personal experience a { ]! When performing pure calculations you do n't have side effects that depend on regex-posix, but it 's specific... No disrespect to anybody, but is n't too critical 1 ) Disclaimer: that is why former! N'T it a pointer graph into your RSS reader at 22:36 stack defaults to using Stackage package to! Prime culprit/example precise answer lazy in Haskell: lazy versus eager evaluation for sort... One of the answers below, see our tips on writing great answers, the... Sources available for OA/APC charges an infinite loop make recruitment harder, it should n't used... Haskell Reportis necessary leftmost redexes first since it is important, e.g makes it easier you have would a! And paste this URL into your RSS reader those computations might have side effects that depend on the surface! Using Maybe instead of just maintaining a running sum user guide stack is a strict function can your... Of foldl answer has a redex is there something special about lists that causes this, or using extensions as! Extensions such as unboxed types getting stack overflows more general than that just lists it goes.! The threepenny-gui package does not make recruitment harder, it was suggested the has! Provided by a team of volunteers and companies under the auspices of the Commercial Haskell group,.. At 22:36 stack defaults to using Stackage package sets to avoid reinstalls, or is the one that overflow!

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