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Questions tagged [fold]

In functional programming, a fold, also known variously as reduction, accumulation, or catamorphism, is a type of higher-order function that recursively applies a transformation to a data structure, "collapsing" it to a summary value

In functional programming, a fold, also known variously as , accumulation, or catamorphism, is a type of higher-order function that recursively applies a transformation to a data structure, "collapsing" it to a summary value.

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Example of the difference between List.fold and List.foldBack

My understanding of the difference between List.fold and List.foldBack is that foldBack iterates over its list in a reverse order. Both functions accumulate a result from the items in the list. I'm having trouble coming up with a good example where…
Mike Coxeter
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python reduce to find the union of sets

I am trying to find the union of set of sets. Specifically I want the union of the list of nodes for each key in the dictionary of networkx graphs called periodic_gs. I would like to use the reduce function as it seems reasonable to take the union…
CodeKingPlusPlus
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Scala Vector fold syntax (/: and :\ and /:\)

Can someone provide some examples for how /: :\ and /:\ Actually get used? I assume they're shortcuts to the reduce / fold methods, but there's no examples on how they actually get used in the Scala docs, and they're impossible to google / search…
chbrown
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Why does foldl' use a lot of RAM with complex data structures?

Lazy fold uses a lot of RAM. In Data.List, foldl' provides a left fold that uses strict evaluation. For example, the following computes the sum of 10 million zeros with little increase in RAM usage. sum0 = foldl' (+) 0 (replicate 10000000…
W. Zhu
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Can't perform I/O in foldr?

I have a Data.Map structure that maps Strings to Stringss. For whatever reason, I want to print the contents of the map in the format key: value using foldrWithKey, like so: M.foldrWithKey (\k v b -> putStrLn (k++": "++v++"\n")) (return ())…
user500944
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foldr and foldl further explanations and examples

I've looked at different folds and folding in general as well as a few others and they explain it fairly well. I'm still having trouble on how a lambda would work in this case. foldr (\y ys -> ys ++ [y]) [] [1,2,3] Could someone go through that…
Matt
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When is a composition of catamorphisms a catamorphism?

From page 3 of http://research.microsoft.com/en-us/um/people/emeijer/Papers/meijer94more.pdf: it is not true in general that catamorphisms are closed under composition Under what conditions do catamorphisms compose to a catamorphism? More…
Sebastien
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Fold expression with comma operator and variadic template parameter pack

#include using namespace std; template void output_argus(Args&&... args) { ((cout << args << '\n'), ...); // #1 (... , (cout << args << '\n')); // #2 } int main() { output_argus(1, "test",…
Ke Zhang
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Real world examples of using reduceRight in JavaScript

A while ago, I posted a question on StackOverflow showing that the native implementation of reduceRight in JavaScript is annoying. Hence, I created a Haskell-style foldr function as a remedy: function foldr(array, callback, initial) { var length…
Aadit M Shah
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Practical use of `foldl`

Today when I was working on one little script I used foldl instead of foldl'. I got stack overflow, so I imported Data.List (foldl') and was happy with this. And this is my default workflow with foldl. Just use foldl' when lazy version falls to…
d12frosted
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Why does Haskell's foldr NOT stackoverflow while the same Scala implementation does?

I am reading FP in Scala. Exercise 3.10 says that foldRight overflows (See images below). As far as I know , however foldr in Haskell does not. http://www.haskell.org/haskellwiki/ -- if the list is empty, the result is the initial value z; else --…
jhegedus
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Foldr/Foldl for free when Tree is implementing Foldable foldMap?

I am a beginner at Haskell and learning from "Learn You a Haskell". There's something I don't understand about the Tree implementation of Foldable. instance F.Foldable Tree where foldMap f Empty = mempty foldMap f (Node x l r) =…
Roely de Vries
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Code folding is not saved in my vimrc

I added the following code to my .vimrc: " save and restore folds when a file is closed and re-opened autocmd BufWinLeave *.* mkview autocmd BufWinEnter *.* silent loadview HTML and CSS documents save and restore their folds but code folding is…
alexchenco
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Please explain in the simplest, most jargon-free English possible, the "universal property of fold"?

I am working through "Real World Haskell", which led to to a free PDF called "A tutorial on the universality and expressiveness of fold". It makes the point that a "fold" is "universal". I am wrestling with his definition of "universal", and would…
Charlie Flowers
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Is there a way to fold with index in Rust?

In Ruby, if I had an array a = [1, 2, 3, 4, 5] and I wanted to get the sum of each element times its index I could do a.each.with_index.inject(0) {|s,(i,j)| s + i*j} Is there an idiomatic way to do the same thing in Rust? So far, I…
Eli Sadoff
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