Recent questions tagged Haskell

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haskell - How to compare two functions for equivalence, as in (λx.2*x) == (λx.x+x)?

Is there a way to compare two functions for equality? For example, (λx.2*x) == (λx.x+x) should ... true, because those are obviously equivalent. See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Why is lazy evaluation useful?

I have long been wondering why lazy evaluation is useful. I have yet to have anyone explain to me in a way ... Note: I do not mean memoization. See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Ordering of parameters to make use of currying

I have twice recently refactored code in order to change the order of parameters because there was too much code ... the best use of currying? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - What constitutes a fold for types other than list?

Consider a single-linked list. It looks something like data List x = Node x (List x) | End It is ... most commonly-used example of folding. See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Why doesn't this function work if I use "[xs]" instead of "xs"?

split :: [a] -> Int -> ([a], [a]) split [xs] n = (take n [xs], drop n [xs]) The same code works if ... under the hood. Test input: [1,2,3] 2. See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Euler 43 - is there a monad to help write this list comprehension?

Here is a way to solve Euler problem 43 (please let me know if this doesn't give the correct answer). Is ... 9] main = do print $ pandigitals See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Composing function composition: How does (.).(.) work?

(.) takes two functions that take one value and return a value: (.) :: (b -> c) -> (a -> b) -> a - ... say it is that the types don't match up. See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - How to extract value from monadic action

Is there a built-in function with signature :: (Monad m) => m a -> a ? Hoogle tells that there is no such function. Can you explain why? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Scala: How to define "generic" function parameters?

I am trying to learn Scala now, with a little bit of experience in Haskell. One thing that stood out as odd ... type that defines a + operator? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - DatatypeContexts Deprecated in Latest GHC: Why?

I was just doing some Haskell development and I recompiled some old code on a new version of GHC: The ... the same or similar functionality? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - How do I use fix, and how does it work?

I was a bit confused by the documentation for fix (although I think I understand what it's supposed to do now), ... ). What am I doing wrong? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - What is indexed monad?

What is indexed monad and the motivation for this monad? I have read that it helps to keep track of the side ... (or any other valid example)? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Using return vs. not using return in the list monad

I started my Grand Haskell Crusade (GHC :) ) and I am a bit confused with monads and IO functions. Could anyone explain ... f2 [[1,2],[2,3]] See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - IO happens out of order when using getLine and putStr

I'm a Haskell beginner, I'm just beginning to wrap my head around Monads, but I don't really get it ... the interpreter and fails when compiled. See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Positive integer type

In many articles about Haskell they say it allows to make some checks during compile time instead of run time. So, I ... zero. How can I do it? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Confused by the meaning of the 'Alternative' type class and its relationship to other type classes

I've been going through the Typeclassopedia to learn the type classes. I'm stuck understanding Alternative (and ... or means something different -- from Monoid. Question&Answers:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - What are paramorphisms?

Reading through this classic paper, I'm stuck on paramorphisms. Unfortunately the section is quite thin, and ... as references, but not as learning materials. Question&Answers:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Non exhaustive pattern in function in GHCi

I want to make a function that displays the last element of a list. This is my code: ghci> let myLast ... I have included all the possibilities. Any ideas? Question&Answers:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Explicitly import instances

How do I explicitly import typeclass instances? Also, how do I do this with a qualified import? ... because the Monad instance is implicitly imported. Question&Answers:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - How to use (->) instances of Monad and confusion about (->)

At different questions I've found hints in comments concerning using the (->) instance of Monads e.g. for ... Haskell somewhere involves (->) or "Monad". Question&Answers:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Concrete example showing that monads are not closed under composition (with proof)?

It is well-known that applicative functors are closed under composition but monads are not. However, I have been ... a proof of the above example in particular. Question&Answers:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Writing foldl using foldr

In Real World Haskell, Chapter 4. on Functional Programming: Write foldl with foldr: -- file: ch04/Fold.hs ... uses 3 parameters, I'm complelely confused! Question&Answers:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - foldl versus foldr behavior with infinite lists

The code for the myAny function in this question uses foldr. It stops processing an infinite list when the ... the function behaves. How is this wrong? Question&Answers:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - A monad is just a monoid in the category of endofunctors, what's the problem?

Who first said the following? A monad is just a monoid in the category of endofunctors, what's the problem ... who doesn't have much Haskell experience)? Question&Answers:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Finite comprehension of an infinite list

I typed the following into ghci, thinking that one of two things would happen: 1) The interpreter would hang ... a finite comprehension out of an infinite list? Question&Answers:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - What is the purpose of Rank2Types?

I am not really proficient in Haskell, so this might be a very easy question. What language ... functions in Haskell already support polymorphic arguments? Question&Answers:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Multi-line commands in GHCi

I am having problem in entering multi-line commands in ghci. The following 2-line code works from a file: addTwo :: ... I am using WinGHCi, version 2011.2.0.1 Question&Answers:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - What's so bad about Lazy I/O?

I've generally heard that production code should avoid using Lazy I/O. My question is, why? Is it ever OK ... makes the alternatives (e.g. enumerators) better? Question&Answers:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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