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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...
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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...
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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...
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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...
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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...
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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...
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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...
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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...
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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...
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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...
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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...
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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...
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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...
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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...
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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...
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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...
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Oct 17, 2021
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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...
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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...
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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...
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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...
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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...
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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...
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Oct 17, 2021
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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...
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Oct 17, 2021
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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...
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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...
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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...
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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...
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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...
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