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    Is there a way to compare two functions for equality? For example, (λx.2*x) == (λx.x+x) should return 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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    I have long been wondering why lazy evaluation is useful. I have yet to have anyone explain to me in a way that makes sense; mostly it ends up boiling down to "trust me". 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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    I have twice recently refactored code in order to change the order of parameters because there was too much code where hacks like flip or x -> foo bar x ... will help me to make 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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    Consider a single-linked list. It looks something like data List x = Node x (List x) | End It is natural to define a folding function such ... property of the 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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    split :: [a] -> Int -> ([a], [a]) split [xs] n = (take n [xs], drop n [xs]) The same code works if I give the variable as xs instead of ... clear what is happening 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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    Here is a way to solve Euler problem 43 (please let me know if this doesn't give the correct answer). Is there a monad or some other syntatic sugar ... pandigitalUS [0..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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    (.) takes two functions that take one value and return a value: (.) :: (b -> c) -> (a -> b) -> a -> c Since (.) takes two arguments, I ... Maybe a better way to 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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    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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    I am trying to learn Scala now, with a little bit of experience in Haskell. One thing that stood out as odd to me is that all function parameters in ... which works for any 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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    I was just doing some Haskell development and I recompiled some old code on a new version of GHC: The Glorious Glasgow Haskell Compilation System, ... instead to achieve 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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    I was a bit confused by the documentation for fix (although I think I understand what it's supposed to do now), so I looked at the source code. That left ... eating up no CPU time). 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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    What is indexed monad and the motivation for this monad? I have read that it helps to keep track of the side effects. But the type signature and ... of side effects (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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    I started my Grand Haskell Crusade (GHC :) ) and I am a bit confused with monads and IO functions. Could anyone explain simply what is the difference between those two ... ,3] *Main> 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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    I'm a Haskell beginner, I'm just beginning to wrap my head around Monads, but I don't really get it yet. I'm writing a game that ... it works correctly in 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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    In many articles about Haskell they say it allows to make some checks during compile time instead of run time. So, I want to implement the simplest check ... integers greater than 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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    I've been going through the Typeclassopedia to learn the type classes. I'm stuck understanding Alternative (and MonadPlus, for that matter). The problems I ... 's different -- 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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    Reading through this classic paper, I'm stuck on paramorphisms. Unfortunately the section is quite thin, and the Wikipedia page doesn't say anything. My ... may be helpful 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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    I want to make a function that displays the last element of a list. This is my code: ghci> let myLast :: [a] -> a ghci> let myLast [] = ... missing a case, but I think 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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    How do I explicitly import typeclass instances? Also, how do I do this with a qualified import? Currently, I'm doing import Control.Monad.Error ... with either one, 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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    At different questions I've found hints in comments concerning using the (->) instance of Monads e.g. for realizing point-free style. As for me, ... nearly every question about 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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    It is well-known that applicative functors are closed under composition but monads are not. However, I have been having trouble finding a concrete ... looking for 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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    In Real World Haskell, Chapter 4. on Functional Programming: Write foldl with foldr: -- file: ch04/Fold.hs myFoldl :: (a -> b -> a) ... in the myFoldl's implementation 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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    The code for the myAny function in this question uses foldr. It stops processing an infinite list when the predicate is satisfied. I rewrote it using foldl: ... is not the way 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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    Who first said the following? A monad is just a monoid in the category of endofunctors, what's the problem? And on a less important note, is ... can be understood by someone 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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    I typed the following into ghci, thinking that one of two things would happen: 1) The interpreter would hang, searching every member of an ... predicate which makes 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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    I am not really proficient in Haskell, so this might be a very easy question. What language limitation do Rank2Types solve? Don't 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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228 views
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    I am having problem in entering multi-line commands in ghci. The following 2-line code works from a file: addTwo :: Int -> Int -> Int addTwo x y = x ... should not be the case. 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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    I've generally heard that production code should avoid using Lazy I/O. My question is, why? Is it ever OK to use Lazy I/O outside of just toying around? And what 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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