Recent questions tagged Haskell

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haskell - Histomorphisms, Zygomorphisms and Futumorphisms specialised to lists

I ended up figuring it out. See the video and slides of a talk I gave: slides/pdf video Original question: ... histo, zygo and futu for lists? See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - How can I uninstall a version of a Cabal package?

Happstack Lite is breaking on me because it's getting blaze-html version 0.5 and it wants version 0.4. ... the version you wish to eliminate. See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - What is the difference between Cabal and Stack?

Yesterday I learnt about a new Haskell tool called Stack. At the first blush, it looks like it does much the same ... Stack do that Cabal can't? See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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880 views

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haskell - mtl, transformers, monads-fd, monadLib, and the paradox of choice

Hackage has several packages for monad transformers: mtl: Monad transformer library transformers: Concrete functor and monad ... you and I use? See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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737 views

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haskell - What are arrows, and how can I use them?

I tried to learn the meaning of arrows, but I didn't understand them. I used the Wikibooks tutorial. I think ... and how I can use them? See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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766 views

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haskell - Mathematica: what is symbolic programming?

I am a big fan of Stephen Wolfram, but he is definitely one not shy of tooting his own horn. In many ... functional languages (such as Haskell)? See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Type of fun g x = ys where ys = [x] ++ filter (curry g x) ys?

I'm trying to understand why the type of fun g x = ys where ys = [x] ++ filter (curry g x) ys is ( ... c But I don't understand how to continue. See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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924 views

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haskell - Monomorphism Restriction, ghci, and let expression

This expression is incorrect. f = show However, in ghci this is legit let f = show Moreover, its type ... any explanation of this phenomenon? See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Kleisli Arrow in Netwire 5?

I am trying to create a game using Haskell + Netwire 5 (+ SDL). Now I am working on the output part, where I ... testRect = SDL.Rect 100 100 0 0 See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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1.9k views

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haskell - No instance for (Num (Int -> Int)) arising from the literal `5'

I have the following function: f :: (Int -> Int) -> Int f = undefined Now I want to call f with 5 (which ... it = f 5 Why did Num appear here? See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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949 views

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haskell - Why is the `mappend` infix alias `<>` instead of `+`?

In Haskell, why is the infix alias of mappend (from class Monoid) <> instead of +? In algebra courses + is ... the binary operator of a monoid. See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - How to use variable from do block assignment line in a where clause?

I have the following sample of code: {-# LANGUAGE ScopedTypeVariables #-} main = do putStrLn "Please input a ... available in the where clause? See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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846 views

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haskell - It works when loaded from file, but not when typed into ghci. Why?

If I put the following 2 lines into foobar.hs f 1 = 1 f x = f (x-1) then $ ghci > :load foobar. ... ^CInterrupted. then it does not return. Why? See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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1.4k views

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haskell - How to convert arbitrary type to string, without adding extra quotes to strings?

I want to define a function which converts to strings, like the following 'toString': toString 1 = "1" toString ... be better to use instead? See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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1.2k views

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haskell - Implementing take using foldr

This is my take version using foldr: myTake n list = foldr step [] list where step x y | (length ... order. Perhaps something obvious I missed? See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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haskell - Convert a "do" notation with more than two actions to use the bind function

I know that the following "do" notation's "bind" function is equivalent to getLine >>= line -> putStrLn do ... <- getLine return (line1,line2) See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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1.1k views

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haskell - Why DuplicateRecordFields cannot have type inference?

Related post: How to disambiguate selector function? https://ghc.haskell.org/trac/ghc/wiki/Records/ ... current type system? See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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899 views

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haskell - why do these pattern matches overlap?

Why do my pattern matches inside a do block overlap? (q, m) <- newRq let m2 = appendMsg "first" key m (q4, m4) ... q m2, otherwise to (q, m2). See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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1.0k views

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haskell - Generating a Truth Table

If I have some values A B C, is is possible to generate a list of all their possible truth values: the ... for an arbitrary number of variables. See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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839 views

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haskell - How to view higher-order functions and IO-actions from a mathematical perspective?

I am trying to understand functional programming from first principles, yet I am stuck on the interface between ... mental model outlined above? See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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964 views

1 answer

haskell - How to have multiple infinite ranges in list comprehensions?

In haskell I have a list comprehension like this: sq = [(x,y,z) | x <- v, y <- v, z <- ... a way to handle multiple infinite ranges? Thanks See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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843 views

1 answer

haskell - Incomplete type signature

Lets say we've got a function like f below, that returns a monad. However, where you see Int, pretend it ... GHC extensions are okay in answers. See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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869 views

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haskell - Why is the strictness-introducing function called seq?

I understand the seq function and why it's necessary to introduce strictness for efficiency. What I don't ... to do with strictness)? See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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878 views

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haskell - Can I write a higher order type for a -> b -> *?

I understand that (->) a is a higher order type of kind * -> *, that when applied to a type argument b ... because it went over my head :-) See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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872 views

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haskell - Is there any connection between `a :~: b` and `(a :== b) :~: True`?

Is there any connection implemented between propositional and promoted equality? Let's say I have prf :: x :~: y in scope ... 'True ~ (x :== y) See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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933 views

1 answer

haskell - Why does ghci say that 1.1 + 1.1 + 1.1 > 3.3 is True?

I've been going through a Haskell tutorial recently and noticed this behaviour when trying some simple Haskell expressions ... know why that is? See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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962 views

1 answer

haskell - Kind Signatures

I am going through the Haskell wiki books GADTS https://en.wikibooks.org/wiki/Haskell/GADT guide. I was ... is the Kind Signature doing? See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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1.0k views

1 answer

haskell - Can you use pattern matching to bind the last element of a list?

Since there is a way to bind the head and tail of a list via pattern matching, I'm wondering if you can use ... bind the last element of a list? See Question&Answers more detail:os...

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

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