Recent questions tagged Haskell

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haskell - Why "and []" is True and "or []" is False

Why "and" on an empty list returns true, does it imply that an empty list holds True? Sorry but I cannot read ... [] True Prelude> or [] False See Question&Answers more detail:os...

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

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1.0k views

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haskell - Writing in pointfree style f x = g x x

I am learning Haskell. I'm sorry for asking a very basic question but I cant seem to find the answer. ... using a lambda expression. Thanks See Question&Answers more detail:os...

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

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922 views

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haskell - How to create a type bounded within a certain range

I would like to create a new integral type which is bounded to a certain range. I have tried: data ... perform constraint checks are exported? See Question&Answers more detail:os...

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

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762 views

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haskell - What types of problems helps "higher-kinded polymorphism" solve better?

As I read through some sections in History of Haskell, I came across: However, higher-kinded polymorphism has ... the crisper data declaration. See Question&Answers more detail:os...

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

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991 views

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haskell - What does eta reduce mean in the context of HLint

I'm looking at the tutorial http://haskell.org/haskellwiki/How_to_write_a_Haskell_program import System.Environment main : ... in this context? See Question&Answers more detail:os...

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

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1.2k views

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haskell - Why is GHC complaining about non-exhaustive patterns?

When I compile the following code with GHC (using the -Wall flag): module Main where data Tree a = EmptyTree | Node ... x (Node a left right). See Question&Answers more detail:os...

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

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1.3k views

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haskell - Instance Show for function

I'm currently trying to write a little Show instance for primitive arithmetic functions. Goal ist to make a ... could I tackle that problem? See Question&Answers more detail:os...

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

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808 views

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haskell - How to deal with application state in Gtk2Hs

Trying to learn to write applications with Gtk2Hs I'm getting difficulties bridging the gap between the event driven ... solution to my problem. 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 - subsequences of length n from list performance

I implemented a version of this answer https://stackoverflow.com/a/9920425/1261166 (I don't know what was intended ... faster ways of doing 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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808 views

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haskell - How Monads are considered pure?

I am very much new to Haskell, and really impressed by the language's "architecture", but it still bothers ... something? I am really confused. See Question&Answers more detail:os...

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

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892 views

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haskell - Is the composition of an arbitrary monad with a traversable always a monad?

If I have two monads m and n, and n is traversable, do I necessarily have a composite m-over-n monad? ... any monad over a traversable one. See Question&Answers more detail:os...

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

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896 views

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haskell - How do you compute the difference between successive elements of a list of unknown size, functionally?

In a programming language that is purely functional (like Haskell) or where you are only using it in a ... this kind of thing functionally? See Question&Answers more detail:os...

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

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782 views

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haskell - How to install/use a local version of package using Stack?

The situation is, I am trying to install Netwire using Stack. However, there is a problem in the latest ... relative path looks incredibly ugly. See Question&Answers more detail:os...

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

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1.2k views

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haskell - Why does the 2-tuple Functor instance only apply the function to the second element?

import Control.Applicative main = print $ fmap (*2) (1,2) produces (1,4). I would expect it it to produce ( ... post my own answer in a minute.. See Question&Answers more detail:os...

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

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873 views

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haskell - Infinite type error when defining zip with foldr only; can it be fixed?

(for the context to this see this recent SO entry). I tried to come up with the definition of zip using foldr only: ... with GHCi 7.8.3 on Win7. See Question&Answers more detail:os...

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

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844 views

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haskell - Are Functor instances unique?

I was wondering to what extent Functor instances in Haskell are determined (uniquely) by the functor laws. Since ghc ... . Any thoughts on this? See Question&Answers more detail:os...

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

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778 views

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haskell - Should I use typeclasses or not?

I have some difficulties to understand when use and when not use typeclass in my code. I mean create my own, ... should I use functions? Thanks. See Question&Answers more detail:os...

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

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886 views

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haskell - Seeking constructive criticism on monad implementation

I'm learning monads, this is my first working one (aside from the trivial monad). Feel free to criticize everything in ... = mysum xs >>= add x See Question&Answers more detail:os...

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

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883 views

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haskell - GHCi "let" -- what does it do?

I'd appreciate if someone could point to docs on what "let" does in GHCi, or failing that, explain it ... it's not prefixed by colon. See Question&Answers more detail:os...

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

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828 views

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haskell - What does let 5 = 10 do? Is it not an assignment operation?

If I say let 5 = 10, why does 5 + 1 return 6 instead of 11? See Question&Answers more detail:os...

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

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819 views

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haskell - Constructing efficient monad instances on `Set` (and other containers with constraints) using the continuation monad

Set, similarly to [] has a perfectly defined monadic operations. The problem is that they require that the ... make it exponentially longer?) See Question&Answers more detail:os...

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

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924 views

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haskell - Maybe "kind-of" monad in Python

Trying to find a way to clean up some of my code. So, I have something like this in my Python code: ... possible to write it in another way. See Question&Answers more detail:os...

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

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796 views

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haskell - Do ghc-compiled binaries require GHC or are they self-contained?

If a friend wants to run my Haskell binaries, does he have to first install Haskell, or can he immediately run ... on Mac, Windows, and Linux? See Question&Answers more detail:os...

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

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1.1k views

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haskell - how to implement doubly linked lists

Is it possible to have a doubly linked list in Haskell, and what's the ideal solution to implementing them? I'm ... both up and down the graph. See Question&Answers more detail:os...

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

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1.0k views

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haskell - What is the difference between $ (dollar) and $! (dollar exclamation point)

Can anybody explain the difference in Haskell between the operators ($) and ($!) (dollar sign vs dollar sign ... the difference, if at all? See Question&Answers more detail:os...

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

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820 views

1 answer

haskell - Why recursive `let` make space effcient?

I found this statement while studying Functional Reactive Programming, from "Plugging a Space Leak with an Arrow" ... the computation is shared. See Question&Answers more detail:os...

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

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736 views

1 answer

haskell - What’s an example of a Monad which is an Alternative but not a MonadPlus?

In his answer to the question Distinction between typeclasses MonadPlus, Alternative, and Monoid? , Edward Kmett ... weakly prefer the former. See Question&Answers more detail:os...

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

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817 views

1 answer

haskell - How to hack GHCi (or Hugs) so that it prints Unicode chars unescaped?

Look at the problem: Normally, in the interactive Haskell environment, non-Latin Unicode characters (that make a ... pretty, unescaped fashion.) See Question&Answers more detail:os...

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

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