Questions tagged [haskell]
Haskell is a purely functional programming language featuring strong static typing, lazy evaluation, extensive parallelism and concurrency support, and unique abstraction capabilities.
haskell
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What is a monad?
Having briefly looked at Haskell recently, what would be a brief, succinct, practical explanation as to what a monad essentially is?
I have found most explanations I've come across to be fairly ...
930
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What part of Hindley-Milner do you not understand?
I swear there used to be a T-shirt for sale featuring the immortal words:
What part of
do you not understand?
In my case, the answer would be... all of it!
In particular, I often see notation like ...
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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?
And on a less important note, is this true and if so could you give an explanation (...
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What is the difference between . (dot) and $ (dollar sign)?
What is the difference between the dot (.) and the dollar sign ($)?
As I understand it, they are both syntactic sugar for not needing to use parentheses.
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Getting started with Haskell
For a few days I've tried to wrap my head around the functional programming paradigm in Haskell. I've done this by reading tutorials and watching screencasts, but nothing really seems to stick.
Now, ...
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answers
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Speed comparison with Project Euler: C vs Python vs Erlang vs Haskell
I have taken Problem #12 from Project Euler as a programming exercise and to compare my (surely not optimal) implementations in C, Python, Erlang and Haskell. In order to get some higher execution ...
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answers
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Large-scale design in Haskell? [closed]
What is a good way to design/structure large functional programs, especially in Haskell?
I've been through a bunch of the tutorials (Write Yourself a Scheme being my favorite, with Real World Haskell ...
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What is Haskell used for in the real world? [closed]
There is a lot of hype around Haskell, however, it is hard to get information on how it is used in the real world applications. What are the most popular projects / usages of Haskell and why it excels ...
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What are free monads?
I've seen the term Free Monad pop up every now and then for some time, but everyone just seems to use/discuss them without giving an explanation of what they are. So: what are free monads? (I'd say I'...
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Is functional GUI programming possible? [closed]
I've recently caught the FP bug (trying to learn Haskell), and I've been really impressed with what I've seen so far (first-class functions, lazy evaluation, and all the other goodies). I'm no expert ...
408
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Why do we need monads?
In my humble opinion the answers to the famous question "What is a monad?", especially the most voted ones, try to explain what is a monad without clearly explaining why monads are really necessary. ...
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Transitivity of Auto-Specialization in GHC
From the docs for GHC 7.6:
[Y]ou often don't even need the SPECIALIZE pragma in the first place. When compiling a module M, GHC's optimiser (with -O) automatically considers each top-level ...
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votes
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answers
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What does the `forall` keyword in Haskell/GHC do?
I'm beginning to understand how the forall keyword is used in so-called "existential types" like this:
data ShowBox = forall s. Show s => SB s
This is only a subset, however, of how ...
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votes
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What does "coalgebra" mean in the context of programming?
I have heard the term "coalgebras" several times in functional programming and PLT circles, especially when the discussion is about objects, comonads, lenses, and such. Googling this term gives pages ...
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Good Haskell source to read and learn from [closed]
What are some open source programs that use Haskell and can be considered to be good quality modern Haskell? The larger the code base, the better.
I want to learn from their source code. I feel I'm ...
345
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Abusing the algebra of algebraic data types - why does this work?
The 'algebraic' expression for algebraic data types looks very suggestive to someone with a background in mathematics. Let me try to explain what I mean.
Having defined the basic types
Product •
...
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Why is Haskell (GHC) so darn fast?
Haskell (with the GHC compiler) is a lot faster than you'd expect. Used correctly, it can get close-ish to low-level languages. (A favorite thing for Haskellers to do is to try and get within 5% of C (...
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Extremely small or NaN values appear in training neural network
I'm trying to implement a neural network architecture in Haskell, and use it on MNIST.
I'm using the hmatrix package for linear algebra.
My training framework is built using the pipes package.
My code ...
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votes
9
answers
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What is Weak Head Normal Form?
What does Weak Head Normal Form (WHNF) mean? What does Head Normal form (HNF) and Normal Form (NF) mean?
Real World Haskell states:
The familiar seq function evaluates an expression to what we
call ...
user295190
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votes
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answers
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Techniques for Tracing Constraints
Here's the scenario: I've written some code with a type signature and GHC complains could not deduce x ~ y for some x and y. You can usually throw GHC a bone and simply add the isomorphism to the ...
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What does the exclamation mark mean in a Haskell declaration?
I came across the following definition as I try to learn Haskell using a real project to drive it. I don't understand what the exclamation mark in front of each argument means and my books didn't ...
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What's so bad about Template Haskell?
It seems that Template Haskell is often viewed by the Haskell community as an unfortunate convenience. It's hard to put into words exactly what I have observed in this regard, but consider these few ...
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How fundamentally different are push-pull and arrowized FRP?
I want to study FRP in Haskell, but it's a bit difficult to decide on a library to use.
Many seem to be dead attempts, some seem to have been resurrected (such as recent activity on Yampa).
From what ...
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Haskell: Lists, Arrays, Vectors, Sequences
I'm learning Haskell and read a couple of articles regarding performance differences of Haskell lists and (insert your language)'s arrays.
Being a learner I obviously just use lists without even ...
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answer
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Getting associated type synonyms with template Haskell
Can Template Haskell find out the names and/or the declarations of the associated type synonyms declared in a type class? I expected reify would do what I want, but it doesn't seem to provide all the ...
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Haskell: Converting Int to String
I know you can convert a String to an number with read:
Prelude> read "3" :: Int
3
Prelude> read "3" :: Double
3.0
But how do you grab the String representation of an Int value?
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Difference between `data` and `newtype` in Haskell
What is the difference when I write this?
data Book = Book Int Int
versus
newtype Book = Book (Int, Int) -- "Book Int Int" is syntactically invalid
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Comparing Haskell's Snap and Yesod web frameworks
The two Haskell web frameworks in the news recently are Yesod (at 0.8) and Snap (at 0.4).
It's quite obvious that Yesod currently supports a lot more features than Snap. However, I can't stand the ...
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Good examples of Not a Functor/Functor/Applicative/Monad?
While explaining to someone what a type class X is I struggle to find good examples of data structures which are exactly X.
So, I request examples for:
A type constructor which is not a Functor.
A ...
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What is the Haskell response to Node.js?
I believe the Erlang community is not envious of Node.js as it does non-blocking I/O natively and has ways to scale deployments easily to more than one processor (something not even built-in in Node....
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Running a Haskell program on the Android OS
Forenote: This is an extension of the thread started on /r/haskell
Lets start with the facts:
Android is one awesome Operating System
Haskell is the best programming language on the planet
...
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votes
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answers
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When is -XAllowAmbiguousTypes appropriate?
I've recently posted a question about syntactic-2.0 regarding the definition of share. I've had this working in GHC 7.6:
{-# LANGUAGE GADTs, TypeOperators, FlexibleContexts #-}
import Data.Syntactic
...
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What is the difference between traits in Rust and typeclasses in Haskell?
Traits in Rust seem at least superficially similar to typeclasses in Haskell, however I've seen people write that there are some differences between them. I was wondering exactly what these ...
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Are there pronounceable names for common Haskell operators? [closed]
I'm reading Learn You a Haskell for Great Good, and I never know how to pronounce the Haskell operators. Do they have "real" names? ?
For instance, how do you read aloud an expression like this one?
...
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Reading GHC Core
Core is GHC's intermediate language. Reading Core can help you better understand the performance of your program. Someone asked me for documentation or tutorials on reading Core, but I couldn't find ...
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What is the difference between Int and Integer?
In Haskell, what is the difference between an Int and an Integer? Where is the answer documented?
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What is an appropriate type for smart contracts?
I'm wondering what is the best way to express smart contracts in typed languages such as Haskell or Idris (so you could, for example, compile it to run on the Ethereum network). My main concern is: ...
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How to split a string in Haskell?
Is there a standard way to split a string in Haskell?
lines and words work great from splitting on a space or newline, but surely there is a standard way to split on a comma?
I couldn't find it on ...
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What optimizations can GHC be expected to perform reliably?
GHC has a lot of optimizations that it can perform, but I don't know what they all are, nor how likely they are to be performed and under what circumstances.
My question is: what transformations can ...
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How to define a function in ghci across multiple lines?
I'm trying to define any simple function that spans multiple lines in ghci, take the following as an example:
let abs n | n >= 0 = n
| otherwise = -n
So far I've tried pressing Enter ...
- 19k
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Why are side-effects modeled as monads in Haskell?
Could anyone give some pointers on why the impure computations in Haskell are modelled as monads?
I mean monad is just an interface with 4 operations, so what was the reasoning to modelling side-...
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lenses, fclabels, data-accessor - which library for structure access and mutation is better
There are at least three popular libraries for accessing and manipulating fields of records. The ones I know of are: data-accessor, fclabels and lenses.
Personally I started with data-accessor and I'...
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Implications of foldr vs. foldl (or foldl')
Firstly, Real World Haskell, which I am reading, says to never use foldl and instead use foldl'. So I trust it.
But I'm hazy on when to use foldr vs. foldl'. Though I can see the structure of how ...
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Why are λ-calculus optimal evaluators able to compute big modular exponentiations without formulas?
Church numbers are an encoding of natural numbers as functions.
(\ f x → (f x)) -- church number 1
(\ f x → (f (f (f x)))) -- church number 3
(\ f x → (f (f (f (f x))))) -- church ...
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Why is there "data" and "newtype" in Haskell? [duplicate]
It seems that a newtype definition is just a data definition that obeys some restrictions (e.g., only one constructor), and that due to these restrictions the runtime system can handle newtypes more ...
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Why not be dependently typed?
I have seen several sources echo the opinion that "Haskell is gradually becoming a dependently-typed language". The implication seems to be that with more and more language extensions, Haskell is ...
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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 :: Int -> Int -> Int
addTwo x y = x + y
But when I enter in ghci, I get an ...
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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 job as Cabal. So, what is the difference between them? Is stack a replacement for ...
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Difference between `mod` and `rem` in Haskell
What exactly is the difference between mod and rem in Haskell?
Both seems to give the same results
*Main> mod 2 3
2
*Main> rem 2 3
2
*Main> mod 10 5
0
*Main> rem 10 5
0
*Main> mod 1 0
...
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What's the status of multicore programming in Haskell?
What's the status of multicore programming in Haskell? What projects, tools, and libraries are available now? What experience reports have there been?