Questions tagged [haskell]
Haskell is a functional programming language featuring strong static typing, lazy evaluation, extensive parallelism and concurrency support, and unique abstraction capabilities.
1
vote
1answer
21 views
How to run GHCi in command line like a regular shell command
Is there any way I can run GHCi on command line like a regular command in shell?
For example: :browse in GHCi - list all the function for specific module.
but I want to run it on shell, e.g.: ghci -...
0
votes
1answer
41 views
Haskell Conflicting family instance declarations
I try to define a division function for two Int types
class Dnum a b where
type DivType a b
_div::a->b-> DivType a b
instance Dnum Int Integer where
type DivType Int Integer = ...
0
votes
1answer
26 views
Cannot install z3 package for Haskell on OS X
I'm trying to install the z3 package as follows:
stack install z3 --extra-lib-dirs=/usr/local/bin --extra-include-dirs=/usr/local/bin
Which brings up the following errors:
Configuring z3-4.3.1...
...
0
votes
1answer
40 views
Custom monoid with WriterT
I am trying to implement WriterT with custom data type. I have implemented the monoid as the required by runWriterT. But I am unable to compile the code. I get an error
Could not deduce (Semigroup (...
4
votes
0answers
46 views
Haskell DLL causes memory leak
I am working on a C++ project that uses a Haskell DLL (GHC version is 8.0.1 x64). I've noticed, that the executing program consumes much memory. I investigated into the matter and that's what I've ...
0
votes
1answer
39 views
How to get value out of nested synonym type
I have defined my own type called CtrlV:
{-# LANGUAGE TemplateHaskell #-}
import Data.Data (Data, Typeable)
import Happstack.Server (Response, ServerPartT)
import Web.Routes (RouteT)
import Web....
0
votes
1answer
72 views
Discovering more typeclass instances
When I was new to Haskell I had very hard time finding instances for various types.
Motivated by this, much later I noticed reifyInstances. While I know little about Template Haskell, it seems that ...
8
votes
6answers
227 views
Making a basic Haskell type an instance of a new typeclass
Say I'm trying to define a new typeclass in Haskell, and among other things, it must have a + operation:
class Foo a where
(+) :: a -> a -> a
(In practice the typeclass Foo would have more ...
0
votes
1answer
64 views
How to create a newtype of parser?
newtype Parser a = PsrOf{
-- | Function from input string to:
--
-- * Nothing, if failure (syntax error);
-- * Just (unconsumed input, answer), if success.
dePsr :: String ->...
0
votes
1answer
63 views
How does p >>= f works in Parser instance?
class Monad m where
return :: a -> m a
(>>=) :: m a -> (a -> m b) -> m b
instance Monad Parser where
return a = Parser (\cs -> [(a,cs)])
p >>= f = Parser (\...
1
vote
1answer
44 views
How to lens onto field of a record which is a polymorphic function?
I just installed the lens library so I can easily set in a nested data structure. However, i ran into a problem. Here is a minimal example to demonstrate my problem
The following code doesn't compile:...
2
votes
1answer
79 views
How to retry blocking IO Action when timeout?
How does one deal with a blocking IO action in Haskell? How can I put this IO action inside a scope and manage this scope from another method? If the timeout is reached, I would just reinvoke this ...
5
votes
1answer
142 views
Is Behavior a Comonad?
Conal Elliott talks about Streams and Comonads here: http://conal.net/blog/posts/sequences-streams-and-segments
However, he doesn't mention Behavior directly. So.. is Behavior a Comonad, and if so - ...
1
vote
1answer
40 views
How do I parse this GHC type check error message?
I have been stumped by this GHC (version 8.4.3) type check error. This is an extract from a Haskell Servant code base I am working on. If somebody can explain the reason for this message, I would be ...
-2
votes
0answers
44 views
What does empty parenthesis mean in type declaration? [duplicate]
put :: s -> State s ()
put s = StateOf (\s0 -> (s, ()))
Don't understand what the empty parenthesis mean in these two lines.
-1
votes
1answer
94 views
Is there an “of” syntax in Haskell?
newtype State s a = StateOf (s -> (s, a))
deState :: State s a -> (s -> (s, a))
deState (StateOf stf) = stf
instance Functor (State s) where
-- fmap :: (a -> b) -> State s a -> ...
0
votes
1answer
58 views
Share http Manager between requests
I'm using the library http-client and http-client-tls to make http requests.
The documentation https://haskell-lang.org/library/http-client mentioned "It is highly advisable to share your Manager ...
-1
votes
2answers
79 views
Can we declare a newtype with a function?
newtype State s a = StateOf (s -> (s, a))
(s -> (s, a)) is a function, isn't it?
newtype State s a = State { runState :: s -> (s, a) } such expression make sense since record syntax is ...
-1
votes
1answer
65 views
Use where outside the Haskell function
I'm wondering whether we can use where outside a function?
e.g.
fun::Int->Int
fun n = n + 1
main = do
fun x where x = 30
Obviously it does't work when I compile it,
I want to declare x as ...
0
votes
2answers
65 views
Understanding a let-expression in Haskell
I'm new to Haskell and currently studying it for an exam. I have been learning it from learnyouahaskell. I don't understand the following let-expression in the group function.
splitWhen :: (a -> ...
1
vote
3answers
58 views
Haskell type inference (ReaderT and tuple)
Exploring this material: Lens over tea I've encountered an interesting (simple at first) point:
ex3 :: (a, b) -> (b, a)
ex3 = do
a <- fst
b <- snd
return (b, a)
Everything's fine, but ...
1
vote
1answer
55 views
succ not getting removed on pred
/* Define a Prolog predicate replicate/3 which corresponds to
* the Haskell function of the same name, except that the numeric
* argument is expressed symbolically.
*
* For example, replicate(s(s(...
0
votes
1answer
49 views
How to compose curried functions in Scala
Is it possible compose functions in Scala that are curried? For example:
def a(s1: String)(s2: String): Int = s1.length + s2.length
def b(n: Int): Boolean = n % 2 == 0
def x : String => String =&...
5
votes
1answer
113 views
Why do initial algebras correspond to data and final coalgebras to codata?
If I understand correctly, we can model inductive data types as initial F-algebras and co-inductive data types as final F-coalgebras (for an appropriate endofunctor F) [1]. I understand that according ...
0
votes
0answers
35 views
VSCode Haskelly error
I'm trying to use VSCode with the extension Haskelly to work with haskell. When I select a word, right click and choose "Go to definition" or "Peek definition" it displays No definition found for ....
...
-1
votes
0answers
19 views
Haskell HXT splitting up XML data
I'm pretty new to arrows so go easy on me...
I'm trying to count the number of a specific nodes in an XML file. The XML file is layed out so that under the root, we have a list of scenes and under ...
3
votes
2answers
69 views
Haskell currying explanation needed
I'm trying to understand the concept of currying and went to the Haskell documentation. However, it says that
f is the curried form of g
Yet f takes two arguments and g only one. Since currying ...
0
votes
1answer
49 views
Haskell cannot match expected type 'Bool' to type [t0]
New to haskell and I keep running into this cryptic error when I try to pattern match a non empty list
Code:
type Bits = [Bool]
nor :: Bits -> Bits -> Bits
nor [] [_:__] = error "mismatched ...
4
votes
3answers
99 views
How does <*> derived from pure and (>>=)?
class Applicative f => Monad f where
return :: a -> f a
(>>=) :: f a -> (a -> f b) -> f b
(<*>) can be derived from pure and (>>=):
fs <*> as =
...
2
votes
1answer
29 views
How to implement correctly Foreign.Storable to create Vectors?
I have been working in a project that has been using lists to calculate artificial neural network operations. Now, I would like to transform that to Data.Vector to improve its efficiency. However, I ...
0
votes
0answers
49 views
Escaping character for full text search in sqlite and Haskell
I have the following program:
{-# LANGUAGE OverloadedStrings #-}
module Main where
import Lib
import Control.Applicative
import Database.SQLite.Simple
import Database.SQLite.Simple.FromRow
data ...
2
votes
1answer
84 views
Why passing a concrete type to function solves the error?
p.s: I wasn't sure how to name my question, fell free to let me know how I should have named it.
If not specifying the concrete type, I get this error, which is very clear and easy to solve:
...
0
votes
0answers
37 views
Yesod Type Error - AuthId master vs. Key User [duplicate]
I have a Yesod application. My current goal is to have the application use a email / password for authentication. However, I am running into a type error.
I believe I am following the directions in ...
3
votes
2answers
139 views
Simulate a path-dependent type in Haskell
Here is a simplified example of what I want to do. Let's say you have a HList of pairs:
let hlist = HCons (1, "1") (HCons ("0", 2) (HCons ("0", 1.5) HNil))
Now I want to write a function replaceAll ...
3
votes
1answer
86 views
Should Floating-Point RNG be preciser near 0?
The Floating-Point RNG from System.Random looks simple, yet inaccurate to me:
instance Random Double where
randomR = randomRFloating
random rng =
case random rng of
(x,rng') -> ...
1
vote
1answer
32 views
text encoding when combining http-conduit and scalpel-core
{-# LANGUAGE OverloadedStrings #-}
module Main where
import Lib
import Network.HTTP.Simple
import qualified Data.ByteString.Lazy.Char8 as L8
import Text.HTML.Scalpel.Core
import Data.Text.Lazy....
0
votes
1answer
54 views
How can I reconstruct a JSON array from a slice using lens?
I would like to extract a slice of a JSON array using lenses, and get a Value back. More specifically, I am trying to do the following:
$ import qualified Data.ByteString.Lazy as LBS
$ import Data....
1
vote
1answer
38 views
Error while getting current user id
I am running through a minor problem that I cannot solve.
Yesod with postgres template.
My goal is to create a new directory named after current uid.
-- model
-- User
ident Text
-- ...
-- ...
-2
votes
2answers
78 views
Haskell Understand Definition of functions
I am not quite able to find a good manual for the following:
If I want to get the definition of a functionality in Haskell, then I am using :t
So for example with:
:t sqrt
:t (+)
:t truncate
I will ...
3
votes
1answer
66 views
How to define a Diagrams backend that combines several primitive backends
I would like to make the same program use two different Diagrams backends, notably diagrams-rasterific to generate PNGs, and diagrams-svg to generate SVGs, from the same diagram. Since Diagrams seems ...
1
vote
0answers
62 views
How to draw an ellipse in codeworld haskell given co-ordinates
I need to draw an ellipse given two co-ordinates
Ellipse Coords
Coords
but i'm not sure how to do this with vertices, co-vertices, foci, and a centre point. I'm basically confused as to ...
0
votes
2answers
47 views
How to match constructor with codeworld function haskell
so I've looked around for hours and can't find an answer to match what I need, although many are along the same line so I'm sorry if this is somehow a repost.
In haskell, I have this
shapeToPicture :...
-1
votes
1answer
52 views
Haskell: Parse CSV file into list of lists of floats
I'm new to Haskell and trying to read a csv file and make a list of lists of floats and I get 'IO' compiler error. Ubuntu 18.04 with GHCI 8.0.2. CSV file looks like this:
2,112,66,22,0,25.0,0.307,...
1
vote
0answers
46 views
Row count and similar in Esqueleto
Ok, I'll be more concrete
{-# LANGUAGE QuasiQuotes #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE ...
5
votes
1answer
84 views
How do I apply inductive reasoning to `GHC.TypeLits.Nat`?
Consider this definition of zip for the usual vectors length indexed by Peano numerals:
{-# language DataKinds #-}
{-# language KindSignatures #-}
{-# language GADTs #-}
{-# ...
8
votes
1answer
151 views
Proving a type inequality to GHC
For educational purposes, I have been trying to reconstruct an example from the book "Type-Driven Development with Idris" (namely RemoveElem.idr) in Haskell via use of various language extensions and ...
2
votes
1answer
45 views
Can't get rid of this type error
I'm trying to solve this problem.
This is my code:
import Data.List (nub)
main = interact $ unwords . map show . solve . map words . lines
solve :: [[String]] -> [Int]
solve (_:r:_:a) = map (...
1
vote
3answers
65 views
What does <*> do in addRecip x y = fmap (+) (recipMay x) <*> recipMay y?
addRecip :: Double -> Double -> Maybe Double
addRecip x y = fmap (+) (recipMay x) <*> recipMay y
where
recipMay a | a == 0 = Nothing
| otherwise = Just (1 / a)
I look ...
8
votes
3answers
336 views
Is there a shorthand for operations like `fromNewtype . f . toNewtype`?
A pattern that presents itself the more often the more type safety is being introduced via newtype is to project a value (or several values) to a newtype wrapper, do some operations, and then retract ...
-1
votes
1answer
58 views
Plotting a Random walk in Haskell
Im trying to plot a random walk in haskell.
Im useing getStdGen for the random nummbers.
Im useing Chart.Easy for the plotting.
Im a pleb at haskell and I cant understand the compiler error Im getting....
