51,819 questions
1
vote
3
answers
32
views
How to avoid the partial function Map.lookup, when the key is known to be in the map?
I have some programs consuming a list with structured items.
The list may contain an item multiple times.
From these items I extract a subcomponent and
collect all these different subcomponents.
With ...
- 1,793
0
votes
1
answer
47
views
How to use a dependency's extra-lib-dirs in stack.yaml
I'm building a package where one of its dependencies requires an external library. So the stack.yaml looks like this:
snapshot: lts-22.44
packages:
- .
- external/PRECiSA/PRECiSA
PRECiSA's build ...
- 496
0
votes
0
answers
34
views
HLS "Couldn't match type" / version mismatch on Windows 10 while Cabal builds fine
OK, I keep getting HLS errors even though cabal builds fine.
Environment:
OS: Windows 10 Pro
GHC: 9.6.7
Cabal: 3.12.1.0
I'm on Windows 10 Pro Education, using GHC 9.6.7, and cabal 3.12.1.0
I'm ...
- 1,085
Advice
0
votes
4
replies
84
views
Why does Haskell print in this format?
The following function
shower = show "String"
when it is called I get the output
"\"String\""
How come those backslashes and addition quotation marks?
In contrast:
shower ...
- 21
0
votes
1
answer
111
views
Obtaining mutable arrays from immutable
Is it possible to obtain an array with O(1) read and write access (like Haskell's STArray) using a read only array (such as Haskell's Data.Array.Array)?
Or is there a wrapper around pure functions ...
- 1
Best practices
0
votes
5
replies
157
views
Is there a better way to get the length of runs in a list in haskell?
module Trial where
import Data.List
getRunCount :: (Ord a, Eq a) => [a] -> [(a, Integer)]
getRunCount lst = getRunCount' (sort lst) []
where getRunCount' :: Eq a => [a] -> [(a, ...
- 2,338
0
votes
1
answer
150
views
HLS running from terminal and getting cabal error on fresh new project
I'm using HLS from terminal with the command haskell-language-server-9.12.2. I'm getting the following error:
Step 4/4: Type checking the files
2025-12-15T23:04:57.109846Z | Info | Cradle path: app/...
- 493
2
votes
0
answers
114
views
Stack for Haskell unable to remove files?
I am a beginner Haskell programmer on Windows 11. I am writing a Snakes & Ladders program, and am attempting to set up a Stack environment to work in.
I have been following this guide, as well as ...
- 55
2
votes
1
answer
163
views
What is the rank of these 3 functions? And, most importantly, why?
The following questions are in fact exercises 6.3-i, 6.3-ii, 6.3-iii from Thinking with Types
What is the rank of Int -> forall a. a -> a?
What is the rank of (a -> b) -> (forall c. c -&...
- 30.4k
Best practices
2
votes
11
replies
122
views
Terminology for a value that has a particular type in Haskell
In English, I might talk about the category or property or type tree, and then refer to specific trees as tree "instances" of the property tree. (In philosophical writing, sometimes it's ...
- 8,894
Best practices
0
votes
2
replies
71
views
How to refer to heterogeneous list of any size?
I want to make a function call with two arguments: function and its arguments. Arguments are provided as heterogeneous list (Data.HList). Function should have a signature matching types of arguments. ...
- 109
Best practices
1
vote
2
replies
108
views
How to get a type of parameter from parametrized type?
How could I get a type of parameter from parametrized type? For example, for type T = Maybe String I want to get String, for type T = [Bool] I want to get Bool. It should be a function of T. Thanks in ...
- 109
0
votes
2
answers
145
views
Why is this instance overlapping/how to make interacting type classes work together?
I have the following sample code which shows the problem
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE FlexibleContexts #-}
{-# ...
- 8,168
0
votes
1
answer
194
views
How can I map on a list that returns a tuple?
MRE:
MyType = MyDataCtor [MyType] | MyDataCtor2 MyType MyType | ... other cases (none of which require the map operation)
foo :: Type1 -> Type2 -> (Type1, Type2)
For example, for the ...
- 2,338
2
votes
1
answer
81
views
How to make the entire Parsec parsing process fail upon certain conditions?
I'm creating a toy language in Haskell, and using Text.Parsec to parse everything, So far it's worked great, but there's a certain feature that I don't know how to implement:
What I want to implement: ...
- 141