All Questions
2,335 questions
0
votes
3
answers
107
views
Solving Ambiguous type variable errors without adding a specific type annotation
I would like to make this code compile and run:
import Data.List
main = print $ elemIndices [] []
Without adding specific type annotations. The error it gives is:
Ambiguous type variable ‘a0’ arising ...
- 225
1
vote
2
answers
113
views
Take elements of a list up to and including the first value that satisfies some predicate in Haskell
Given a predicate p, takeWhile p xs gives you the longest prefix of elements satisfying p. How can I get the shortest prefix containing an element that satisfies p?
1
vote
2
answers
103
views
How do I join a list of chars using a separator?
I am basically looking for the Haskell equivalent of the following python code:
' '.join(['_','a','b','1'])
I understand that python treats those as strings instead of chars, but... I digress.
MRE:
[...
- 2,280
0
votes
2
answers
86
views
How do I overload a certain operator in haskell to take different types on either side?
MRE:
class Foo s where
myCons :: Char -> s -> s
myCons c xs = <my definition of however I wish to interpret this>
instance (Eq, Show) Foo where
(:) x y = x `myCons` y
Error:
...
- 2,280
4
votes
3
answers
183
views
Why is `tranpose [[]] == []` in Haskell?
Consider the following?
ghci> transpose []
[]
ghci> transpose [[]]
[]
ghci> transpose [[[]]]
[[[]]]
ghci> transpose [[[[]]]]
[[[[]]]]
ghci> transpose [[[[[]]]]]
[[[[[]]]]]
ghci> ...
0
votes
2
answers
78
views
How to create a list of elements of an indexed data type, whose length depends on the index
I am busy formalising a theorem using a library which introduces an indexed datatype. For simplicity one can think of it to have the form data idx (n : ℕ).
Now I want to create a list of elements over ...
- 275
2
votes
2
answers
87
views
Filtering a List based on State in Haskell
I've been looking at the implementations of ListT to find a good way to do something like this:
func list = do
set <- get -- Data.Set
el <- list
guard $ Set.notMember el set
return el
I ...
1
vote
2
answers
78
views
Haskell foldr1 lambda function which adds tuple values
I have been scratching my head trying to figure this out. How do I use foldr1 (or any other fold for that matter) in order to get the sum of tuples in a list.
Example:
list = [(1,2), (3,4)]
sum = 10
...
- 13
3
votes
3
answers
93
views
Haskell basic - pattern matching vs list tail
I'm doing Set7.hs from haskell.mooc.fi/ Exercises7
-- Ex 5: reverse a NonEmpty list.
--
-- PS. The Data.List.NonEmpty type has been imported for you
-- below doesn't work
-- reverseNonEmpty :: ...
- 2,071
1
vote
0
answers
71
views
Replacing single elements in list
So, I am currently doing an exercise where I have a list of employees, with the data type Employee.
data Employee = Employee Name Age Salary deriving (Show)
employees :: [Employee]
employees =
[ ...
- 21
1
vote
2
answers
110
views
understand haskell logic behind odd's and even's index lists
I had a question in haskell that asks a function to divide a list in two different lists so the even indexs filled a list and the odd ones another.
Example:
func :: [Int] -> ([Int], [Int])
Then, ...
2
votes
1
answer
169
views
Is There an O(1) Way To Give Haskell Read Access to C-style Arrays?
I want to call a Haskell function from C++:
a = f(x);
where f is a Haskell function.
x is double[] in cpp.
Whatever stateful mutations happen to x will happen in C++. For example,
x[42] = x[41] + pi;
...
- 819
3
votes
3
answers
96
views
Define variable in list comprehension format function
I have this pseudocode that calculates the factorial of a number given by the user.
Process without_title
Define i, tmp, x As Integers;
tmp<-1;
Write "Enter a number to ...
- 1,090
0
votes
4
answers
150
views
Functor over multiple levels
I have this lame attempt:
fmap2 :: (Functor f, Functor g) => (a -> b) -> f (g a) -> f (g b)
fmap2 f f2 = (fmap2 f . fmap f2)
It is supposed to work like this:
fmap2 negate [[1,2], [3]] -- ...
- 997
0
votes
0
answers
71
views
Joining NonEmpty types in Haskell
If I have two generic objects of type (NonEmpty a), how can I join those two objects into a new (NonEmpty a), just like ++ does with lists?
I was expecting to be able to do something like the ...
- 13