All Questions
2,329
questions
1
vote
1
answer
81
views
How does the arrow operator in haskell make sense in functions that take more than one parameter?
FYI: Second day into Haskell
MRE (what the function does, does not matter):
foo :: Int -> Int -> Int
foo x y
| x == y = x + y
| x < y = x
| x > y = y
This is what ...
- 2,102
1
vote
1
answer
83
views
Finding the type of functions
I have a problem regarding finding the type in Haskell.
The question is as follows:
Find the type of uncurry curry
Does anyone know a way to find the type?
The answer should be ((b1, b2) -> c, b1)...
8
votes
1
answer
314
views
A type with `Eq a` but not `Ord a`
In (an idealized version of) Haskell, is there a concrete type a such that we can implement Eq a lawfully, but it is impossible to implement Ord a? The order doesn't need to be meaningful, and any ...
- 430
2
votes
1
answer
83
views
Writing foldLeft equivalent for recursion schemes
This is a definition of foldr and foldl in terms of foldr:
foldr :: (a -> b -> b) -> b -> [a] -> b
foldr f z [] = z
foldr f z (x:xs) = f x (foldr f z xs)
foldl :: (a -> b -> ...
- 139
2
votes
0
answers
66
views
Multiparameter classes versus data types [closed]
It often happens that we get to choose between writing a (often multiparameter) class or using some (sometimes n-ranked) data type. Let me give two examples to clarify my point:
Example 1
An action of ...
- 689
2
votes
1
answer
50
views
Are fold and unfold required when recursive types appear in a language?
Are fold and unfold required when recursive types appear in a language? I'm reading "Types and Programming Languages" book by Benjamin C. Pierce. Section 4, “Recursive Types,” introduces ...
- 163
1
vote
0
answers
30
views
Haskell: Rigid type binding [duplicate]
The following code works as expected:
data Test a = Test {
sample :: [a],
underflow :: a,
width :: a,
bounds :: Int -> a
}
test :: Num a => [a] -> a -> a -> ...
- 177
2
votes
2
answers
84
views
Extract a Maybe from a heterogeneous collection
I have a project where I define a "heterogenous list" like the following. I'm not sure what this trick is called but I've found it to be useful:
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE ...
- 1,132
1
vote
2
answers
95
views
Confusion about list types in Haskell
As far as I understand [a] means that there can be a list that can have any number of nested lists inside.
f :: [a] -> a
f (x:xs) = x
It is possible to call f [[1,2]] or f [[[True]]]] without ...
2
votes
2
answers
107
views
Iterate over a type level list and call a function based on each type in the list
Given a list of types
{-# LANGUAGE DataKinds #-}
type MyTypes = '[String, Int, Char]
and a type class similar to the following:
class MyClass a where
describe :: Proxy a -> String
instance ...
- 1,256
-1
votes
1
answer
110
views
Non-associativity in a Haskell AST
I have been given the task of writing a parser for which I shall be able to write a composit function f(x)(y). In the specific parser I am to write, this is unlike functions in most languages a non-...
- 647
4
votes
2
answers
71
views
Admissble type role overrides
In GHC Haskell, Map k v has a type role declaration for k to be nominal. This is because the user can otherwise coerce k to another type with the same representation but different Ord instance, ...
- 430
4
votes
2
answers
205
views
rankN type equivalent for mypy in python
In Haskell we can use rankN types like so:
rankN :: (forall n. Num n => n -> n) -> (Int, Double)
rankN f = (f 1, f 1.0)
Is the same thing possible in python with mypy?
I tried the following ...
- 508
1
vote
1
answer
67
views
How to convert between custom constant types in Haskell?
I am doing a problem on Exercism.org (specifically meetup). To reduce it to a minimal example, I am going to solve a slightly simpler problem that illustrates the same issue.
Added for context after ...
- 163
1
vote
2
answers
104
views
Haskell binary datatype of specific bitwidth
I would like to create a datatype representing a binary number of specific bit width.
So a type bin3 --> 0bXXX, bin7 --> 0bXXXXXXX
Does anyone know how to do this? I have tried to install ...
- 300