CODEWITHSUNDEEP
haskelltupleslist-comprehension
Niklas R
Niklas R

Reputation: 16860

Haskell list comprehension with tuple input

Is it possible to somehow use a tuple as input for a list comprehension? Or maybe a tuple comprehension? I expected the following to work, but it does not. 

[x * 2 | x <- (4, 16, 32)]

I can not use lists from the very beginning as the given signature of my homework function is

success :: (Int, Int, Int) -> Int -> (Int, Int, Int) -> Bool

But working with lists would be so much simpler as one part of the task requires me to count how many 1s and 20s there are in the tuples.

Upvotes: 0

Views: 583

Answers (2)

Andr&#225;s Kov&#225;cs
Andr&#225;s Kov&#225;cs

Reputation: 30103

Control.Lens has overloaded traversal support for homogeneous tuples of all lengths:

import Control.Lens

-- Convert to list:
(3, 4, 5)^..each -- [3, 4, 5]
(1, 2)^..each -- [1, 2]

-- modify elements:
(4, 16, 32)& each %~ \x -> x * 2 -- (8, 32, 64)
(1, 2)& each %~ (+1) -- (2, 3)

-- operator notation for common modifications (see Control.Lens.Operators):
(1, 2, 3)& each +~ 2 -- (3, 4, 5)
(1, 2, 3)& each *~ 2 -- (2, 4, 6)

-- monadic traversals (here each works like `traverse` for the list monad)
each (\x -> [x, x + 1]) (1, 2) -- [(1,2),(1,3),(2,2),(2,3)]

-- `each` is basically an overloaded "kitchen sink" traversal for 
-- common containers. It also works on lists, vectors or maps, for example
[(3, 4), (5, 6)]& each . each +~ 1 -- [(4, 5), (6, 7)]

Upvotes: 5

Lee
Lee

Reputation: 144126

You could just create a function to convert a triple into a list:

tripleToList :: (a, a, a) -> [a]
tripleToList (a, b, c) = [a, b, c]

then you can do

[x * 2 | x <- tripleToList (4, 16, 32)]

Upvotes: 2

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