Haskell: List comprehension unique char from string

Question

Here is my current list comprehension: [((a,b),a+1) | a <- [1,2], b <- "ab"]

Current output: [((1,'a'),2),((1,'b'),2),((2,'a'),3),((2,'b'),3)]

Intended output: [((1,'a'),2),((2,'b'),3)]

How can I edit my list comprehension to achieve the intended output?

Thanks in advance!

chepner · Accepted Answer · 2022-10-14 19:28:19Z

3

Having two generators produces all possible combinations of the items. You want a single generator produced by zipping the two lists together.

[((a,b), a+1) | (a, b) <- zip [1,2] "ab"]

You can also write

[(t, a+1) | t@(a, _) <- zip [1,2] "ab"]

or

[(t, fst t + 1) | t <- zip [1,2] "ab"]

because you don't care about b except to reconstruct the tuple you just unpacked from zip.

GHC also provides an extension, ParallelListComp, to zip the generators implicitly.

> :set -XParallelListComp
> [((a,b), a+1) | a <- [1,2] | b <- "ab"]

Note the use of | instead of , to separate the two generators.

answered Oct 14, 2022 at 19:10

chepner

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1 Comment

If the string is not fixed at a length of 2 characters, it may be beneficial to: zip [1..] "ab"