How to properly generate a list of String through list comprehension in Haskell?

Question

I need to use List Comprehension to generate a list like this one: ["AaBB", "AbBB", "AcBB", "AdBB", "AeBB", "AfBB","AgBB"]. But I've been running into some problems creating a expression to do it

I've tried to create a list which every element would be a string concatenation, something like this "A" + x + "BB" where x is a element from a range of letters starting with "a" and ending with "g"

module C where
    genList :: [String]
    genList = [ "A" ++ x ++ "BB" | x <- ["a" .. "g"]]

So, I was expecting to generate a list similar to the one asked in the problem. But instead I just got this compiling error:

Prelude> :l exC
[1 of 1] Compiling C                ( exC.hs, interpreted )

exC.hs:3:41: error:
    • No instance for (Enum [Char])
        arising from the arithmetic sequence ‘"a" .. "g"’
    • In the expression: ["a" .. "g"]
      In a stmt of a list comprehension: x <- ["a" .. "g"]
      In the expression: ["A" ++ x ++ "BB" | x <- ["a" .. "g"]]
  |
3 |     genList = [ "A" ++ x ++ "BB" | x <- ["a" .. "g"]] 
  |                                         ^^^^^^^^^^^^
Failed, no modules loaded.

Joseph Sible-Reinstate Monica · Accepted Answer · 2019-09-01 22:44:57Z

4

You can't use the .. syntax to build a list of strings. Luckily, you're using it for single-character strings here, so you can just use it to build a list of characters instead: [ "A" ++ x : "BB" | x <- ['a' .. 'g']]

answered Sep 1, 2019 at 22:44

Joseph Sible-Reinstate Monica

49k7 gold badges55 silver badges118 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Robin Zigmond Over a year ago

To expand on this (I had just started on my own answer, now redundant, but I believe this point may be helpful to the OP): as the error message says, the .. syntax is only defined for members of the Enum typeclass. Specifically it's syntactic sugar for the enumFromTo function. And there's a very good reason why Char is an instance of Enum while String isn't. What would you say the "next" string after say "hello" is?

willeM_ Van Onsem · Accepted Answer · 2019-09-02 11:42:02Z

0

Strings are not an instance of the Enum typeclass. Stricly speaking one could make it an instance, but that would likely not be beneficial much. Since that would yield "a", "a\NUL", "a\NUL\NUL", etc. if we yield these in lexicographical order.

You probably want to use a Character here, like @JosephSible wrote in his answer:

[ 'A': x : "BB" | x <- ['a' .. 'g']]

or we can make use of a functor map here:

('A':) . (:"BB") <$> ['a' .. 'g']

This gives us:

Prelude> ('A':) . (:"BB") <$> ['a' .. 'g']
["AaBB","AbBB","AcBB","AdBB","AeBB","AfBB","AgBB"]

edited Sep 2, 2019 at 11:42

answered Sep 2, 2019 at 11:36

willeM_ Van Onsem

482k33 gold badges483 silver badges624 bronze badges

Collectives™ on Stack Overflow

How to properly generate a list of String through list comprehension in Haskell?

2 Answers 2

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related