How can I combine two different types of lists and traverse the result in Haskell?
For example:
input: [1,2,3] ['A','B','C'],
output: ["A1","A2","A3","B1","B2","B3","C1","C2","C3"].
I tried making an example using Int and Char, like:
combine :: Int -> Char -> String
combine a b = show b ++ show a
This doesn't work, however, because if I use this function for combine 3 'A', the output will be "'A'3", not "A3".
show :: Char -> String will indeed put single quotes around the character:
*Main> show 'A'
"'A'"
But since a type String = [Char], we can use:
combine :: Int -> Char -> String
combine i c = c : show i
So here we construct a list of characters with c as the head (the first character), and show i (the representation of the integer) as tail.
Now we can use list comprehension to make a combination of two lists:
combine_list :: [Int] -> [Char] -> [String]
combine_list is cs = [combine i c | c <- cs, i <- is]
This then generates the output:
*Main> combine_list [1,2,3] ['A','B','C']
["A1","A2","A3","B1","B2","B3","C1","C2","C3"]
You may do as follows;
combiner :: [Char] -> [Int] -> [String]
combiner cs xs = (:) <$> cs <*> (show <$> xs)
*Main> combiner ['A','B','C'] [1,2,3]
["A1","A2","A3","B1","B2","B3","C1","C2","C3"]
Here (:) <$> cs (where <$> is infix fmap) will construct an applicative list functor while show <$> xs is like map show xs yielding ["1","2","3"] and by <*> we just apply the applicative list to ["1","2","3"] resulting ["A1","A2","A3","B1","B2","B3","C1","C2","C3"]
