如何在Haskell中合并两种不同类型的列表

Question

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" .

Answer 1

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"]

Answer 2

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"]