如何在haskell中组合两个字符串中的字母

Question

I am learning Haskell and following the guide on http://learnyouahaskell.com/starting-out . I am at the point where it is shown:

ghci> let nouns = ["hobo","frog","pope"]  
ghci> let adjectives = ["lazy","grouchy","scheming"]  
ghci> [adjective ++ " " ++ noun | adjective <- adjectives, noun <- nouns]  
["lazy hobo","lazy frog","lazy pope","grouchy hobo","grouchy frog",  
"grouchy pope","scheming hobo","scheming frog","scheming pope"]   

What I'd like to achieve, it is something similar, but combining the letters contained in two strings, and since strings are basically lists of char in Haskell, this is what I tried:

 [x ++ ' ' ++ y | x <- "ab", y <- "cd"]

But the compiler is complaining:

Prelude> [y ++ ' ' ++ y | x <- "abd", y <- "bcd"]

<interactive>:50:2:
    Couldn't match expected type ‘[a]’ with actual type ‘Char’
    Relevant bindings include it :: [[a]] (bound at <interactive>:50:1)
    In the first argument of ‘(++)’, namely ‘y’
    In the expression: y ++ ' ' ++ y

<interactive>:50:7:
    Couldn't match expected type ‘[a]’ with actual type ‘Char’
    Relevant bindings include it :: [[a]] (bound at <interactive>:50:1)
    In the first argument of ‘(++)’, namely ‘' '’
    In the second argument of ‘(++)’, namely ‘' ' ++ y’
    In the expression: y ++ ' ' ++ y

<interactive>:50:14:
    Couldn't match expected type ‘[a]’ with actual type ‘Char’
    Relevant bindings include it :: [[a]] (bound at <interactive>:50:1)
    In the second argument of ‘(++)’, namely ‘y’
    In the second argument of ‘(++)’, namely ‘' ' ++ y’

I did a number of tries, such as wrapping the expression in brackets to get a list, changed the space to be a String rather than a char... How can I get it working?

Thanks

Answer 1

++ works only for lists, but x and y are only Char . After all, they're elements from a String (= [Char] ), whereas the LYAH example had lists of lists of Char : [String] = [[Char]] :

-- [a] -> [a] -> [a]
-- vv     vv
[y ++ ' ' ++ y | x <- "abd", y <- "bcd"]
--           ^   ^           ^
--           Char           Char

-- vs

--                                        [String]          [String]
--                                       vvvvvvvvvv          vvvvv
[adjective ++ " " ++ noun | adjective <- adjectives, noun <- nouns]  
-- ^^^^^^^           ^^^^
-- String           String

Instead, use (:) to cons the characters on each other and onto the empty list:

[x : ' ' : y : [] | x <- "abd", y <- "bcd"]

Answer 2

x ++ ' ' ++ y

The actual problem here is, you are trying to concatenate three characters, with a function defined only for list of items. ++ will actually concatenate two lists, not two individual items and give a list.

1. So, you can fix your program either by converting all the characters to strings, like this

    > [[x] ++ " " ++ [y] | x <- "ab", y <- "cd"] ["ac","ad","bc","bd"] 

   Note the " " , not ' ' . Because " " means a string with just a space character, but ' ' means just the space character.

2. Or, convert y to a String, use cons operator with the ' ' , and concatenate it to x converted to a string, like this

    > [[x] ++ (' ' : [y]) | x <- "ab", y <- "cd"] ["ac","ad","bc","bd"] 

3. Or, even simpler and intutive, as suggested by chi , create a list of characters, like this

    > [[x, ' ', y] | x <- "ab", y <- "cd"] ["ac","ad","bc","bd"] 

Note: Wrapping a character with [] makes it a list of characters with just one character in it. It basically becomes a String.