从字符串列表中过滤奇数长度的字符串

Question

I am solving this problem: Using map, filter, and (.) (function composition operator), define a function that examines a list of strings, keeping only those whose length is odd, converts them to upper case letters, and concatenates the results to produce a single string.

I know I can do this with a list comprehension but I have been explicitly instructed to use filter. Right now, my code looks like this:

concatenateAndUpcaseOddLengthStrings :: [String] -> String
concatenateAndUpcaseOddLengthStrings [] = ""
concatenateAndUpcaseOddLengthStrings xs = filter (\x -> length x `mod` 2 == 1) xs

I receive this error: Couldn't match type '[Char]' with 'Char' Expected type: String Actual type: [[Char]]

I am testing the filter function before adding the other components, but I can't get past this error. Any ideas?

Answer 1

The reason this produces an error is because the filter (…) xs expression will return a list fo strings, not a single string. But your type signature specifies:

concatenateAndUpcaseOddLengthStrings :: [String] -> 

so it expects a String , not a [String] . You can make use of concat :: [[a]] -> [a] here to concatenate strings:

concatenateAndUpcaseOddLengthStrings :: [String] -> String
concatenateAndUpcaseOddLengthStrings xs =  (filter (odd . length) xs)

or even shorter:

concatenateAndUpcaseOddLengthStrings :: [String] -> String
concatenateAndUpcaseOddLengthStrings =  . filter (odd . length)

You however still need to implement the fact that it will convert the characters in the strings that are retained to their uppercase variant. I leave that as an exercise.