Haskell：从字符串列表中删除空格

Question

The question is: Write a function that will delete leading white space from a string. Example: cutWhitespace [" x","y"," z"] Expected answer: ["x","y","z"]

Heres what I have:

cutWhitespace (x:xs) = filter (\xs -> (xs /=' ')) x:xs

This returns ["x", " y"," z"] when the input is [" x"," y", " z"] . Why is it ignoring the space in the second and third string and how do I fix it?

We are allowed to use higher-order functions which is why I implemented filter.

Answer 1

The reason the OP cutWhitespace function only works on the first string, is that due to operator precedence, it's actually this function:

cutWhitespace (x:xs) = (filter (\xs -> (xs /=' ')) x) : xs

Here, I've put brackets around most of the body to make it clear how it evaluates. The filter is only applied on x , and x is the first element of the input list; in the example input " x" .

If you filter " x" as given, you get "x" :

Prelude> filter (\xs -> (xs /=' ')) " x"
"x"

The last thing cutWhitespace does, then, is to take the rest of the list ( [" y", " z"] ) and cons it on "x" , so that it returns ["x"," y"," z"] .

In order to address the problem, you could write the function with the realisation that a list of strings is a nested list of characters, ie [[Char]] .

As a word of warning, pattern-matching on (x:xs) without also matching on [] is dangerous, as it'll fail on empty lists.

Answer 2

The question you asked, on how to delete leading whitespace from a string, you can do by simply doing dropWhile on a string:

deleteLeadingWhitespace = dropWhile (\c -> c == ' ')

though you should be more clever if you consider other things "whitespace". You could use the "isSpace" function defined in Data.Char for example.

From your sample data, it looks like you are really trying to do this for a list of strings, in which case you can map the dropWhile over your array:

map deleteLeadingWhitespace

The filter approach you are taking is a little bit dangerous, because even if you had it doing what you think it should, it would be deleting all the spaces, not just the leading ones.

Answer 3

Instead of writing a custom function that checks if a character is whitespace, I would advice to use isSpace :: Char -> Bool . This function does not only returns True for a space ( ' ' ), but for a new line ( '\\n' ), a carriage return ( '\\r' ), a tab ( '\\t' ), a vertical tab ( '\\v' ) and form feed ( '\\f' ) as well. Usually it is better to work with such functions since the odds of forgetting certain cases is lower.

We can thus remove the spacing of a single string with:

dropWhile isSpace

Where we thus dropWhile in such way that all chacters where isSpace .

We can then perform a mapping with this filter to filter the spaces out of all the strings, like:

import Data.Char(isSpace)

cutWhitespace = map (dropWhile isSpace)