List of Strings to single String

Question

I'm trying to do this exercise in a book,

Define a function:

onSeperateLines :: [String] -> String

which takes a list of string and returns a single string which when printed shows the strings on separate lines.

I'm struggling with converting a list to a single string

onSeperateLines :: [String] -> String
onSeperateLines ls = [x | x <- ls]

I can write functions that take a String and convert it to a list and also a list that outputs a list, but I can't figure out how to take a list and convert it to a single string.

leftaroundabout · Accepted Answer · 2016-04-19 21:33:13Z

5

A String is nothing but a list of characters:

type String = [Char]

Hence,

onSeperateLines :: [[Char]] -> [Char]

Now, if you actually need this for some application, it's a good idea to first ask Hoogle if there's already something there. You get a whole lot of results:

unlines :: [String] -> String -- that's exactly the function you're trying to implement!
unwords :: [String] -> String -- similar, but only insert spaces, not newlines
joinPath :: [FilePath] -> FilePath -- Not relevant here
concat :: [[a]] -> [a] -- this does the general task of flattening a nested list to a simple one like a string.

concat is a good choice if you don't just want to use a standard function for the specific task, but also don't want to make life more difficult than necessary.

Of course, it also can't hurt to write a function like that once completely by yourself. To do that, you'll need to recursively^† deconstruct a list. This is extremely simple thanks to pattern matching:

onSeperateLines [] = ... --- no lines to concatenate... what's the result?
onSeperateLines (l:ls) = ...
 where otherLines = onSeperateLines ls

Think a little about what goes in these gaps.

^†_{The equivalent solution with foldr is also a good excercise.}

answered Apr 19, 2016 at 21:33

leftaroundabout

121k6 gold badges185 silver badges337 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

w13rfed Over a year ago

I've not used the (l:ls) notation before, if I entered a list of ["first","second","third"] how would I use 'l' to get the last two strings because 'ls' is a list and 'l' refers to the first string in the list doesn't it?

dfeuer Over a year ago

Another good one: intercalate, which easily generalizes unwords and unlines.

leftaroundabout Over a year ago

@W22: right. That's why you do a recursive call on ls: in one of these calls, any line will be the l once!

Collectives™ on Stack Overflow

List of Strings to single String

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related