haskell - List of Strings to single String -

i'm trying exercise in book,

define function:

onseperatelines :: [string] -> string 

which takes list of string , returns single string when printed shows strings on separate lines.

i'm struggling converting list single string

onseperatelines :: [string] -> string onseperatelines ls = [x | x <- ls] 

i can write functions take string , convert list , list outputs list, can't figure out how take list , convert single string.

a string nothing list of characters:

type string = [char] 

hence,

onseperatelines :: [[char]] -> [char] 

now, if need application, it's idea first ask hoogle if there's there. whole lot of results:

unlines :: [string] -> string -- that's exactly function you're trying implement!
unwords :: [string] -> string -- similar, insert spaces, not newlines
joinpath :: [filepath] -> filepath -- not relevant here
concat :: [[a]] -> [a] -- general task of flattening nested list simple 1 string.

concat choice if don't want use standard function specific task, don't want make life more difficult necessary.

of course, can't hurt write function once completely yourself. that, you'll need recursively^† deconstruct list. extremely simple pattern matching:

onseperatelines [] = ... --- no lines concatenate... what's result? onseperatelines (l:ls) = ...  otherlines = onseperatelines ls 

think little goes in these gaps.

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^†_{the equivalent solution foldr excercise.}