Question

Im learning haskell and I got a problem.
The type must be: sentences :: [String] -> [String]

I want to convert strings into a sentence

["something","","Asd dsa abc","hello world..",""] 

to look like this: ["Something.","Asd dsa abc.","Hello world..."]

And I want to use a higher-order function like map.
I just cant figure out how to make this.

I managed to work with a single string:

import Data.Char
sentences :: String -> String
sentences [] = []
sentences (a:as) = (( toUpper a):as) ++ "."

So I get from this:

sentences "sas das asd"

this: "Sas das asd."

I hope someone can help me with this problem.
Thanks for your help!

Edit: Thanks for your help now it looks like this:

import Data.Char
sentences :: [String] -> [String]
sentence (a:as) = ((toUpper a):as)++['.']
sentences = map sentence

But i dont know where to put the filter

Answer 1

Your function coupled with map gets you half of the way, but it does not remove the empty strings from your list of strings. You can do this with filter, so in total

sentences ss = map sentence $ filter (/="") ss

Note that the core of sentences (plural) is simply the mapping of sentence (singular) over your list of strings. The filter is only there to remove the empty strings. Without this requirement, it would simply be sentences ss = map sentence ss.

Now you can call sentences with your list of strings to have each element transformed, except the empty strings that are removed by filter

In general, if you have a function foo that transforms bar into baz, you can use map foo to transform [bar] into [baz]

filter, like map, is a higher order function which, given a predicate function and a list, returns a list consisting of the elements for which the predicate is True. In this case, we give the predicate function (/=""), which is True for all strings that are not empty.