Haskell-Custom data structure from an array

Question

I need some advice on how to list only words from Text branches in this code I have programmed.

data Article = Text String
             | Section String [Article] deriving (Show)

myArticle :: Article
myArticle = Section "Document" [
                 Section "Introduction" [
                       Text "My intoduction",
                       Section "Notation" [Text "alpha beta gamma"]],
                 Section "Methods" [
                       Section "Functional Programming" [Text "FPR"],
                       Section "Logical Programming" [Text "LPR"]],
                 Section "Results" [Text "All is great"]]

tex :: Article -> [String]
tex (Text x) = [x]
tex (Section x (l:ls)) = tex l

I tried to call ls in the tex function, but it throws me an error. I don't know how to proceed.

Answer 1

You're looking for the concatMap function . This will run the function you give it on each element of the input list, and concatenate all of the results together into a single list. Also, you don't need to use : to bind a list unless you actually want to break it into its head and tail (and in your case, you don't). So change the last line of your code to this:

tex (Section x ls) = concatMap tex ls

And then tex myArticle will be ["My intoduction","alpha beta gamma","FPR","LPR","All is great"] .

Answer 2

You pattern match with (l:ls) , which means it is a non-empty list, and you only work with l , so the first item, you "ignore" the rest of the Article s (which can be Text s and Section s).

You can match with any list, and then have to process that list by generating a list of String s for each subitem, and concatenating these, so:

tex :: Article -> [String]
tex (Text x) = [x]
tex (Section _ ls) = … ls