Haskell: Traverse through a String/Text File - parsing

Question

I am trying to read a script file then process and output it to a html file. In my script file, whenever there is a #title(this is a title), I will add tag [header] this is a title [/header] in my html output. So my approach is to first read the script file, write the content to a string, process the string, then write the string to html file.
In other to recognize the #title, I will need to read character by character in the string. When I read '#', I will need to detect the next character to see if they are t i t l e.
QUESTION: How do I traverse through a string (which is a list of char) in Haskell?

Answer 1

You could use a simple recursion trick, for example
findTag [] = -- end of list code.
findTag ('#':xs)
| take 5 xs == "title" = -- your code for #title
| otherwise = findTag xs
findTag (_:xs) = findTag xs
so basically you just pattern match if the next char (head of list) is '#' and then you check if the next 5 characters form "title". if so you can then continue your parsing code. if next character isnt '#' you just continue the recursing. Once the list is empty you reach the first pattern match.
Someone else might have a better solution.
I hope this answers your question.
edit:
For a bit more flexibility, if you want to find a specific tag you could do this:
findTag [] _ = -- end of list code.
findTag ('#':xs) tagName
| take (length tagName) xs == tagName = -- your code for #title
| otherwise = findTag xs
findTag (_:xs) _ = findTag xs
This way if you do
findTag text "title"
You'll specifically look for the title, and you can always change the tagname to whatever you want.
Another edit:
findTag [] _ = -- end of list code.
findTag ('#':xs) tagName
| take tLength xs == tagName = getTagContents tLength xs
| otherwise = findTag xs
where tLength = length tagName
findTag (_:xs) _ = findTag xs
getTagContents :: Int -> String -> String
getTagContents len = takeWhile (/=')') . drop (len + 1)
to be honest, it's getting a bit messy but here's what's happening:
You first drop the length of the tagName, then one more for the open bracket, and then you finish off by using takeWhile to take the characters until the closing bracket.

Answer 2

Evidently your problem falls into parsing category. As wisely stated by Daniel Wagner, for maintainability reasons you're much better off approaching it generally with a parser.
Another thing is if you want to work with textual data efficiently, you're better off using Text instead of String.
Here's how you could solve your problem using the Attoparsec parser library:
-- For autocasting of hardcoded strings to `Text` type
{-# LANGUAGE OverloadedStrings #-}
-- Import a way more convenient prelude, excluding symbols conflicting
-- with the parser library. See
-- http://hackage.haskell.org/package/classy-prelude
import ClassyPrelude hiding (takeWhile, try)
-- Exclude the standard Prelude
import Prelude ()
import Data.Attoparsec.Text
-- A parser and an inplace converter for title
title = do
string "#title("
r <- takeWhile $ notInClass ")"
string ")"
return $ "[header]" ++ r ++ "[/header]"
-- A parser which parses the whole document to parts which are either
-- single-character `Text`s or modified titles
parts =
(try endOfInput >> return []) ++
((:) <$> (try title ++ (singleton <$> anyChar)) <*> parts)
-- The topmost parser which concats all parts into a single text
top = concat <$> parts
-- A sample input
input = "aldsfj#title(this is a title)sdlfkj#title(this is a title2)"
-- Run the parser and output result
main = print $ parseOnly top input
This outputs
Right "aldsfj[header]this is a title[/header]sdlfkj[header]this is a title2[/header]"
P.S. ClassyPrelude reimplements ++ as an alias for Monoid's mappend, so you can replace it with mappend, <> or Alternative's <|> if you want.

Answer 3

For pattern search-and-replace, you can use
streamEdit.
import Replace.Megaparsec
import Text.Megaparsec
import Text.Megaparsec.Char
title :: Parsec Void String String
title = do
void $ string "#title("
someTill anySingle $ string ")"
editor t = "[header]" ++ t ++ "[/header]"
streamEdit title editor " #title(this is a title) "
" [header]this is a title[/header] "