Read An Input.md file and output a .html file Haskell - html

Question

I had a question concerning some basic transformations in Haskell.
Basically, I have a written Input file, named Input.md. This contains some markdown text that is read in my project file, and I want to write a few functions to do transformations on the text. After completing these functions under a function called convertToHTML, I have output the file as an .html file in the correct format.
module Main
(
convertToHTML,
main
) where
import System.Environment (getArgs)
import System.IO
import Data.Char (toLower, toUpper)
process :: String -> String
process s = head $ lines s
convertToHTML :: String -> String
convertToHTML str = do
x <- str
if (x == '#')
then "<h1>"
else return x
--convertToHTML x = map toUpper x
main = do
args <- getArgs -- command line args
let (infile,outfile) = (\(x:y:ys)->(x,y)) args
putStrLn $ "Input file: " ++ infile
putStrLn $ "Output file: " ++ outfile
contents <- readFile infile
writeFile outfile $ convertToHTML contents
So,
How would I read through my input file, and transform any line that starts with a # to an html tag
How would I read through my input file once more and transform any WORD that is surrounded by _word_ (1 underscore) to another html tag
Replace any Character with an html string.
I tried using such functions such as Map, Filter, ZipWith, but could not figure out how to iterate through the text and transform each text. Please if anybody has any suggestions. I've been working on this for 2 days straight and have a bunch of failed code to show for a couple of weeks and have a bunch of failed code to show it.

Answer 1

I tried using such functions such as Map, Filter, ZipWith, but could not figure out how to iterate through the text and transform each text.
Because they work on appropriate element collection. And they don't really "iterate"; you simply have to feed the appropriate data. Let's tackle the # problem as an example.
Our file is one giant String, and what we'd like is to have it nicely split in lines, so [String]. What could do it for us? I have no idea, so let's just search Hoogle for String -> [String].
Ah, there we go, lines function! Its counterpart, unlines, is also going to be useful. Now we can write our line wrapper:
convertHeader :: String -> String
convertHeader [] = [] -- that prevents us from calling head on an empty line
convertHeader x = if head x == '#' then "<h1>" ++ x ++ "</h1>"
else x
and so:
convertHeaders :: String -> String
convertHeaders = unlines . map convertHeader . lines
-- ^String ^[String] ^[String] ^String
As you can see the function first converts the file to lines, maps convertHeader on each line, and the puts the file back together.
See it live on Ideone
Try now doing the same with words to replace your formatting patterns. As a bonus exercise, change convertHeader to count the number of # in front of the line and output <h1>, <h2>, <h3> and so on accordingly.