How to read text files into a haskell program?

Question

I have a text file which contains two lists on each line. Each list can contain any number of alphanumeric arguments. eg [t1,t2,...] [m1,m2,...] I can read the file into ghc, but how can I read this into another main file and how can the main file recognise each argument separately to then process it?

Answer 1

I think it's best for you to figure out most of this for yourself, but I've got some pointers for you. Firstly, try not to deal with the file access until you've got the rest of the code working, otherwise you might end up having IO all over the place. Start with some sample data:

sampleData = "[m1,m2,m3][x1,x2,x3,x4]\n[f3,f4,f5][y7,y8,y123]\n[m4,m5,m6][x5,x6,x7,x8]"

You should not mention sampleData anywhere else in your code, but you should use it in ghci for testing.

Once you have a function that does everything you want, eg processLists::String->[(String,String)], you can replcae readFile "data.txt" :: IO String with

readInLists :: FilePath -> IO [(String,String)]
readInLists filename = fmap processLists (readFile filename)

If fmap makes no sense to you, you could read a tutorial I accidentally wrote.

If they really are alphanumeric, you can split them quite easily. Here are some handy functions, with examples.

tail :: [a] -> [a]
tail "(This)" = "This)"

You can use that to throw away something you don't want at the front of your string.

break :: (Char->Bool) -> String -> (String,String)
break (== ' ') "Hello Mum" = ("Hello"," Mum")

So break uses a test to find the first character of the second string, and breaks the string just before it. Notice that the break character is still there at the front of the next string. span is the same but uses a test for what to have in the first list, so

span :: (Char->Bool) -> String -> (String,String)
span (/= ' ') "Hello Mum" = ("Hello"," Mum")

You can use these functions with things like (==','), or isAlphaNum (you'll have to import Data.Char at the top of your file to use it).

You might want to look at the functions splitWith and splitOn that I have in this answer. They're based on the definitions of split and words from the Prelude.