Question

So I’ve read the theory, now trying to parse a file in Haskell – but am not getting anywhere. This is just so weird…

Here is how my input file looks:

        m n
        k1, k2...

        a11, ...., an
        a21,....   a22
        ...
        am1...     amn

Where m,n are just intergers, K = [k1, k2...] is a list of integers, and a11..amn is a “matrix” (a list of lists): A=[[a11,...a1n], ... [am1... amn]]

Here is my quick python version:

def parse(filename):
    """
    Input of the form:
        m n
        k1, k2...

        a11, ...., an
        a21,....   a22
        ...
        am1...     amn

    """

    f = open(filename)
    (m,n) = f.readline().split()
    m = int(m)
    n = int(n)

    K = [int(k) for k in f.readline().split()]

    # Matrix - list of lists
    A = []
    for i in range(m):
        row = [float(el) for el in f.readline().split()]
        A.append(row)

    return (m, n, K, A)

And here is how (not very) far I got in Haskell:

import System.Environment
import Data.List

main = do
    (fname:_) <- getArgs
    putStrLn fname --since putStrLn goes to IO ()monad we can't just apply it
    parsed <- parse fname
    putStrLn parsed

parse fname = do
    contents <- readFile fname
    -- ,,,missing stuff... ??? how can I get first "element" and match on it?

    return contents

I am getting confused by monads (and the context that the trap me into!), and the do statement. I really want to write something like this, but I know it’s wrong:

firstLine <- contents.head
(m,n) <- map read (words firstLine)

because contents is not a list – but a monad.

Any help on the next step would be great.

So I’ve just discovered that you can do:

 liftM lines . readFile

to get a list of lines from a file. However, still the example only only transforms the ENTIRE file, and doesn’t use just the first, or the second lines…

Answer 1

The very simple version could be:

import Control.Monad (liftM)

-- this operates purely on list of strings
-- and also will fail horribly when passed something that doesn't 
-- match the pattern
parse_lines :: [String] -> (Int, Int, [Int], [[Int]])
parse_lines (mn_line : ks_line : matrix_lines) = (m, n, ks, matrix)
    where [m, n] = read_ints    mn_line
          ks     = read_ints    ks_line
          matrix = parse_matrix matrix_lines

-- this here is to loop through remaining lines to form a matrix
parse_matrix :: [String] -> [[Int]]
parse_matrix lines = parse_matrix' lines []
    where parse_matrix' []       acc = reverse acc
          parse_matrix' (l : ls) acc = parse_matrix' ls $ (read_ints l) : acc

-- this here is to give proper signature for read
read_ints :: String -> [Int]
read_ints = map read . words

-- this reads the file contents and lifts the result into IO
parse_file :: FilePath -> IO (Int, Int, [Int], [[Int]])
parse_file filename = do
    file_lines <- (liftM lines . readFile) filename
    return $ parse_lines file_lines

You might want to look into Parsec for fancier parsing, with better error handling.

*Main Control.Monad> parse_file "test.txt"
(3,3,[1,2,3],[[1,2,3],[4,5,6],[7,8,9]])