How do I parse a matrix from file in Haskell? - file

Question

I'm new in Haskell.
I trying to parse a text file with two matrices. The insides of a text file:
m n
a11 a12 ...
a21 a22 ...
...
b11 b12 ...
b21 b22 ...
...
where m is number of rows of the 1st matrix, n is number of rows of the 2nd matrix.
For instance:
3 2
1 2 3
4 5 6
7 8 9
1 2
3 4
I know, looks stupid, but I have a task parse a text file with 2 matrices and I only came up with it.
There is the code:
readLine :: Read a => Handle -> IO [a]
readLine = fmap (map read . words) . hGetLine
parse :: Handle -> IO (Matrix a, Matrix a)
parse = do
[m, n] <- readLine
xss1 <- replicateM m readLine
xss2 <- replicateM n readLine
return (fromLists xss1, fromLists xss2)
main = do
[input, output] <- getArgs
h <- openFile input ReadMode
(m1, m2) <- parse h
print $ mult m1 m2
There is a log from console:
Prelude> :r
[1 of 1] Compiling Matrix ( lab.matrix.hs, interpreted )
lab.matrix.hs:156:5:
Couldn't match expected type `IO [a0]' with actual type `[t0]'
In the pattern: [m, n]
In a stmt of a 'do' block: [m, n] <- readLine
In the expression:
do { [m, n] <- readLine;
xss1 <- replicateM m readLine;
xss2 <- replicateM n readLine;
return (fromLists xss1, fromLists xss2) }
Failed, modules loaded: none.
Most likely, there are still a few bugs.
Help me please, I'm exhausted already...

Answer 1

You need to supply a Handle as an argument to every call of readLine, so parse could look like this:
parse h = do
[m, n] <- readLine h
xss1 <- replicateM n $ readLine h
xss2 <- replicateM m $ readLine h
return (fromLists xss1, fromLists xss2)
Another note - it's probably safer to check the number of arguments returned by getArgs, rather than just assuming there will be two. For example:
main = do
args <- getArgs
case args of
[input, output] -> do
h <- openFile input ReadMode
(m1, m2) <- parse h
hClose h
print $ show mult m1 m2
_ -> putStrLn "expected two arguments"