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POHow do I parse a matrix of integers in Haskell?
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copied!<p>So I've read the theory, now trying to parse a file in Haskell - but am not getting anywhere. This is just so weird...</p> <p>Here is how my input file looks:</p> <pre><code> m n k1, k2... a11, ...., an a21,.... a22 ... am1... amn </code></pre> <p>Where m,n are just intergers, <code>K = [k1, k2...]</code> is a list of integers, and <code>a11..amn</code> is a "matrix" (a list of lists): <code>A=[[a11,...a1n], ... [am1... amn]]</code></p> <p>Here is my quick python version:</p> <pre class="lang-py prettyprint-override"><code>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) </code></pre> <p>And here is how (<strong>not very</strong>) far I got in Haskell:</p> <pre class="lang-hs prettyprint-override"><code>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 </code></pre> <p>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:</p> <pre><code>firstLine <- contents.head (m,n) <- map read (words firstLine) </code></pre> <p>because contents is not a list - but a monad.</p> <p>Any help on the <em>next</em> step would be great.</p> <p>So I've just <a href="https://stackoverflow.com/questions/215748/haskell-inserting-every-line-from-a-file-into-a-list">discovered</a> that you can do:</p> <pre><code> liftM lines . readFile </code></pre> <p>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...</p>

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