Assignment 7: Haskell grade summary

Due: 5:00pm, Friday, October 19. Value: 30 pts.

Complete the problem from Assignment 4 using Haskell rather than Ruby.

To do this, you'll need to read a file. Below is an example Haskell program for counting the number of lines, words, and characters in a file. This example program defines forEachLine function, a handy function that I strongly suggest using exactly as written (though passing in another function than incrCounts).

import System.IO

import Data.Map as Map

import Data.Set as Set

import Data.List as List



-- Given a file handle, a function, and a starting value,

-- execute a fold across the lines of the file. That is, return

-- f line0 x0 >>= f line1 >>= f line2 >>= ... >>= f lineZ

forEachLine :: Handle -> (String -> a -> IO a) -> a -> IO a

forEachLine handle f x0 =

 do atEof <- hIsEOF handle

    if atEof then

        return x0

     else

        do line <- hGetLine handle

           x1 <- f line x0

           forEachLine handle f x1



incrCounts :: String -> (Int, Int, Int) -> IO (Int, Int, Int)

incrCounts line (ls, ws, cs) =

    return (ls + 1, ws + length (words line), cs + length line + 1)



main :: IO ()

main =

 do handle <- openFile "essay.txt" ReadMode

    (ls, ws, cs) <- forEachLine handle incrCounts (0, 0, 0)

    putStrLn (show ls ++ " " ++ show ws ++ " " ++ show cs)

Another useful utility function that you may want to include is diffLists: Given two lists of strings, diffLists returns a list of each element from the first list that is not in the second.

diffLists :: Ord a => [a] -> [a] -> [a]

diffLists allStrings exclStrings =

    let exclSet = Set.fromList exclStrings

    in List.filter (\x -> Set.notMember x exclSet) allStrings

Another function that you'll definitely want is the words function included in the standard Prelude: words line constructs a list containing each space-separated word from line as a separate list element, very much like Python's line.split().

Finally, I suggest using a Map for what the Python implementation calls totals. In particular, I found Map.empty, Map.insertWith, Map.keys, and Map.toList to be particularly helpful. [Map documentation]