Foo.hs

module Foo where

import System.IO

-- define fold

myfold :: (Num b) => [(t, b)] -> b
myfold ((t,b):xs) = foldr (\(t,b) x -> x + b) 0 ((t,b):xs)

-- define writing results to file

savefile :: (Num b) => b -> IO()
savefile b = do
    let result = show b
    writeFile "fold_result.txt" ("fold=" ++ result)

-- input data

foobarlist = [("foo", 1), ("bar", 2), ("foo", 4), ("bar", -1), ("bar", -1)]

Main.hs

module Main where

import Foo

main = savefile $ myfold foobarlist

Pattern matching, file IO, a lambda, lists and tupels. Something for everybody ;-)