I'm having some trouble getting the InteractWith example from Real World Haskell's Chapter 4 to compile. When I use spaces (like below), it says "Interact.hs:10:30: Empty 'do' construct." When I use tabs (4 spaces per tab), I get "Interact.hs:16:13: parse error on input `='." In either case, I don't see the problem. Any help would be greatly appreciated.

I'm also confused about the formatting of the example in the book (page 72). The last line ("myFunction = id") seems to be indented between the where and the next line, why is that? Is that simply a layout problem? To me, it seems it should be on the same level as the where.

Below is my program:

-- Interact.hs, simple filter in Haskell

import System.Environment (getArgs)

interactWith function inputFile outputFile = do

    input <- readFile inputFile

    writeFile outputFile (function input)

main = mainWith myFunction

    where mainWith function = do

        args <- getArgs

        case args of

            [input, output] -> interactWith function input output

            _ -> putStrLn "Usage: Interact inputFile outputFile"

        myFunction = id

Regards,

Robert

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