[Haskell-cafe] Re: Functional version of this OO snippet

[Thomas Davie]

> You don't even need a type class, a simple data type is enough.

Very true, but I disagree that you've made it functional in any way,  
IO is all about sequencing things, it's very much not a functional style
>
>  data Engine = Engine { foo :: IO (), bar :: String -> IO () }
>
>  run e = processCommand e =<< getLine
>
>  processCommand e c
>    | "foo" `isPrefixOf` c = foo e   >> run e
>    | "bar" `isPrefixOf` c = bar e c >> run e
>    | otherwise            = return ()

This is much nicer done as functions from String -> String, it becomes  
much more compassable, removes a sequential style from your code and  
stops processCommand depending on always working with the "run"  
function making it a bit more orthogonal.

data Engine = Engine {foo :: String, bar :: String -> String}

run e = unlines . map (proccesCommand e) . lines

processCommand e c
   | "foo" `isPrefixOf` c = foo e
   | "bar" `isPrefixOf` c = bar e c
   | otherwise           = ""

Bob