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
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