Thank you all for your contributions so far. Plenty of food for thought.
I though I'd try to put it into practice and have a go at the motivating example I gave: essentially a EDSL for defining simple maths tests.
If you have a Python version that has other features you would like, you can send that too.
But it all goes pear shaped as soon as I try to cater for questions dealing with fractions, for example: Now the type system requires me to duplicate all the question-making utilities and give them different names. I tried to mitigate this by using type classes but got walloped by the No Monomorphism Restriction, and so on, and so forth. Wherever I turned, the type system was getting in the way.
NoMonomorphismRestriction is the one extension I used. I suppose I could have replaced def = beautiful combinators by def x = beautiful combinators x Dealing with curried functions of varying arity is one thing that does tend to be fairly annoying, but in this case addParam was possible. The rest was pretty straightforward, mostly avoiding duplication by making more specific helpers rather than more generic functions. {-# LANGUAGE NoMonomorphismRestriction #-} import System.IO (hFlush, stdout) data Result = Correct | Improve String | Huh String | Incorrect String deriving Show data Question = Question { ask :: String , answer :: String , check :: String -> Result } bool2result True = Correct bool2result False = Incorrect "" readCheckBy :: (Read a) => (a -> Bool) -> String -> Result readCheckBy pred str = case reads str of [(val,"")] -> bool2result (pred val) _ -> Huh "" readCheck :: (Read a, Eq a) => a -> String -> Result readCheck v s = readCheckBy (==v) s -- helpers value val prompt = Question prompt (show val) (readCheck val) infix2 op symbol a b = value (op a b) (unwords [show a, symbol, show b]) addParam :: (Show a) => (funTy -> String -> qty) -> (a -> funTy) -> String -> (a -> qty) addParam qmakr fun string v = qmakr (fun v) (string++" "++show v) prefix1 = addParam value prefix2 = addParam prefix1 prefix3 = addParam prefix2 -- question 'types' addition = infix2 (+) "+" subtraction = infix2 (-) "-" multiplication = infix2 (*) "x" power = infix2 (^) "^" square = (flip power) 2 cube = (flip power) 3 square' = prefix1 (^2) "square" pi1 = value pi "pi" pi2 = Question "pi" (show pi) (readCheckBy (\v -> abs (pi - v) / pi < 0.0001)) questions = [ addition 1 2 , subtraction 3 2 , multiplication 4 5 , square 3 , cube 3 , square' 7 , value 3.14 "pi" ] test :: Question -> IO () test q = do putStr $ ask q ++ " = " hFlush stdout reply <- getLine putStrLn $ show $ check q reply main = mapM_ test questions