Talk:Haskell Quiz/Bytecode Compiler/Solution Michael Sloan
Some refactoring ideas. Often just to be cute.
import Char (digitToInt)
data Tok = C Char
| N Int
| Op Char
| App Tok Tok Tok
| G {members :: [Tok]}
--Debugging porpoises
instance Show Tok where
show (C ch) = ch : ""
show (N i) = show i
show (Op ch) = ch : ""
show (App a o b) = '(' : show a ++ show o ++ show b ++ ")"
show (G xs) = '[' : (concatMap (\x -> show x) xs) ++ "]"
compile = bytes . parse
parse = stripG . head . foldl (\f c -> prec c . f) id "+-*/^" . pToks . parseNOp . map C
parseNOp [] = []
parseNOp ((N num):(C ch):inp)
| ch `elem` "0123456789" = parseNOp (N (num * 10 + digitToInt ch) : inp)
parseNOp ((C ch):inp) | ch `elem` "0123456789" = parseNOp (N (digitToInt ch) : inp)
parseNOp ((C ch):inp) | ch `elem` "+-*/^%" = parseNOp (Op ch : inp)
parseNOp ((C ch):inp) | ch `elem` " \t\r\n" = parseNOp inp
parseNOp (a:inp) = a : parseNOp inp
--Groups an array of tokens if it needs to be
g (x:xs) | null xs = x
g xs = G xs
--Returns parsed tokens/stream remainder
pToks = fst . parseG
pRem = snd . parseG
parseG [] = ([], [])
parseG [(G ts)] = (ts, [])
parseG ((C '(') : inp) = (g (pToks inp) : (pToks $ pRem inp), [])
parseG ((C ')') : inp) = ([], inp)
parseG (i:inp) = (i : (pToks inp), pRem inp)
--Traverses groups, applies operators to their immediate arguments
prec mo [] = []
prec mo ((G xs):inp) = (g $ prec mo xs) : (prec mo inp)
prec mo (a:(Op o):b:inp) | o == mo = prec mo ((App (g $ prec mo [a]) (Op o) (g $ prec mo [b])) : inp)
prec mo ((App a o b):inp) = (App (g $ prec mo [a]) o (g $ prec mo [b])) : (prec mo inp)
prec mo (x:inp) = x : (prec mo inp)
--Removes any vestigial grouping
stripG (App a o b) = App (stripG a) o (stripG b)
stripG (G (x:xs)) | null xs = error $ "error: " ++ show (x:xs) ++ " <- improper infix nesting here"
stripG (G (x:xs)) = stripG x
stripG x = x
--Converts to a byte format
bytes (N n) = 1 + x : toByteArray n (2^x) where x = fromEnum (abs n >= 2^15)
bytes (Op ch) = return $ case ch of '+' -> 10; '*' -> 11; '^' -> 12; '/' -> 14; '%' -> 15
bytes (App a o b) = concat $ map bytes [a,o,b]
bytes (C x) = error $ "Invalid character: " ++ [x]
bytes x = error $ "Error, invalid: " ++ show x
toByteArray x n = map (\i -> (x `div` 2^(i*8)) `mod` 256 ) [(n-1),(n-2)..0]