As part of the Ruby Quiz in Haskell solutions appearing on the wiki recently, I just a solution to Ruby Quiz #100 - create a bytecode interpreter for a simple expression language. Like I said, the code below parses simple integer arithmetic expressions and generates byte codes for a hypothetical stack-based intepreter that would evaluate the expressions. To run it, save it as a literate haskell file and run "interpret_tests". That just shows correctness, though. Other output can be obtained by running "compile_tests" (shows bytes for all tests), "generate_tests" (symbolic bytecodes for all tests), and "eval_tests" (evaluate ASTs for all tests). To see the AST generated for a example expression, try something like 'parse "2-2-2"'. I'm just learning Haskell (about a month in) and if anyone has time and desire to critique the code below, I'd love to hear it. I come from an OOP (primarily C# & Ruby) background, so I'm really interested in getting a handle on the functional/Haskell "way" of coding. Thanks for any feedback! Justin p.s. This code is also available on the wiki: http://www.haskell.org/haskellwiki/Haskell_Quiz/Bytecode_Compiler/Solution_J... p.p.s. The original ruby quiz is available at: http://www.rubyquiz.com/quiz100.html \begin{code} import Text.ParserCombinators.Parsec hiding (parse) import qualified Text.ParserCombinators.Parsec as P (parse) import Text.ParserCombinators.Parsec.Expr import Data.Bits import Data.Int -- Represents various operations that can be applied -- to expressions. data Op = Plus | Minus | Mult | Div | Pow | Mod | Neg deriving (Show, Eq) -- Represents expression we can build - either numbers or expressions -- connected by operators. This structure is the basis of the AST built -- when parsing data Expression = Statement Op Expression Expression | Val Integer | Empty deriving (Show) -- Define the byte codes that can be generated. data Bytecode = NOOP | CONST Integer | LCONST Integer | ADD | SUB | MUL | POW | DIV | MOD | SWAP deriving (Show) -- Using imported Parsec.Expr library, build a parser for expressions. expr :: Parser Expression expr = buildExpressionParser table factor <?> "expression" where -- Recognizes a factor in an expression factor = do{ char '(' ; x <- expr ; char ')' ; return x } <|> number <?> "simple expression" -- Recognizes a number number :: Parser Expression number = do{ ds <- many1 digit ; return (Val (read ds)) } "number" -- Specifies operator, associativity, precendence, and constructor to execute -- and built AST with. table = [[prefix "-" (Statement Mult (Val (-1)))], [binary "^" (Statement Pow) AssocRight], [binary "*" (Statement Mult) AssocLeft, binary "/" (Statement Div) AssocLeft, binary "%" (Statement Mod) AssocLeft], [binary "+" (Statement Plus) AssocLeft, binary "-" (Statement Minus) AssocLeft] ] where binary s f assoc = Infix (do{ string s; return f}) assoc prefix s f = Prefix (do{ string s; return f}) -- Parses a string into an AST, using the parser defined above parse s = case P.parse expr "" s of Right ast -> ast Left e -> error $ show e -- Take AST and evaluate (mostly for testing) eval (Val n) = n eval (Statement op left right) | op == Mult = eval left * eval right | op == Minus = eval left - eval right | op == Plus = eval left + eval right | op == Div = eval left `div` eval right | op == Pow = eval left ^ eval right | op == Mod = eval left `mod` eval right -- Takes an AST and turns it into a byte code list generate stmt = generate' stmt [] where generate' (Statement op left right) instr = let li = generate' left instr ri = generate' right instr lri = li ++ ri in case op of Plus -> lri ++ [ADD] Minus -> lri ++ [SUB] Mult -> lri ++ [MUL] Div -> lri ++ [DIV] Mod -> lri ++ [MOD] Pow -> lri ++ [POW] generate' (Val n) instr = if abs(n) > 32768 then LCONST n : instr else CONST n : instr -- Takes a statement and converts it into a list of actual bytes to -- be interpreted compile s = toBytes (generate $ parse s) -- Convert a list of byte codes to a list of integer codes. If LCONST or CONST -- instruction are seen, correct byte representantion is produced toBytes ((NOOP):xs) = 0 : toBytes xs toBytes ((CONST n):xs) = 1 : (toConstBytes (fromInteger n)) ++ toBytes xs toBytes ((LCONST n):xs) = 2 : (toLConstBytes (fromInteger n)) ++ toBytes xs toBytes ((ADD):xs) = 0x0a : toBytes xs toBytes ((SUB):xs) = 0x0b : toBytes xs toBytes ((MUL):xs) = 0x0c : toBytes xs toBytes ((POW):xs) = 0x0d : toBytes xs toBytes ((DIV):xs) = 0x0e : toBytes xs toBytes ((MOD):xs) = 0x0f : toBytes xs toBytes ((SWAP):xs) = 0x0a : toBytes xs toBytes [] = [] -- Convert number to CONST representation (2 element list) toConstBytes n = toByteList 2 n toLConstBytes n = toByteList 4 n -- Convert a number into a list of 8-bit bytes (big-endian/network byte order). -- Make sure final list is size elements long toByteList :: Bits Int => Int -> Int -> [Int] toByteList size n = reverse $ take size (toByteList' n) where toByteList' a = (a .&. 255) : toByteList' (a `shiftR` 8) -- All tests defined by the quiz, with the associated values they should evaluate to. test1 = [(2+2, "2+2"), (2-2, "2-2"), (2*2, "2*2"), (2^2, "2^2"), (2 `div` 2, "2/2"), (2 `mod` 2, "2%2"), (3 `mod` 2, "3%2")] test2 = [(2+2+2, "2+2+2"), (2-2-2, "2-2-2"), (2*2*2, "2*2*2"), (2^2^2, "2^2^2"), (4 `div` 2 `div` 2, "4/2/2"), (7`mod`2`mod`1, "7%2%1")] test3 = [(2+2-2, "2+2-2"), (2-2+2, "2-2+2"), (2*2+2, "2*2+2"), (2^2+2, "2^2+2"), (4 `div` 2+2, "4/2+2"), (7`mod`2+1, "7%2+1")] test4 = [(2+(2-2), "2+(2-2)"), (2-(2+2), "2-(2+2)"), (2+(2*2), "2+(2*2)"), (2*(2+2), "2*(2+2)"), (2^(2+2), "2^(2+2)"), (4 `div` (2+2), "4/(2+2)"), (7`mod`(2+1), "7%(2+1)")] test5 = [(-2+(2-2), "-2+(2-2)"), (2-(-2+2), "2-(-2+2)"), (2+(2 * -2), "2+(2*-2)")] test6 = [((3 `div` 3)+(8-2), "(3/3)+(8-2)"), ((1+3) `div` (2 `div` 2)*(10-8), "(1+3)/(2/2)*(10-8)"), ((1*3)*4*(5*6), "(1*3)*4*(5*6)"), ((10`mod`3)*(2+2), "(10%3)*(2+2)"), (2^(2+(3 `div` 2)^2), "2^(2+(3/2)^2)"), ((10 `div` (2+3)*4), "(10/(2+3)*4)"), (5+((5*4)`mod`(2+1)), "5+((5*4)%(2+1))")] -- Evaluates the tests and makes sure the expressions match the expected values eval_tests = concat $ map eval_tests [test1, test2, test3, test4, test5, test6] where eval_tests ((val, stmt):ts) = let eval_val = eval $ parse stmt in if val == eval_val then ("Passed: " ++ stmt) : eval_tests ts else ("Failed: " ++ stmt ++ "(" ++ show eval_val ++ ")") : eval_tests ts eval_tests [] = [] -- Takes all the tests and displays symbolic bytes codes for each generate_tests = concat $ map generate_all [test1,test2,test3,test4,test5,test6] where generate_all ((val, stmt):ts) = (stmt, generate (parse stmt)) : generate_all ts generate_all [] = [] -- Takes all tests and generates a list of bytes representing them compile_tests = concat $ map compile_all [test1,test2,test3,test4,test5,test6] where compile_all ((val, stmt):ts) = (stmt, compile stmt) : compile_all ts compile_all [] = [] interpret_tests = concat $ map f' [test1, test2, test3, test4, test5, test6] where f' tests = map f'' tests f'' (expected, stmt) = let value = fromIntegral $ interpret [] $ compile stmt in if value == expected then "Passed: " ++ stmt else "Failed: " ++ stmt ++ "(" ++ (show value) ++ ")" fromBytes n xs = let int16 = (fromIntegral ((fromIntegral int32) :: Int16)) :: Int int32 = byte xs byte xs = foldl (\accum byte -> (accum `shiftL` 8) .|. (byte)) (head xs) (take (n - 1) (tail xs)) in if n == 2 then int16 else int32 interpret [] [] = error "no result produced" interpret (s1:s) [] = s1 interpret s (o:xs) | o < 10 = interpret ((fromBytes (o*2) xs):s) (drop (o*2) xs) interpret (s1:s2:s) (o:xs) | o == 16 = interpret (s2:s1:s) xs | otherwise = interpret (((case o of 10 -> (+); 11 -> (-); 12 -> (*); 13 -> (^); 14 -> div; 15 -> mod) s2 s1):s) xs \end{code}