Difference between revisions of "Haskell Quiz/Bytecode Compiler/Solution Justin Bailey"
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Mod > lri ++ [MOD] 
Mod > lri ++ [MOD] 

Pow > lri ++ [POW] 
Pow > lri ++ [POW] 

−  generate' (Val n) instr 
+  generate' (Val n) instr 
−  +   abs n > 32768 = LCONST n : instr 

−  +   otherwise = CONST n : instr 

−  else CONST n : instr 

 Takes a statement and converts it into a list of actual bytes to 
 Takes a statement and converts it into a list of actual bytes to 
Latest revision as of 02:32, 19 February 2010
This solution should work correctly. All test strings from the quiz evaluate to the correct values. To see it for yourself, execute the interpret_tests
function.
To see the (symbolic) byte codes generated, run generate_tests
. To see the actual byte codes, run compile_tests
. To see that the values produced by each expression match those expected, run eval_tests
. The last actually evaluates the AST, without generating any bytescodes. The tests are contained in the variables test1,test2, ..., test6
, which correspond to the six "test_n" methods found in the quiz's test program.
The byte codes aren't optimized. For example, SWAP is never used. However, they should produce correct results (even for negative and LCONST/CONST values).
The code below is literate Haskell.
\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
 abs n > 32768 = LCONST n : instr
 otherwise = 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 8bit bytes (bigendian/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"), (22, "22"), (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"), (222, "222"), (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+22, "2+22"), (22+2, "22+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+(22), "2+(22)"), (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+(22), "2+(22)"), (2(2+2), "2(2+2)"), (2+(2 * 2), "2+(2*2)")]
test6 = [((3 `div` 3)+(82), "(3/3)+(82)"), ((1+3) `div` (2 `div` 2)*(108), "(1+3)/(2/2)*(108)"),
((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 = concatMap 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 = concatMap generate_all [test1,test2,test3,test4,test5,test6]
where generate_all = map (\(val, stmt) > (stmt, generate (parse stmt)))
 Takes all tests and generates a list of bytes representing them
compile_tests = concatMap compile_all [test1,test2,test3,test4,test5,test6]
where compile_all = map (\(val, stmt) > (stmt, compile stmt))
interpret_tests = concatMap f' [test1, test2, test3, test4, test5, test6]
where
f' = map f''
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}