Haskell Quiz/Bytecode Compiler

The Problem

Create a bytecode compiler as described on this ruby quiz page: http://www.rubyquiz.com/quiz100.html

Use this tester by Michael Sloan:

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

test :: (String -> [Int]) -> IO ()
test f = assert "2+5" 7 >> assert "2-1" 1 >> assert "2*12" 24 >> assert "2^3" 8 >> assert "5/2" 2 >> assert "15%4" 3 >>
         assert "2+2+2" 6 >> assert "2^8/4" 64 >> assert "3*11%3" 0 >>
         assert "2*(3+4)" 14 >> assert "(3/3)+(8-2)" 7 >> assert "(1+3)/(2/2)*(10-8)" 8 >> assert "(10%3)*(2+2)" 4 >>
         assert "(10/(2+3)*4)" 8 >> assert "5+((5*4)%(2+1))" 7 >> assert "-(2-3-5)" 6 >> assert "-1*-1" (1) >>
         assert "1*-1" (-1) >> assert "1*-1" (-1) >> assert "-1*1" (-1) >>
         assert "-1" (-1)
  where assert str val = print ((if (interpret [] $ f str) == val then "Passed: " else "Failed: ") ++ str)

Or you can use the original Ruby test suite via this Ruby wrapper for your Haskell solution:

class Compiler 
 def Compiler.compile(arith)
   result = `runghc compiler.hs #{arith}`
   eval (result.strip.delete '"')
 end
end

Solutions

• Michael Sloan

• Justin Bailey

• Pepe Iborra

• A (non-monadic) solution to the parsing and eval part of this quiz is a case study in Chapter 17 of The Craft of Functional Programming by Simon Thompson.