# Haskell Quiz/Bytecode Compiler/Solution Pepe Iborra

### From HaskellWiki

This extremely simple solution declares the type of Expressions as an instance of Num, thus don't really need to define a parser as long as the compiler is launched interpreted via 'ghc -e' . This trick is inspired from the Ruby solution.

As far as I know it passes all the tests in the original suite, but due to the parsing trick some expressions need parentization. Namely expressions with negations such as1*-1

1*(-1)

In order to launch the compiler from the command line you should use the script:

```
```

```
```ghc bytecode.hs -fno-warn-missing-methods -e "process ($1)"

```
And then:
```

```
```

```
```sh compiler.sh 1+2

```
```

The solution:

import Data.Bits import Prelude hiding ((**), mod,div,const) process :: Exp -> String process = output . flip generate [] data Exp = Exp :+ Exp | Exp :/ Exp | Exp :* Exp | Exp :- Exp | Exp :^ Exp | Exp :% Exp | Val Int deriving (Show, Eq) data ByteCode = Const Int | LConst Int | ADD | SUB | MUL | POW | DIV | MOD | SWAP deriving (Show,Eq) type Stack = [ByteCode] ------------------- -- The "Parser" ------------------- instance Fractional Exp where (/) = (:/) instance Num (Exp) where (+) = (:+) (-) = (:-) (*) = (:*) negate (Val i) = Val (negate i) fromInteger = Val . fromIntegral (**) = (:^) (%) = (:%) ---------------------- -- Smart constructors ---------------------- min_small = -32768 max_small = 32767 i `inBounds` (min,max) = i >= min && i <= max add,sub,mul,pow,div,mod,swap :: Stack -> Stack const i = if i `inBounds` (min_small,max_small) then Const i else LConst i add = (++[ADD]) sub = (++[SUB]) mul = (++[MUL]) pow = (++[POW]) div = (++[DIV]) mod = (++[MOD]) swap = (++[SWAP]) --------------------- generate :: Exp -> Stack -> Stack generate (Val i) = (++[const i]) generate (x :+ y) = binaryOp x y add generate (x :- y) = binaryOp x y sub generate (x :* y) = binaryOp x y mul generate (x :/ y) = binaryOp x y div generate (x :^ y) = binaryOp x y pow generate (x :% y) = binaryOp x y mod binaryOp :: Exp -> Exp -> (Stack -> Stack) -> Stack -> Stack binaryOp x y f = f . generate y . generate x bytes :: Int -> [Int] bytes a = a .&. 255 : bytes (a `shiftR` 8) represent :: ByteCode -> [Int] represent (Const i) = 1 : reverse( take 2 (bytes i)) represent (LConst i) = 2 : reverse( take 4 (bytes i)) represent ADD = [10] represent SUB = [11] represent MUL = [12] represent POW = [13] represent DIV = [14] represent MOD = [15] represent SWAP= [160] output :: Stack -> String output = show . concatMap represent