Compiler example from section 13.7 of Programming in Haskell,
Graham Hutton, Cambridge University Press, 2007.


Compiler for simple arithmetic expressions
------------------------------------------

> data Expr                     =  Val Int | Add Expr Expr
>
> eval                          :: Expr -> Int
> eval (Val n)                  =  n
> eval (Add x y)                =  eval x + eval y
>
> type Stack                    =  [Int]
>
> type Code                     =  [Op]
>
> data Op                       =  PUSH Int | ADD
>
> exec                          :: Code -> Stack -> Stack
> exec []           s           =  s
> exec (PUSH n : c) s           =  exec c (n:s)
> exec (ADD    : c) (m:n:s)     =  exec c (n+m:s)
>
> comp'                         :: Expr -> Code -> Code
> comp' (Val n)   c             =  PUSH n : c
> comp' (Add x y) c             =  comp' x (comp' y (ADD : c))
>
> comp                          :: Expr -> Code
> comp e                        =  comp' e []