mini-mini-compiler

源代码来自：http://www.cs.nott.ac.uk/~gmh/compiler.lhs

> 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 []

它只支持整型和求和，表达式转换成指令的列表后在堆栈上执行，结果放在栈顶。假设expr为 Add (Val 1) (Val 2)，则 comp expr 会得到 [PUSH 1,PUSH 2,ADD]，把这个结果丢给exec 则得到[3]。

*Main> let e = Add (Val 1) (Val 2)
*Main> exec (comp e) []
[3]

区区23行代码，代码之美，莫过于此！