Peter Norvig 在他的個人網站上頭，寫了一篇 (How to Write a (Lisp) Interpreter (in Python))（註），他寫了約莫 100 行的 python script 來小小模擬一個簡化版的 Lisp Interpreter，看了不禁拍案叫絕。下頭這個表格截取自他的網頁，指出他這 100 行的程式碼，所實作的 syntax 只有 9 個（註）。

Example: x
constant literal | number | A number evaluates to itself.
Examples: 12 or -3.45e+6
quotation | (quoteexp) | Return the exp literally; do not evaluate it. Example: (quote (a b c)) ⇒ (a b c)
conditional | (if test conseq alt) | Evaluate test; if true, evaluate and return conseq; otherwise evaluate and return alt.
Example: (if (< 10 20) (+ 1 1) (+ 3 3)) ⇒ 2
assignment | (set! var exp) | Evaluate exp and assign that value to var, which must have been previously defined (with a define or as a parameter to an enclosing procedure).
Example: (set! x2 (* x x))
definition | (define var exp) | Define a new variable in the innermost environment and give it the value of evaluating the expression exp.
Examples: (define r 3) or (define square (lambda (x) (* x x))).
procedure | (lambda (var…) exp) | Create a procedure with parameter(s) named var… and the expression as the body.
Example: (lambda (r) (* 3.141592653 (* r r)))
sequencing | (beginexp…) | Evaluate each of the expressions in left-to-right order, and return the final value.
Example: (begin (set! x 1) (set! x (+ x 1)) (* x 2)) ⇒ 4
procedure call | (proc exp…) | If proc is anything other than one of the symbols if, set!, define, lambda, begin, or quote then it is treated as a procedure. It is evaluated using the same rules defined here. All the expressions are evaluated as well, and then the procedure is called with the list of expressions as arguments. Example: (square 12) ⇒ 144

整個 interpreter 的核心：evaluator 只有如下的數行而已，實在很巧妙。

def eval(x, env=global_env):
    "Evaluate an expression in an environment."
    if isa(x, Symbol):             # variable reference
        return env.find(x)[x]
    elif not isa(x, list):         # constant literal
        return x                
    elif x[0] == 'quote':          # (quote exp)
        (_, exp) = x 
        return exp 
    elif x[0] == 'if':             # (if test conseq alt)
        (_, test, conseq, alt) = x 
        return eval((conseq if eval(test, env) else alt), env)
    elif x[0] == 'set!':           # (set! var exp)
        (_, var, exp) = x 
        env.find(var)[var] = eval(exp, env)
    elif x[0] == 'define':         # (define var exp)
        (_, var, exp) = x 
        env[var] = eval(exp, env)
    elif x[0] == 'lambda':         # (lambda (var*) exp)
        (_, vars, exp) = x 
        return lambda *args: eval(exp, Env(vars, args, env))
    elif x[0] == 'begin':          # (begin exp*)
        for exp in x[1:]:
            val = eval(exp, env)
        return val 
    else:                          # (proc exp*)
        exps = [eval(exp, env) for exp in x]
        proc = exps.pop(0)
        return proc(*exps)

測試方式：

• 下載程式碼。
• 執行 python lis.py。
• 呼叫 repl() 這個函式，進入 pseudo Lisp interactive shell。

• 一一試試下頭的 lisp expression。

  repl() lis.py> (define area (lambda ® (* 3.141592653 (* r r)))) lis.py> (area 3) 28.274333877 lis.py> (define fact (lambda (n) (if (<= n 1) 1 (* n (fact (- n 1)))))) lis.py> (fact 10) 3628800 lis.py> (fact 100) 93326215443944152681699238856266700490715968264381621468592963895217599993229915608941463976156518286253697920827223758251185210916864000000000000000000000000 lis.py> (area (fact 10)) 4.1369087198e+13 lis.py> (define first car) lis.py> (define rest cdr) lis.py> (define count (lambda (item L) (if L (+ (equal? item (first L)) (count item (rest L))) 0))) lis.py> (count 0 (list 0 1 2 3 0 0)) 3 lis.py> (count (quote the) (quote (the more the merrier the bigger the better))) 4

• 連文章的標題都搞了一堆括號，可惜無法拿來執行 XD。

• 其實還有 +, -, *, / 算術運算子; not, >, <, >=, <=, =, equal?, eq? 邏輯運算子; length, car, cdr, cons, append, list, list?, null 這幾個 list 運算子。