1970 Pascal：為教學而生的完美刀刃#

program Payroll;
type
    TMonth = 1..12;                          { 子界型別：值域限制 }
    TDate = record                           { 記錄型別：承襲 COBOL 的資料描述 }
        Year: integer;
        Month: TMonth;
        Day: 1..31
    end;
var
    salary: real;
    hireDate: TDate;
begin
    hireDate.Year := 2026;
    hireDate.Month := 2;
    hireDate.Day := 30;                      { 編譯錯誤！Day 不能超過 31 }
    salary := hireDate;                      { 編譯錯誤！型別不匹配 }
end

program Fibonacci;
type
    TSmall = 0..30;              { 子界型別：防止溢位 }
var
    i: TSmall;
    fib: array[0..30] of integer;

function Fib(n: integer): integer;
begin                             { 遞迴：ALGOL 傳統的堆疊式執行 }
    if n <= 1 then
        Fib := n
    else
        Fib := Fib(n - 1) + Fib(n - 2)
end;

begin
    for i := 0 to 10 do
        writeln('F(', i, ') = ', Fib(i))
end.

import re

# 1. 模擬遞迴下降剖析器：每條文法規則對應一個程序
tokens = []
pos = 0

def tokenize(src):
    global tokens, pos
    tokens = re.findall(r"\d+|[a-zA-Z_]\w*|[()+\-*/;:]", src)
    pos = 0

def peek():  return tokens[pos] if pos < len(tokens) else None
def advance():
    global pos
    t = tokens[pos]; pos += 1; return t

# 文法（Wirth 的 PL/0 風格）:
# <expr>  ::= <term> {( + | - ) <term>}
# <term>  ::= <factor> {( * | / ) <factor>}
# <factor ::= <number> | ( <expr> )
def parse_expr():
    node = parse_term()
    while peek() in ("+", "-"):
        op = advance()
        node = (op, node, parse_term())
    return node

def parse_term():
    node = parse_factor()
    while peek() in ("*", "/"):
        op = advance()
        node = (op, node, parse_factor())
    return node

def parse_factor():
    if peek() == "(":
        advance()
        node = parse_expr()
        advance()                      # 吃掉 ')'
        return node
    return ("num", advance())

def eval_tree(node):
    if node[0] == "num": return int(node[1])
    op, l, r = node
    return {"+": lambda: eval_tree(l) + eval_tree(r),
            "-": lambda: eval_tree(l) - eval_tree(r),
            "*": lambda: eval_tree(l) * eval_tree(r),
            "/": lambda: eval_tree(l) // eval_tree(r)}[op]()

tokenize("(a + 3) * (b - 2)")
tree = parse_expr()
print("剖析樹:", tree)
print("求值 (a=10, b=5):", eval_tree(tree) and
      eval_tree((tree[0], ("num", "10") if tree[1][0] == "num" else tree[1], tree[2])) or
      "以數值代入驗證：")

# 更直接的數值驗證：把識別字換成數值再剖析
tokenize("(10 + 3) * (5 - 2)")
print("數值代入求值:", eval_tree(parse_expr()))

# 2. 模擬 Pascal 的子界型別檢查：編譯期攔截值域錯誤
class Subrange:
    def __init__(self, lo, hi): self.lo, self.hi = lo, hi
    def check(self, value):
        if not (self.lo <= value <= self.hi):
            raise TypeError(f"編譯錯誤：值 {value} 超出子界 [{self.lo}..{self.hi}]")
        return value

month = Subrange(1, 12)
day = Subrange(1, 31)
print("Month := 2 ->", month.check(2))
try:
    month.check(13)
except TypeError as e:
    print(e)
try:
    day.check(30) if False else day.check(31)
    print("Day := 30 -> OK (在 1..31 之內)")
except TypeError as e:
    print(e)

# 3. 模擬 Pascal-P 的自我編譯循環：直譯器 -> 編譯器 -> 本地碼
def pcode_interpret(pcode, env):        # 第一步：P-code 直譯器
    stack, output = [], []
    for op, *args in pcode:
        if op == "PUSH": stack.append(args[0])
        elif op == "ADD": b, a = stack.pop(), stack.pop(); stack.append(a + b)
        elif op == "PRINT": output.append(stack.pop())
    return output

fib_pcode = [("PUSH", 1), ("PUSH", 2), ("ADD",), ("PRINT",)]
print("P-code 直譯器執行:", pcode_interpret(fib_pcode), "— 中介碼路線，Java JVM 的祖先")