三句 let 建出的表

（每道题开头都有同一段：上一站的 parser（多记了位置）、show()，和错误类 SemError——错误写成一行「原因@行:列」。）

贯穿的源码：

let rate = 12 ;
let msg = "hi" ;
let total = rate * 3 ;
if ( total >= 30 ) { let big = 1 ; total = total - big ; } else { total = 0 ; }

先只看顶层的三句 let，把名字和类型登记进一个字典（类型先用最简单的判断：右边是数字 num、字符串 str、别的先算 num）：

KEYWORDS = ("let", "if", "else", "while")
CMP = (">=", "<=", "==", "!=", "<", ">")

def T(src):
    toks = []
    for ln, line in enumerate(src.split("\n"), 1):
        pos = 0
        for w in line.split():
            pos = line.index(w, pos)
            if w in KEYWORDS:
                k = "KEYWORD"
            elif w[0] == '"':
                k = "STRING"
            elif w.isdigit():
                k = "NUMBER"
            elif w[0].isalpha() or w[0] == "_":
                k = "IDENT"
            else:
                k = "OP"
            toks.append((k, w, ln, pos + 1))
            pos += len(w)
    toks.append(("EOF", "", ln, pos + 1))
    return toks


class ParseError(Exception):
    def __init__(self, want, got, line, col):
        super().__init__("期待「" + want + "」遇到「" + got + "」@" + str(line) + ":" + str(col))


class Parser:
    def __init__(self, toks):
        self.toks, self.i = toks, 0

    def peek(self):
        return self.toks[self.i]

    def advance(self):
        self.i += 1
        return self.toks[self.i - 1]

    def at(self, text):
        return self.peek()[1] == text

    def expect(self, text):
        t = self.peek()
        if t[1] != text:
            raise ParseError(text, t[1] or "文件结尾", t[2], t[3])
        return self.advance()

    def expect_kind(self, kind):
        t = self.peek()
        if t[0] != kind:
            raise ParseError(kind, t[1] or "文件结尾", t[2], t[3])
        return self.advance()

    def program(self):
        stmts = []
        while self.peek()[0] != "EOF":
            stmts.append(self.stmt())
        return ("program", stmts)

    def stmt(self):
        t = self.peek()
        if self.at("let"):
            self.advance()
            name = self.expect_kind("IDENT")
            self.expect("=")
            e = self.expr()
            self.expect(";")
            return ("let", name[1], e, name[2], name[3])
        if self.at("if"):
            self.advance()
            self.expect("(")
            c = self.expr()
            self.expect(")")
            body = self.block()
            alt = None
            if self.at("else"):
                self.advance()
                alt = self.block()
            return ("if", c, body, alt, t[2], t[3])
        if self.at("while"):
            self.advance()
            self.expect("(")
            c = self.expr()
            self.expect(")")
            return ("while", c, self.block(), t[2], t[3])
        name = self.expect_kind("IDENT")
        self.expect("=")
        e = self.expr()
        self.expect(";")
        return ("assign", name[1], e, name[2], name[3])

    def block(self):
        self.expect("{")
        stmts = []
        while not self.at("}"):
            if self.peek()[0] == "EOF":
                self.expect("}")
            stmts.append(self.stmt())
        self.expect("}")
        return stmts

    def expr(self):
        left = self.sum_()
        if self.peek()[1] in CMP:
            t = self.advance()
            return ("bin", t[1], left, self.sum_(), t[2], t[3])
        return left

    def sum_(self):
        left = self.term()
        while self.peek()[1] in ("+", "-"):
            t = self.advance()
            left = ("bin", t[1], left, self.term(), t[2], t[3])
        return left

    def term(self):
        left = self.factor()
        while self.peek()[1] in ("*", "/"):
            t = self.advance()
            left = ("bin", t[1], left, self.factor(), t[2], t[3])
        return left

    def factor(self):
        t = self.peek()
        if t[0] == "NUMBER":
            self.advance()
            return ("num", int(t[1]))
        if t[0] == "STRING":
            self.advance()
            return ("str", t[1][1:-1])
        if t[0] == "IDENT":
            self.advance()
            return ("var", t[1], t[2], t[3])
        if t[1] == "(":
            self.advance()
            e = self.expr()
            self.expect(")")
            return e
        raise ParseError("表达式", t[1] or "文件结尾", t[2], t[3])


def parse(src):
    return Parser(T(src)).program()


def show(n):
    if n[0] == "num" or n[0] == "var":
        return str(n[1])
    if n[0] == "str":
        return '"' + n[1] + '"'
    if n[0] == "bin":
        return "(" + show(n[2]) + n[1] + show(n[3]) + ")"
    if n[0] == "let":
        return "let:" + n[1] + "=" + show(n[2])
    if n[0] == "assign":
        return n[1] + "=" + show(n[2])
    if n[0] == "if":
        s = "if[" + show(n[1]) + "]{" + ";".join(show(x) for x in n[2]) + "}"
        if n[3] is not None:
            s += "else{" + ";".join(show(x) for x in n[3]) + "}"
        return s
    if n[0] == "while":
        return "while[" + show(n[1]) + "]{" + ";".join(show(x) for x in n[2]) + "}"
    if n[0] == "program":
        return "|".join(show(x) for x in n[1])
    return "?"


class SemError(Exception):
    def __init__(self, msg, line, col):
        super().__init__(msg + "@" + str(line) + ":" + str(col))
        self.msg, self.line, self.col = msg, line, col


SRC = ('let rate = 12 ;\nlet msg = "hi" ;\nlet total = rate * 3 ;\n'
       'if ( total >= 30 ) { let big = 1 ; total = total - big ; } else { total = 0 ; }')
sym = {}
for s in parse(SRC)[1]:
    if s[0] == "let":
        e = s[2]
        sym[s[1]] = "str" if e[0] == "str" else "num"
print(",".join(k + ":" + v for k, v in sym.items()))