用python写语言的解释器
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我花了一下午的时间完成了一个简单语言的解释器,我会在最后帖出所有代码,但是今天不打算详细解释程序的每一个步骤,最近考虑找实习、做论文,要花一些时间。有时间了我会解释每一部分,在这里只提醒一下读者,程序的写作过程和它呈现出来的不一样,总体来说我的写作过程是先写一个只包含一条指令的解释器,然后逐渐加入其他指令。ps:我是多么的想看看高手们写程序的过程,而不只是结果,但是就像graham说的“写的过程往往显得冗长”,所以这方面的书籍不多。我觉得比较接近的书包括《clean code》、《paip》、《software tools》。只可惜现在只看完了一本半。
想法的来源
昨天圣诞节,坐在不能上网的实验室,觉得好无聊。然后想起了学习《可计算性与计算复杂性》的时候,里面用到了一种含有5条指令的原语言,老师也要求我们用这个语言写程序,所以我就想写一个解释器,来执行用该语言,并把这个解释器作为自己的圣诞礼物。
原语言描述
书中说它是一个fortran式的简单语言,所以我就给它起了个简单的名字Metafor,表示meta fortran。下面来看一下它的具体描述,然后贴上我在代码里更详细的描述,我为每一种指令都起了个名字(注意其中把赋值语句命名为setf是由于我最近在写common lisp多一些)。
+-------------------------------------------------------------------------------------------+
指令 描述
x = x + 1 变量x的值加1
x = x - 1 变元x的值减1。若x的值为0,则结果仍为0
TO A IF x≠0 若x≠0,则转标号为A的指令;否则执行下一条指令
TO A 无条件转到标号为A的指令
y=x 把x的值赋给变元y,x值保持不变
+-------------------------------------------------------------------------------------------+
1. Metafor has five instructions:name instruction descriptioninc: x = x + 1, Increase by 1 the value of the variable x.dec: x = x - 1, If the value of x is 0, leave it unchanged; otherwise decrease by 1 the value of x.con_goto: TO A IF x != 0, If the value of x is nonzero, perform the instruction with the label A next; otherwise proceed to the next instruction in the list.goto: TO A, Perform the instruction with the label A next.setf: y = x, Change the value variable y to the value of variable x.2. Some specification:(1) Input variables are represented as:x1, x2, x3, ...(2) Local variables are represented as:z1, z2, z3, ...(3) Output variable is represented as:ynote: the num 1 is often omitted(i.e., x stands for x1 and z standsfor z1).3. Labeled instructions:Instructions may or may not have labels. When an instruction is labeled,the label is written to its left in square brackets. For example,[B] Z = Z - l4. A smaple program:"Program to compute y = x1+ x2" y = x1[B] TO A IF x2 != 0 TO E[A] x2 = x2 - 1 y = y + 1 TO B4. For more information, please refer to 《可计算性与计算复杂性》,周长林、李占山著.整体思路
由于该语言中含有goto语句,所以我不能一句一句的解释执行,我需要把整个程序读进来,转换成一种方面的内部格式后,作为整体执行。例如,这是计算y = x + x2的语言程序:
y = x[B] TO A IF x2 != 0 TO E[A] x2 = x2 - 1 y = y + 1 TO B
它转换为内部格式后,为:
[['setf', 'y', 'x'], ['labeled_exp', 'B', ['con_goto', 'A', 'x2']], ['goto', 'E'], ['labeled_exp', 'A', ['dec', 'x2']], ['inc', 'y'], ['goto', 'B']]运行演示——从文件获得程序
还是前面那个y = x + x2的程序,首先我需要为两个输入参数(x, x2)给定值,如果没有为其赋值,那么它们的值默认为0。另外,我们还可以查看程序的内部表示结构,以及简单调试(就查看其他环境变量的值)。其演示结果如下:
======================================================*Metafor 1.0, Welecome to Metafor shell environment. **Author: Zhu zhaolong(zzljlu@gmail.com) *======================================================Metafor> 2 35Metafor> 5 611Metafor> code[['setf', 'y', 'x'], ['labeled_exp', 'B', ['con_goto', 'A', 'x2']], ['goto', 'E'], ['labeled_exp', 'A', ['dec', 'x2']], ['inc', 'y'], ['goto', 'B']]>>> ========================================== RESTART ==========================================>>> ======================================================*Metafor 1.0, Welecome to Metafor shell environment. **Author: Zhu zhaolong(zzljlu@gmail.com) *======================================================Metafor> 234 5239Metafor> debugdebug> x234debug> x20debug> y239debug> exit>>>运行演示——从终端用户直接输入原语言程序
我写了一个简单的repl交互环境,这样我们可以方便的测试简单的原语言程序,例如下面给出了一个y = x + 1的程序,示例如下:
>>> repl()======================================================*Metafor 1.0, Welecome to Metafor shell environment. **Author: Zhu zhaolong(zzljlu@gmail.com) *======================================================Input your program:x = x + 1y = xinputs> 5=>6Input your program:全部代码
在这里我贴出所有python代码:
""" Metafor: meta fortran, is a small language used in"Computabilit and Complexity" course in Jilin University.@Author: Zhu Zhaolong(zzljlu@gmail.com)@Date: 2011.12.251. Metafor has five instructions:name instruction descriptioninc: x = x + 1, Increase by 1 the value of the variable x.dec: x = x - 1, If the value of x is 0, leave it unchanged; otherwise decrease by 1 the value of x.con_goto: TO A IF x != 0, If the value of x is nonzero, perform the instruction with the label A next; otherwise proceed to the next instruction in the list.goto: TO A, Perform the instruction with the label A next.setf: y = x, Change the value variable y to the value of variable x.2. Some specification:(1) Input variables are represented as:x1, x2, x3, ...(2) Local variables are represented as:z1, z2, z3, ...(3) Output variable is represented as:ynote: the num 1 is often omitted(i.e., x stands for x1 and z standsfor z1).3. Labeled instructions:Instructions may or may not have labels. When an instruction is labeled,the label is written to its left in square brackets. For example,[B] Z = Z - l4. A smaple program:"Program to compute y = x1+ x2" y = x1[B] TO A IF x2 != 0 TO E[A] x2 = x2 - 1 y = y + 1 TO B4. For more information, please refer to 《可计算性与计算复杂性》,周长林、李占山著."""###========================================================import refrom pprint import pprint###===========================parse========================def parse(program): return [read(e) for e in program]def read(s): """Read a metafor insctruction from a string. and change it to internal represtation.""" return read_from(tokenize(s))def tokenize(s): "Convert a string into alist of tokens." return s.split()def read_from(tokens): "Read an exp from a sequence of tokens." if len(tokens) == 0: raise SyntaxError("uncexpected EOF while reading.") if is_setf(tokens): return make_setf(tokens[0], tokens[2]) elif is_inc(tokens): return make_inc(tokens[0]) elif is_dec(tokens): return make_dec(tokens[0]) elif is_goto(tokens): return make_goto(tokens[1]) elif is_con_goto(tokens): return make_con_goto(tokens[1], tokens[3]) elif is_labeled_exp(tokens): return make_labeled_exp(get_label(tokens[0]), read_from(tokens[1:])) else: raise SyntaxError("unexpected instructions: %s" % " ".join(tokens))#=======================================================def is_variable(token): if token.startswith("x") or \ token.startswith("z") or \ token == "y": return True else: return Falsedef get_label(token): "Token is like [A1]. we want get A1." return token.replace('[', '').replace(']', '')## setf def is_setf(tokens): if tokens[1] == "=" and len(tokens) == 3: if is_variable(tokens[0]) and is_variable(tokens[2]): return True else: raise SyntaxError("unexpected setf instruction: %s" % " ".join(tokens)) else: return Falsedef make_setf(des_var, src_var): return ["setf", des_var, src_var]## inc def is_inc(tokens): if len(tokens) == 5 and tokens[1] == "=" \ and tokens[3] == "+" and tokens[4] == "1": if tokens[0] == tokens[2] and is_variable(tokens[0]): return True else: raise SyntaxError("unexpected inc instruction: %s" % " ".join(tokens)) else: return Falsedef make_inc(var): return ["inc", var]## decdef is_dec(tokens): if len(tokens) == 5 and tokens[1] == "=" \ and tokens[3] == "-" and tokens[4] == "1": if tokens[0] == tokens[2] and is_variable(tokens[0]): return True else: raise SyntaxError("unexpected dec instruction: %s" % " ".join(tokens)) else: return Falsedef make_dec(var): return ["dec", var]## gotodef is_goto(tokens): if len(tokens) == 2 and tokens[0] == "TO": return True else: return Falsedef make_goto(lable): return ["goto", lable]## con_gotodef is_con_goto(tokens): if len(tokens) == 6 and tokens[0] == "TO" \ and tokens[2] == "IF" and is_variable(tokens[3]) \ and tokens[4] == "!=" and tokens[5] == "0": return True else: return Falsedef make_con_goto(lable, var): return ["con_goto", lable, var]## labeled expdef is_labeled_exp(tokens): return tokens[0].startswith('[')def make_labeled_exp(label, exp): return ["labeled_exp", label, exp]###==========================CodeSeg======================def is_label(s): return s == "labeled_exp" class CodeSeg: """ To store internal instructions (exps). N : the number of instructions. exps: the list of instructions. label_2_exps: a dict of {'label': num} pairs.""" def __init__(self, exps): self.N = len(exps) self.exps = [] self.label_2_exp={} for (i, e) in enumerate(exps): if is_label(e[0]): (_, label, exp) = e self.label_2_exp[label] = i self.exps.append(exp) else: self.exps.append(e) def get_instruction_num(self, label): "Return instruction num with the label." if label in self.label_2_exp.keys(): return self.label_2_exp[label] # IF there is no instruction with that label, # return a instruction num that doesn't exit. else: return self.N def get_instruction(self, pc): return self.exps[pc] ###=============================Env======================class Env(): "An enviromnet: a dict of {'var': val} paris, var's default value is 0." def __init__(self, args): self.pairs = {} for (i, v) in enumerate(args): if i == 0: self.pairs['x'] = int(v) else: self.pairs['x'+str(i+1)] = int(v) def get_v(self, var): if var in self.pairs.keys(): return self.pairs[var] else: return 0 def set_v(self, var, value): self.pairs[var] = value###=========================mainloop=======================def mainloop(codeseg, env): "pc refers to the current instruction." pc = 0 while pc < codeseg.N: e = codeseg.get_instruction(pc) if e[0] == 'inc': (_, v) = e env.set_v(v, env.get_v(v) + 1) pc += 1 elif e[0] == 'dec': (_, v) = e env.set_v(v, max(env.get_v(v) - 1, 0)) pc += 1 elif e[0] == 'setf': (_, x, y) = e env.set_v(x, env.get_v(y)) pc += 1 elif e[0] == 'goto': (_, label) = e pc = codeseg.get_instruction_num(label) elif e[0] == "con_goto": (_, label, var) = e val = env.get_v(var) if val != 0: pc = codeseg.get_instruction_num(label) else: pc += 1 else: raise SyntaxError("unexpected instructions: %s" % " ".join(e)) return env.get_v('y')###========================repl================================def repl(input_prompt='inputs> '): "A prompt-read-eval-print loop." print_info() while True: program = get_prog_from_shell() if program == []: return "Return successfully!" codes = CodeSeg(parse(program)) inputs = raw_input(input_prompt) env = Env(inputs.split()) print("=>" + str(mainloop(codes, env))) def get_prog_from_shell(): program = [] exp = raw_input("\n\nInput your program:\n") while exp: program.append(exp) exp = raw_input() return programdef print_info(): print("======================================================\n" "*Metafor 1.0, Welecome to Metafor shell environment. *\n" "*Author: Zhu zhaolong(zzljlu@gmail.com) *\n" "======================================================\n")###========================run=============================def run(filepath, prompt='Metafor> '): "This function can run Metafor program from file." print_info() program = get_prog_from_file(filepath) parsed_prog = parse(program) codes = CodeSeg(parsed_prog) inputs = raw_input(prompt) while True: env = Env(inputs.split()) print(mainloop(codes, env)) inputs = raw_input(prompt) if inputs == "exit": return "Return successfully!" if inputs == "debug": while True: var = raw_input('debug> ') if var == "exit": return None print(env.get_v(var)) if inputs == "code": pprint(parsed_prog) return Nonedef get_prog_from_file(filepath): f = open(filepath, "r") program = f.readlines() f.close() return program###===========================test==================================# y = x + x2test_file_1 = "metafor_test.mf"# y = 2 * xtest_file_2 = "metafor_test2.mf"if __name__ == "__main__": run(test_file_1)
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