[用python自制编程语言]第2篇

前言

我做过一个编程语言:mylang

开始

还是一样，看mylang中的代码:

def parse_for(block):
    global status
    try:
        s = block.split('\n')[0]
        init,foo,expression = s[4:-2].split(';')
    except:
        return ['error','ForSentenceError:too many expressions']
    val = comp(init)
    if(val[0] == 'error'):
        return val
    while(1):
        F = comp(foo)
        if(F[0] == 'error'):
            return F
        if(not F[0]):
            break
        for i in block.split('\n')[1:-1]:
            i = i.strip()
            val = comp(i)
            if(val[0] == 'error'):
                return val
            if(status == MYL_BREAK_STATUS):
                status = MYL_NONE_STATUS
                return ['']
            if(status == MYL_CONTINUE_STATUS):
                status = MYL_NONE_STATUS
                break
        val = comp(expression)
        if(val[0] == 'error'):
            return val
    return ['']

参数
block:代码段

第一行

    global status

status是全局变量，有三种状态:

MYL_NONE_STATUS = 0
MYL_BREAK_STATUS = 1
MYL_CONTINUE_STATUS = 2
status = MYL_NONE_STATUS

其中:
MYL_NONE_STATUS表示尚未发现有break或continue
MYL_BREAK_STATUS表示发现有break
MYL_CONTINUE_STATUS表示发现有continue
status初始为MYL_NONE_STATUS，即尚未发现有break或continue

往后

    try:
        s = block.split('\n')[0]
        init,foo,expression = s[4:-2].split(';')
    except:
        return ['error','ForSentenceError:too many expressions']

要理解此处先要看for的语法:

for(<init>;<foo>;<expression>){
    <code>
}

s是第一行:

for(<init>;<foo>;<expression>){

s[4:-2]即为<init>;<foo>;<expression>
将其分割，成为<init>,<foo>,<expression>
最后unpack掉
如果有报错，说明在unpack时for中有不止或不到3个表达式
那么报错:ForSentenceError:too many expressions

继续

    val = comp(init)
    if(val[0] == 'error'):
        return val

执行<init>表达式

后面

    while(1):
        F = comp(foo)
        if(F[0] == 'error'): 
            return F
        if(not F[0]):
            break
        for i in block.split('\n')[1:-1]:
            i = i.strip()
            val = comp(i)
            if(val[0] == 'error'):
                return val
            if(status == MYL_BREAK_STATUS):
                status = MYL_NONE_STATUS
                return ['']
            if(status == MYL_CONTINUE_STATUS):
                status = MYL_NONE_STATUS
                break
        val = comp(expression)
        if(val[0] == 'error'):
            return val
    return ['']

分解看

第一部分

        F = comp(foo)
        if(F[0] == 'error'): 
            return F
        if(not F[0]):
            break

检查条件式<foo>，如有报错或值为假，退出

第二部分

        for i in block.split('\n')[1:-1]:
            i = i.strip()
            val = comp(i)
            if(val[0] == 'error'):
                return val
            if(status == MYL_BREAK_STATUS):
                status = MYL_NONE_STATUS
                return ['']
            if(status == MYL_CONTINUE_STATUS):
                status = MYL_NONE_STATUS
                break

            i = i.strip()
            val = comp(i)

此处在执行<code>部分的第i行

            if(val[0] == 'error'):
                return val

如果第i行有报错，退出

            if(status == MYL_BREAK_STATUS):
                status = MYL_NONE_STATUS
                return ['']
            if(status == MYL_CONTINUE_STATUS):
                status = MYL_NONE_STATUS
                break

如果是break，退出
如果是continue，放弃执行这次程序循环，但不退出

第三部分

        val = comp(expression)
        if(val[0] == 'error'):
            return val

执行<expression>
如果有报错，退出

最后

	return ['']

结束

总结一下

作者

hit-road

拜拜,下课!

hit-road不定期跟新，不见不散！

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