无限循环或if语句的第四语言EBNF规则 [英] Forth language EBNF rule for an infinite loop or if statement

问题描述

是否存在 EBNF 规则，该规则描述了Forth无限循环或如果声明?

Is there an EBNF rule that describes a Forth infinite loop or if statement?

推荐答案

EBNF用于描述语法.循环是无限循环或其他情况通常不会落入它所描述的范围内.这样，您可能会在EBNF中看到一个不确定的循环，看起来像:

EBNF is used to describe syntax. A loop being infinite or otherwise wouldn't normally fall within what it would describe. As such, you'd be looking at the EBNF for an indefinite loop, which looks something like:

indefinite_loop ::= 'BEGIN' statements cond 'UNTIL'

通常，cond将在堆栈上压入0或1，以确定是否继续循环(0表示继续循环，1表示退出).这样，如果您直接插入0，则循环将永远执行:

Normally the cond will be something that pushes a 0 or 1 on the stack to determine whether to continue the loop (0 means continue the loop, 1 means exit). As such, if you just insert a 0 directly, the loop will execute forever:

: infinite_loop BEGIN do_whatever 0 UNTIL ;

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