Prolog是带有列表的vs = [英] Prolog is vs = with lists

问题描述

为什么它失败L is [1,2,3,4]，并且可以正常工作:L = [1,2,3]?

Why does this fail L is [1,2,3,4], and this works: L = [1,2,3]?

但是L is 1和L = 1的工作原理相同.

But L is 1, and L = 1 both work the same.

推荐答案

is仅应在评估右侧的算术运算时使用. 即:

is should only be used when evaluating arithmetic operations on the right-hand side. i.e.:

X is 1 + 2 + 3

is/2将右侧结构评估为算术表达式.如果它不是有效的算术表达式或数字，则is/2失败.否则，将算术表达式求值的数字与[大概]未绑定的左手值统一.

is/2 evaluates the right-hand structure as an arithmetic expression. If it is not a valid arithmetic expression or a number, is/2 fails. Otherwise, the number to which the arithmetic expression evaluted is unified with the [presumably] unbound left-hand value.

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