如果...否则，如果...否则，在REBOL中 [英] If...else if...else in REBOL

问题描述

我注意到REBOL没有内置的if...elsif...else语法，就像这样:

I've noticed that REBOL doesn't have a built in if...elsif...else syntax, like this one:

theVar: 60

{This won't work}
if theVar > 60 [
    print "Greater than 60!"
]
elsif theVar == 3 [
    print "It's 3!"
]
elsif theVar < 3 [
    print "It's less than 3!"
]
else [
    print "It's something else!"
]

我找到了一种解决方法，但是它非常冗长:

I have found a workaround, but it's extremely verbose:

theVar: 60

either theVar > 60 [
     print "Greater than 60!"
 ][
        either theVar == 3 [
            print "It's 3!"
        ][
            either theVar < 3 [
                print "It's less than 3!"
            ][
                print "It's something else!"
            ]
        ]
 ]

在REBOL中是否有更简洁的方法来实现if...else if...else链?

Is there a more concise way to implement an if...else if...else chain in REBOL?

推荐答案

您要查找的结构是CASE.它需要一系列条件和代码块进行评估，仅在条件为真时才评估这些块，并在第一个条件为真后停止.

The construct you would be looking for would be CASE. It takes a series of conditions and code blocks to evaluate, evaluating the blocks only if the condition is true and stopping after the first true condition is met.

theVar: 60

case [
    theVar > 60 [
        print "Greater than 60!"
    ]

    theVar == 3 [
        print "It's 3!"
    ]

    theVar < 3 [
        print "It's less than 3!"
    ]

    true [
        print "It's something else!"
    ]
]

如您所见，获得默认值就像满足TRUE条件一样简单.

As you see, getting a default is as simple as tacking on a TRUE condition.

也:如果您愿意，可以运行所有情况，而不会因CASE/ALL而短路.这样可以防止案件在第一个真实条件下停止；它将按顺序运行它们，评估任何块的任何真实条件.

Also: if you wish, you can have all of the cases run and not short circuit with CASE/ALL. That prevents case from stopping at the first true condition; it will run them all in sequence, evaluating any blocks for any true conditions.

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