REBOL 3 Functions: until

Description

The until function evaluates a block until the block returns true. It is different from while because it only requires a single block, which also serves as the condition. However, the block is always evaluated at least once.

The general form is:

while [cond] [body]

For example:

num: 0
until[
    print num
    num: num + 1
    num >= 2
]
0
1
2

Another example:

str: "test"
until [
    print str
    tail? str: next str
]
test
est
st
t

Return Value

The last value of the block is returned from the until function. Because this is also the termination condition, it will always be non-none non-false, but that can be useful at times.

Other Notes

• A break can be used to escape from the loop at any time.
• A common mistake is to forget that block must return the test condition for the loop, which could result in an infinite loop.