REBOL 3 Concepts: Tour: Evaluation

Pending Revision

This document was written for R2 and has yet to be revised for R3.

OLD DOCS

This section is obsolete and being replaced. It is kept only as a source for new content.

Blocks are evaluated to compute their results. When a block is evaluated the values of its variables are obtained. The following examples evaluate the variables age, snack-time, birthday, and friends that were defined in the previous section:

print age
22

if current-time > snack-time [print snack-time]
12:32

print third friends
Georgia

A block can be evaluated multiple times by using a loop, as shown in the following examples:

loop 10 [prin "*"]  ;("prin" is not a typo, see manual)
**********

loop 20 [
    wait 8:00
    send friend@rebol.com read http://www.cnn.com
]

repeat count 3 [print ["count:" count]]
count: 1
count: 2
count: 3

The evaluation of a block returns a result. In the following examples, 5 and PM are the results of evaluating each block:

print do [2 + 3]
5

print either now/time < 12:00 ["AM"]["PM"]
PM

In REBOL there are no special operator precedence rules for evaluating blocks. The values and words of a block are always evaluated from first to last, as shown in the following example:

print 2 + 3 * 10
50

Parentheses can be used to control the order of evaluation, as shown in the following examples:

2 + (3 * 10)
32

(length? "boat") + 2
6

You can also evaluate a block and return each result that was computed within it. This is the purpose of the reduce function:

reduce [1 + 2  3 + 4  5 + 6]
3 7 11