REBOL 3 Concepts: Blocks: Composing Blocks

Pending Revision

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

The compose function is handy for creating blocks from dynamic values. It can be used for creating both data and code.

The compose function takes a block as an argument and returns a block that has each value in the argument block. Values in parentheses are evaluated before the block is returned. For example:

probe compose [1 2 (3 + 4)]
[1 2 7]

probe compose ["The time is" (now/time)]
["The time is" 10:32:45]

If the values in parentheses return a block, that block's individual values are used:

probe compose [a b ([c d])]
[a b c d]

To prevent this, you need to enclose the result in an extra block:

probe compose [a b ([[c d]])]
[a b [c d]]

An empty block inserts nothing:

probe compose [a b ([]) c d]
[a b c d]

When compose is given a block that contains sub-blocks, the sub-blocks are not evaluated, even if they contain parentheses:

probe compose [a b [c (d e)]]
[a b [c (d e)]]

If you would like the sub-blocks to be evaluated, use the /deep refinement. The /deep refinement causes all parentheses to be evaluated, regardless of where they are:

probe compose/deep [a b [c (d e)]]
[a b [c d e]]