REBOL 3 Datatypes: Paren!

Contents

Concept

A paren! datatype is a block that is immediately evaluated. It is identical to a block in every way, except that it is evaluated when it is encountered and its result is returned.

When used within an evaluated expression, a paren! allows you to control the order of evaluation:

print 1 + (2 * 3)
7

print 1 + 2 * 3
9

The value of a paren! can be accessed and modified in the same way as any block. However, when referring to a paren!, care must be taken to prevent if from being evaluated. If you store a paren in a variable, you will need to use a get-word form (:word) to prevent it from being evaluated.

Parens are a type of series, thus anything that can be done with a series can be done with paren values.

paren: first [(1 + 2 * 3 / 4)]

print type? :paren
paren!

print length :paren
7

print first :paren
1

print last :paren
4

insert :paren [10 + 5 *]
probe :paren
(10 + 5 * 1 + 2 * 3 / 4)

print paren
12.75

Format

Parens are identified by their open and closing parenthesis. They can span multiple lines and contain any data, including other paren values.

Creation

The make function can be used to allocate a paren value:

paren: make paren! 10
insert :paren 10
insert :paren `+
insert :paren 20

print :paren
20 + 10

print paren
30

The to-paren function converts data to the paren! datatype:

probe to-paren "123 456"
(123 456)

probe to-paren [123 456]
(123 456)

Related

Use paren? to test the datatype.

blk: [(3 + 3)]
probe pick blk 1
(3 + 3)

probe paren? pick blk 1
true

As parens are a subset of the [bad-link:datatypes/series.txt] pseudotype, use series? to check this:

probe series? pick blk 1
true

Using form on a paren value creates a string from the contents contained in the paren:

probe form pick blk 1
3 + 3