Full text search
Functions: the differences between value and read-only arguments
Prerequisite Concepts		Related Syntax

The fundamental difference between value and read-only arguments is that a value argument is used to pass the result of a single expression, and read-only is used to pass an entire shelf.

Use a value argument when:

• the argument may be a constant (like 42 or foo)
• the argument may be the result of an expression (like a + b)

Use a read-only argument when a shelf is being passed as an argument and:

• items other than the currently selected one may need to be examined. (For example, if the argument is the subject of a repeat over, it should likely be passed as read-only.)
• the key of any item on the shelf may not be accessed.
• information about the shelf itself (like the number of items) is required.
• the argument is a stream shelf and the item may be anything other than a closed buffer.

In general, if none of the conditions that require a read-only argument apply, then it is probably better to make the argument a value argument.

This sample shows how to use a read-only argument to count the number of active switch values on a switch shelf, without modifying the shelf.

  define counter function count-active-switches
                          (read-only switch switch-shelf) as
     local counter active-count
     set active-count to 0
     repeat over switch-shelf
        increment active-count when switch-shelf
     again
     return active-count

Prerequisite Concepts      Functions    Functions: argument classes

Related Syntax    define function    value

----

[CONTENTS] [CONCEPTS] [SYNTAX] [LIBRARIES] [SAMPLES] [ERRORS] [INDEX]