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Order of evaluation
Prerequisite Concepts

The components of OmniMark actions and operators are evaluated in order, from left to right, and the action or operator is performed only after all its components (arguments) have been evaluated.

In particular, this means that the side effects of functions called during the evaluation of an action occur in the lexical order of the function calls in the action.

More particularly, the order of evaluation of the components of an action, operator or function call is as follows:

• First, all the value-returning expressions, including indexers of reference-returning expressions, are evaluated, in order, left-to-right. Value returning expressions are those that produce counter, switch, or stream values. Reference-returning expressions identify the item of a counter, switch, or stream shelf rather than producing the value of that item. Reference-returning expressions are used as the left-hand side of set actions and as modifiable and read-only function arguments.
• Second, all the reference-returning expressions are evaluated to produce shelf item references. No side effects, other than the reporting of fatal errors when referenced items are found not to exist, can occur at this stage.
• Third and last, the action or operator itself is performed, or the function is called, using the values of its components, as produced by the previous two steps.

These steps are applied recursively to the component expressions of an action, operator or function call, as those component expressions are evaluated.

The first and third stages are those at which function side effects may occur.

The reason for this order of evaluation is to constrain the places at which checks for non-existent shelf items need be done. Note that a side effect of a function may be to create or delete a shelf item.

• how OmniMark actions and operators are evaluated in order, from left to right.
• that an action or operator is performed only after all of its components (arguments) have been evaluated.

In this example:

  set counter target item f1 to f2

1. f1 is called.
2. f2 is called.
3. an item of counter target is selected using the result of f1 as an item number.
4. The selected item's value is set to the result of f2.

As a consequence of this left-to-right evaluation, the selected item may not exist prior to the set (or "set counter") action, but actually be created by either "f1" or "f2". It could even be removed by "f1" and recreated by "f2".

Note that programming in this style is generally deprecated.

Prerequisite Concepts      Arithmetic and comparison operators    Functions: argument classes

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