Preface - Adobe

CHAPTER 3
Literal and Executable
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Language
Every object is either literal or executable. This distinction comes into play when
the interpreter attempts to execute the object.
• If the object is literal, the interpreter treats it strictly as data and pushes it on
the operand stack for use as an operand of some subsequent operator.
• If the object is executable, the interpreter executes it.
What it means to execute an object depends on the object’s type; this is described
in Section 3.5, “Execution.” For some object types, such as integers, execution
consists of pushing the object on the operand stack; the distinction between literal
and executable integers is meaningless. But for other types, such as names,
operators, and arrays, execution consists of performing a different action.
• Executing an executable name causes it to be looked up in the current dictionary
context and the associated value to be executed.
• Executing an executable operator causes some built-in action to be performed.
• Executing an executable array (otherwise known as a procedure) causes the elements
of the array to be executed in turn.
As described in Section 3.2, “Syntax,” some tokens produce literal objects and
some produce executable ones.
• Integer, real, and string constants are always literal objects.
• Names are literal if they are preceded by / and executable if they are not.
• The [ and ] operators, when executed, produce a literal array object with the enclosed
objects as elements. Likewise, > (LanguageLevel 2) produce a
literal dictionary object.
• { and } enclose an executable array or procedure.
Note: As mentioned above, it does not matter whether an object is literal or executable
when it is accessed as data, only when it is executed. However, referring to an
executable object by name often causes that object to be executed automatically; see
Section 3.5.5, “Execution of Specific Types.” To avoid unintended behavior, it is best
to use the executable attribute only for objects that are meant to be executed, such as
procedures.