SIMSCRIPT II.5 Programming Language

SIMSCRIPT II.5 Programming Language
As seen in this example, a subprogram variable can be used instead of an actual routine name in
a call statement. When a subprogram variable appears in a call statement, the effect is the
same as a direct call on the routine named in the assignment to the subprogram variable. This
provides a powerful mechanism for directing the selection of routines to be called during program
execution.
Subprogram variables can be global or local, saved or recursive, and subscripted or unsubscripted.
They are initialized to zero in the normal way, and should not be used in a call until a
value has been assigned. Obviously, the numbers of arguments passed through a subprogram
variable cannot be checked against any one routine definition in the preamble. Execution time
checking, however, may still be carried out.
While subprogram arrays can be defined and values assigned to the subscripted elements, subscripted
elements cannot be used directly to reference a routine. This is because of the ambiguity
in the notation used for both subscripts and routine arguments. Any parenthesized variables or
expressions following the subprogram variable are interpreted as given arguments to the routine
being referenced.
Subprogram variables can also be used to call functions. The mode of the subprogram variable
must be declared in a statement of the form:
define variable list as a mode subprogram variable
All functions called indirectly through this variable must be of the declared mode. If no mode is
declared, the current background mode is assumed.
An indirect function call must be indicated by putting a dollar sign ($) before the subprogram variable
name. This is required to distinguish between assignment of values between subprogram
variables and actual function references. If FVAR1 and FVAR2 are declared as subprogram variables,
the statement:
let FVAR2 = FVAR1
assigns to FVAR2 a copy of the value in FVAR1, while:
let VAR = $FVAR1
assigns to VAR a value computed by the function referenced by FVAR1.
Program 2-6 illustrates several of the permissible usages of variables defined as subprogram. The
first example shows a number of routine name literals passed as arguments to a subprogram, which
in turn calls the selected routine indirectly. The second example is similar, but illustrates the use of
a subprogram variable array, comparison operations, and the function notation used with
subprogram variables.
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