A Pro/3 function is mapping from zero, one or more parameters to a function value. The parameters and the function value (return value) belong to a specified data element domain.

A function consists of three parts:

• terminology
• sentence model declaration
• function definition

Normally, you have to insert these three elements in the KB in the sequence over, and remove (delete) from the KB in the reversed over. 

Functions are used two ways in Pro/3, in sentence rules and in certainty rules (i.e. in the query part of data rules). The general rule of function usage is to use them as little as possible. Functions should be resorted on only if the desired rule logic cannot be achieved without functions.

Terminological definition - Sentence model declaration

The faculty function example illustrates several aspects of Pro/3 functions:

• The function definition consists of a header (terminated by ':'), and one or more statements separated by ';' - the last statement is terminated by '!'.
• There can be several function definitions defined for a function each handling e.g. unique ranges of direct or indirect parameter values. The definitions are processed in the same sequence as they are defined (there is accordingly no need to have a condition N>1 in the last definition in the example above). It is however strongly recommended to program the function definitions such that the function works correctly regardless of the sequence of the definitions in the KB.
• One and only one statement assigns a value to return value. This is usually, but not necessarily, the last statement. return value is a pre-defined function return variable. (The return value can also be used in conditional statements such as function value > 0.
• A function can call other functions and it can call itself.
• The integer error function is an error handler function which takes an error message string as an input parameter. The function opens an error message dialog box (with the error message) and asks if the execution is to be continued or canceled. The function will return 0 if the execution is continued. The integer error function is an example of a built-in function.

Function Window

Function statements

There are three types of function statements: assignment statements, comparison statements and true/false statements.

Assignment statements have the general format VARIABLE = RIGHT SIDE, where the variable is a local variable or the function return value variable. The function return value variable has the predefined name return value. The right side of the assignment statement is a local variable, a function call, a data element reference, a constant from any of the data element domains or an expression.

The NL syntax describes the expression syntax in detail. Note that:

• expressions must always be in parentheses
• expressions can not contain function calls
• expressions can not contain data element references

One, and only one, statement must assign a value to the return value predefined variable. This is usually but not necessarily the last statement in the function definition.

A comparison statement has the format VARIABLE <comparison operator> COMPARATOR. The comparator is a local variable or a constant from any of the data element domains. The execution of the function definition fails if the comparison is not true.

True/false statements have the general format true(UNARY PREDICATE CONDITION) or false(UNARY PREDICATE CONDITION). It is similar to the comparison statement in the way that the function interpreter simply needs to decide whether or not the statement is true or false.

Deterministic and non-deterministic functions

A function is defined as either deterministic or non-deterministic. Deterministic functions return one value (or none if it fails). Non-deterministic functions can return many values. Refer to non-determinism for further information on the use of non-determinism.

The built-in function integer series is an example of a non-deterministic function. The call integer series given 1, 2 and 20 will return, consecutively, the integers 1, 3, 5, ..., 19. You can define non-deterministic functions by building in at least on non-deterministic statement, i.e. a call to another non-deterministic function or a non-deterministic data element reference. You can also achieve non-determinism by using multiple function definitions.

The months function returns non-deterministically the name of the twelve months. The function is not very meaningful, but it illustrates the three methods of making a defining a non-deterministic function.

Terminology and structure are defined by the following sentences:

the syntagm with language English, KB - name "fMonths", syntax class FUNCTION, syntax form SINGULAR / UNDETERMINED and token list "months" is in the terminology with segment name "EXAMPLES"!

the non-deterministic function with NL - name months and domain IDENTIFIER is in the sentence model with segment name "EXAMPLES"!

Function Calls

Actual parameters (i.e. the parameters specified in given call) must match the formal parameters (i.e. the parameters in the function declaration) in number and domain compatibility.  Actual parameters can be parameters, variables and constants. Note that:

• function calls and certainty rule calls are not permitted as actual parameters
• expressions are not allowed as actual parameters

Built-in functions

Pro/3 has a number of built in functions. Refer to the following files in the /STD directory for details:

Function libraries (standard Pro/3 functions)