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6 Subprograms


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A subprogram is a program unit or intrinsic operation whose execution is invoked by a subprogram call. There are two forms of subprogram: procedures and functions. A procedure call is a statement; a function call is an expression and returns a value. The definition of a subprogram can be given in two parts: a subprogram declaration defining its interface, and a subprogram_body defining its execution. [Operators and enumeration literals are functions.]

To be honest: A function call is an expression, but more specifically it is a name.
glossary entry
A subprogram is a section of a program that can be executed in various contexts. It is invoked by a subprogram call that may qualify the effect of the subprogram through the passing of parameters. There are two forms of subprograms: functions, which return values, and procedures, which do not.
glossary entry
A function is a form of subprogram that returns a result and can be called as part of an expression.
glossary entry
A procedure is a form of subprogram that does not return a result and can only be called by a statement.

A callable entity is a subprogram or entry (see Section 9). A callable entity is invoked by a call; that is, a subprogram call or entry call. A callable construct is a construct that defines the action of a call upon a callable entity: a subprogram_body, entry_body, or accept_statement.

Note that “callable entity” includes predefined operators, enumeration literals, and abstract subprograms. “Call” includes calls of these things. They do not have callable constructs, since they don't have completions.