• Subprograms are classified as procedures or functions.

• Procedures are parameterized statements. They may be declared, and then called with various actual parameters. Formal parameter declarations resemble incomplete constant and variable declarations.

• Functions are essentially parameterized expressions.

• The semantics of subprogram declarations and calls illustrates the use of the reflective action notation introduced in Chapter 9, as well as the declarative action notation introduced in Chapter 7.

• Semantic entities now include subprogram entities, which are formed from abstractions, and data that are used to distinguish subprogram returns from other reasons for abnormal termination of statement execution.

A procedure is a construct that incorporates a statement, which is known as the body of the procedure. A procedure declaration binds an identifier to a procedure. A procedure call statement causes the body of the identified procedure to be executed. In a few languages, procedures are provided as expressions; then a procedure declaration may be written just like an ordinary constant declaration.

A function is like a procedure, but a function body is essentially an expression, rather than a statement: it has to return a result. A function call is a kind of expression. In practice, most conventional programming languages allow impure functions, where the body is a mixture of statements and an expression.