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Pure Procedures

A pure function is one that obeys certain syntactic constraints that ensure it produces no side effects. This means that the only effect of a pure function reference on the state of a program is to return a result-it does not modify the values, pointer associations, or data mapping of any of its arguments or global data, and performs no external I/O. A pure subroutine is one that produces no side effects except for modifying the values and/or pointer associations of INTENT(OUT) and INTENT(INOUT) arguments. These properties are declared by a new attribute (the PURE attribute) of the the procedure.

A pure procedure (i.e., function or subroutine) may be used in any way that a normal procedure can. However, a procedure is required to be pure if it is used in any of the following contexts:

• The mask or body of a FORALL statement or construct;
• Within the body of a pure procedure; or
• As an actual argument in a pure procedure reference.
• []Rationale. The freedom from side effects of a pure function allows the function to be invoked concurrently in a FORALL without such undesirable consequences as nondeterminism, and additionally assists the efficient implementation of concurrent execution. Syntactic constraints (rather than semantic constraints on behavior) are used to enable compiler checking.
• The HPF Journal of Development also proposes allowing elemental invocation of pure procedures with scalar arguments.
• (End of rationale.)

• Pure Procedure Declaration and Interface
  • Pure function definition
  • Pure subroutine definition
  • Pure procedure interfaces
• Pure Procedure Reference
• Examples of Pure Procedure Usage
• Comments on Pure Procedures