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Pure procedure interfaces

To define interface specifications for pure procedures, the following constraints are added to Rule R1204 in Section 12.3.2.1 of the Fortran 90 standard (defining interface-body):

• An interface-body of a pure procedure must specify the intents of all dummy arguments except POINTER and procedure arguments.

The procedure characteristics defined by an interface body must be consistent with the procedure's definition. Regarding pure procedures, this is interpreted as follows:

• A procedure that is declared pure at its definition may be declared pure in an interface body, but this is not required.
• A procedure that is not declared pure at its definition must not be declared pure in an interface body.

That is, if an interface body contains a PURE attribute, then the corresponding procedure definition must also contain it, though the reverse is not true. When a procedure definition with a PURE attribute is compiled, the compiler may check that it satisfies the necessary constraints.