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Pure Procedure Declaration and Interface

If a user-defined procedure is used in a context that requires it to be pure, then its interface must be explicit in the scope of that use, and that interface must specify the PURE attribute. This attribute is specified in the function-stmt or subroutine-stmt by an extension of rules R1217 (for prefix) and R1220 (for subroutine-stmt) in the Fortran 90 standard. Rule R1216 (for function-stmt) is not changed, but is rewritten here as Rule for clarity.

XBNF prefix -to -rule to to is prefix-spec [ prefix-spec ] ...

prefix-spec -to -rule to to is type-spec -to or RECURSIVE -to or PURE -to or extrinsic-prefix

function-stmt -to -rule to to is [ prefix ] FUNCTION function-name function-stuff

function-stuff -to -rule to to is ( [ dummy-arg-name-list ] ) [ RESULT ( result-name ) ]

subroutine-stmt -to -rule to to is [ prefix ] SUBROUTINE subroutine-name subroutine-stuff

subroutine-stuff -to -rule to to is [ ( [ dummy-arg-list ] ) ] XBNF

• A prefix must contain at most one of each variety of prefix-spec.

• The prefix of a subroutine-stmt must not contain a type-spec.

(For a discussion of the extrinsic-prefix (Rule ), see Section .)

A procedure with the PURE attribute is referred to as a ``pure procedure'' in the following constraints.

• Pure function definition
• Pure subroutine definition
• Pure procedure interfaces