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Pure Procedures

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.
• The specification-part of a pure function must specify that all dummy arguments have INTENT(IN) except procedure arguments and arguments with the POINTER attribute.
• A local variable declared in the specification-part or internal-subprogram-part of a pure function must not have the SAVE attribute.
  • Advice to users. Note local variable initialization in a type-declaration-stmt or a data-stmt implies the SAVE attribute; therefore, such initialization is also disallowed. (End of advice to users.)
• The execution-part and internal-subprogram-part of a pure function may not use a dummy argument, a global variable, or an object that is storage associated with a global variable, or a subobject thereof, in the following contexts:
  • As the assignment variable of an assignment-stmt;
  • As a DO variable or implied DO variable, or as an index-name in a forall-triplet-spec;
  • As an input-item in a read-stmt;
  • As an internal-file-unit in a write-stmt;
  • As an IOSTAT= or SIZE= specifier in an I/O statement.
  • In an assign-stmt;
  • As the pointer-object or target of a pointer-assignment-stmt;
  • As the expr of an assignment-stmt whose assignment variable is of a derived type, or is a pointer to a derived type, that has a pointer component at any level of component selection;
  • As an allocate-object or stat-variable in an allocate-stmt or deallocate-stmt, or as a pointer-object in a nullify-stmt; or
  • As an actual argument associated with a dummy argument with INTENT (OUT) or INTENT(INOUT) or with the POINTER attribute.
• Any procedure referenced in a pure function, including one referenced via a defined operation or assignment, must be pure.
• A dummy argument or the dummy result of a pure function may be explicitly aligned only with another dummy argument or the dummy result, and may not be explicitly distributed or given the INHERIT attribute.
• In a pure function, a local variable may be explicitly aligned only with another local variable, a dummy argument, or the result variable. A local variable may not be explicitly distributed.
• In a pure function, a dummy argument, local variable, or the result variable must not have the DYNAMIC attribute.
• In a pure function, a global variable must not appear in a realign-directive or redistribute-directive.
• A pure function must not contain a print-stmt, open-stmt, close-stmt, backspace-stmt, endfile-stmt, rewind-stmt, inquire-stmt, or a read-stmt or write-stmt whose io-unit is an external-file-unit or *.
• A pure function must not contain a pause-stmt or stop-stmt.
• The specification-part of a pure subroutine must specify the intents of all dummy arguments except procedure arguments and arguments that have the POINTER attribute.
• A local variable declared in the specification-part or internal-function-part of a pure subroutine must not have the SAVE attribute.
• The execution-part or internal-subprogram-part of a pure subroutine must not use a dummy parameter with INTENT(IN), a global variable, or an object that is storage associated with a global variable, or a subobject thereof, in the following contexts:

• • As the assignment variable of an assignment-stmt;
  • As a DO variable or implied DO variable, or as a index-name in a forall-triplet-spec;
  • As an input-item in a read-stmt;
  • As an internal-file-unit in a write-stmt;
  • As an IOSTAT= or SIZE= specifier in an I/O statement.
  • In an assign-stmt;
  • As the pointer-object or target of a pointer-assignment-stmt;
  • As the expr of an assignment-stmt whose assignment variable is of a derived type, or is a pointer to a derived type, that has a pointer component at any level of component selection;
  • As an allocate-object or stat-variable in an allocate-stmt or deallocate-stmt, or as a pointer-object in a nullify-stmt;
  • As an actual argument associated with a dummy argument with INTENT (OUT) or INTENT(INOUT) or with the POINTER attribute.
• Any procedure referenced in a pure subroutine, including one referenced via a defined operation or assignment, must be pure.
• A dummy argument of a pure subroutine may be explicitly aligned only with another dummy argument, and may not be explicitly distributed or given the INHERIT attribute.
• In a pure subroutine, a local variable may be explicitly aligned only with another local variable or a dummy argument. A local variable may not be explicitly distributed.
• In a pure subroutine, a dummy argument or local variable must not have the DYNAMIC attribute.
• In a pure subroutine, a global variable must not appear in a realign-directive or redistribute-directive.
• A pure subroutine must not contain a print-stmt, open-stmt, close-stmt, backspace-stmt, endfile-stmt, rewind-stmt, inquire-stmt, or a read-stmt or write-stmt whose io-unit is an external-file-unit or *.
• A pure subroutine must not contain a pause-stmt or stop-stmt.
• An interface-body of a pure procedure must specify the intents of all dummy arguments except POINTER and procedure arguments.
• In a reference to a pure procedure, a procedure-name actual-arg must be the name of a pure procedure.

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