Manual Reference Pages - pure (7)

NAME

pure(7f) - [FORTRAN:SPECIFICATION ATTRIBUTE] Pure procedures (LICENSE:PD)

SYNOPSIS

pure subroutine() pure function()

DESCRIPTION

A pure procedure is

o a pure intrinsic procedure (13.1), defined by a pure subprogram,

o a dummy procedure that has been specified to be PURE, or

o a statement function that references only pure functions.

A pure subprogram is a subprogram that has the prefix-spec PURE, or has the prefix-spec ELEMENTAL and does not have the prefix-spec IMPURE. The following additional constraints apply to pure subprograms.

o The specification-part of a pure function subprogram shall specify that all its nonpointer dummy data objects have INTENT(IN).

o The specification-part of a pure subroutine subprogram shall specify the intents of all its nonpointer dummy data objects.

o A local variable of a pure subprogram, or of a BLOCK construct within a pure subprogram, shall not have the SAVE attribute.
Variable initialization in a type-declaration-stmt or a data-stmt implies the SAVE attribute; therefore, such initialization is also disallowed.

o The specification-part of a pure subprogram shall specify that all its dummy procedures are pure.

o If a procedure that is neither an intrinsic procedure nor a statement function is used in a context that requires it to be pure, then its interface shall be explicit in the scope of that use. The interface shall specify that the procedure is pure.

o All internal subprograms in a pure subprogram shall be pure.

o In a pure subprogram any designator with a base object that is in common or accessed by host or use association, is a dummy argument of a pure function, is a dummy argument with INTENT(IN) of a pure subroutine, is a coindexed object, or an object that is storage associated with any such variable, shall not be used

1. in a variable definition context (16.6.7),

2. as the data-target in a pointer-assignment-stmt,

3. as the expr corresponding to a component with the POINTER attribute in a structure-constructor ,

4. as the expr of an intrinsic assignment statement in which the variable is of a derived type if the derived type has a pointer component at any level of component selection, or

5. as an actual argument corresponding to a dummy argument with INTENT(OUT) or INTENT(INOUT) or with the POINTER attribute.

NOTE 12.50 Item 3 requires that processors be able to determine if entities with the PRIVATE attribute or with private components have a pointer component.

o Any procedure referenced in a pure subprogram, including one referenced via a defined operation, defined assignment, defined input/output, or finalization, shall be pure.

o A pure subprogram shall not contain a print-stmt, open-stmt, close-stmt, backspace-stmt, endfile-stmt, rewind-stmt, flush-stmt, wait-stmt, or inquire-stmt.

o A pure subprogram shall not contain a read-stmt or write-stmt whose io-unit is a file-unit-number or *.

o A pure subprogram shall not contain a stop-stmt or allstop-stmt.

o A pure subprogram shall not contain an image control statement (8.5.1).
NOTE 12.51
The above constraints are designed to guarantee that a pure procedure is free from side effects (modifications of data visible outside the procedure), which means that it is safe to reference it in constructs such as a FORALL assignment-stmt or a DO CONCURRENT construct, where there is no explicit order of evaluation.
The constraints on pure subprograms may appear complicated, but it is not necessary for a programmer to be intimately familiar with them. From the programmer’s point of view, these constraints can be summarized as follows: a pure subprogram shall not contain any operation that could conceivably result in an assignment or pointer assignment to a common variable, a variable accessed by use or host association, or an INTENT (IN) dummy argument; nor shall a pure subprogram contain any operation that could conceivably perform any external file input/output or STOP operation. Note the use of the word conceivably; it is not sufficient for a pure subprogram merely to be side-effect free in practice. For example, a function that contains an assignment to a global variable but in a block that is not executed in any invocation of the function is nevertheless not a pure function. The exclusion of functions of this nature is required if strict compile-time checking is to be used.
It is expected that most library procedures will conform to the constraints required of pure procedures, and so can be declared pure and referenced in FORALL statements and constructs, DO CONCURRENT constructs, and within user-defined pure procedures.
NOTE 12.52
Pure subroutines are included to allow subroutine calls from pure procedures in a safe way, and to allow forall-assignment-stmts to be defined assignments. The constraints for pure subroutines are based on the same principles as for pure functions, except that side effects to INTENT (OUT), INTENT (INOUT), and pointer dummy arguments are permitted.

pure (7)

March 11, 2021

Generated by manServer 1.08 from 6d545469-7cf9-4aec-a3bf-e62eacd18ad4 using man macros.

o	a pure intrinsic procedure (13.1), defined by a pure subprogram,
o	a dummy procedure that has been specified to be PURE, or
o	a statement function that references only pure functions.

o	Any procedure referenced in a pure subprogram, including one referenced via a defined operation, defined assignment, defined input/output, or finalization, shall be pure.
o	A pure subprogram shall not contain a print-stmt, open-stmt, close-stmt, backspace-stmt, endfile-stmt, rewind-stmt, flush-stmt, wait-stmt, or inquire-stmt.
o	A pure subprogram shall not contain a read-stmt or write-stmt whose io-unit is a file-unit-number or *.
o	A pure subprogram shall not contain a stop-stmt or allstop-stmt.
o	A pure subprogram shall not contain an image control statement (8.5.1). NOTE 12.51 The above constraints are designed to guarantee that a pure procedure is free from side effects (modifications of data visible outside the procedure), which means that it is safe to reference it in constructs such as a FORALL assignment-stmt or a DO CONCURRENT construct, where there is no explicit order of evaluation. The constraints on pure subprograms may appear complicated, but it is not necessary for a programmer to be intimately familiar with them. From the programmer’s point of view, these constraints can be summarized as follows: a pure subprogram shall not contain any operation that could conceivably result in an assignment or pointer assignment to a common variable, a variable accessed by use or host association, or an INTENT (IN) dummy argument; nor shall a pure subprogram contain any operation that could conceivably perform any external file input/output or STOP operation. Note the use of the word conceivably; it is not sufficient for a pure subprogram merely to be side-effect free in practice. For example, a function that contains an assignment to a global variable but in a block that is not executed in any invocation of the function is nevertheless not a pure function. The exclusion of functions of this nature is required if strict compile-time checking is to be used. It is expected that most library procedures will conform to the constraints required of pure procedures, and so can be declared pure and referenced in FORALL statements and constructs, DO CONCURRENT constructs, and within user-defined pure procedures. NOTE 12.52 Pure subroutines are included to allow subroutine calls from pure procedures in a safe way, and to allow forall-assignment-stmts to be defined assignments. The constraints for pure subroutines are based on the same principles as for pure functions, except that side effects to INTENT (OUT), INTENT (INOUT), and pointer dummy arguments are permitted.

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Manual Reference Pages - pure (7)

NAME

SYNOPSIS

DESCRIPTION