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9.62 CO_REDUCE — Reduction of values on the current set of images

CO_REDUCE determines element-wise the reduction of the value of A on all images of the current team. The pure function passed as OPERATOR is used to pairwise reduce the values of A by passing either the value of A of different images or the result values of such a reduction as argument. If A is an array, the deduction is done element wise. If RESULT_IMAGE is present, the result values are returned in A on the specified image only and the value of A on the other images become undefined. If RESULT_IMAGE is not present, the value is returned on all images. If the execution was successful and STAT is present, it is assigned the value zero. If the execution failed, STAT gets assigned a nonzero value and, if present, ERRMSG gets assigned a value describing the occurred error.
Technical Specification (TS) 18508 or later
Collective subroutine

A is an INTENT(INOUT) argument and shall be nonpolymorphic. If it is allocatable, it shall be allocated; if it is a pointer, it shall be associated. A shall have the same type and type parameters on all images of the team; if it is an array, it shall have the same shape on all images.
OPERATOR pure function with two scalar nonallocatable arguments, which shall be nonpolymorphic and have the same type and type parameters as A. The function shall return a nonallocatable scalar of the same type and type parameters as A. The function shall be the same on all images and with regards to the arguments mathematically commutative and associative. Note that OPERATOR may not be an elemental function, unless it is an intrisic function.
RESULT_IMAGE (optional) a scalar integer expression; if present, it shall have the same the same value on all images and refer to an image of the current team.
STAT (optional) a scalar integer variable
ERRMSG (optional) a scalar character variable

          program test
            integer :: val
            val = this_image ()
            call co_reduce (val, result_image=1, operator=myprod)
            if (this_image() == 1) then
              write(*,*) "Product value", val  ! prints num_images() factorial
            end if
            pure function myprod(a, b)
              integer, value :: a, b
              integer :: myprod
              myprod = a * b
            end function myprod
          end program test

While the rules permit in principle an intrinsic function, none of the intrinsics in the standard fulfill the criteria of having a specific function, which takes two arguments of the same type and returning that type as result.
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