To: J3                                                     J3/23-188
From: Brad Richardson
Subject: Possible Solutions to Long Templates
Date: 2023-June-13

#Reference:

Introduction:
-------------

As many plenary members have observed, the CONTAINS section of
template definitions appears in the specification section of their
surrounding scope.  For larger templates and especially in the case of
nested templates, this can significantly impair the legibility of the
source code.  Note that the "obvious" analog of SUBMODULE does not work
for technical reasons.  Subgroup intends to explore mechanisms to improve
this situation.

As example, consider a template like the following:

  TEMPLATE A(T, plus)
    REQUIRES binary_op(T, plus)

    TEMPLATE B(times)
      REQUIRES binary_op(T, times)
    CONTAINS
      PURE FUNCTION product(array)
        TYPE(T), intent(in) :: array(:)
        TYPE(T) :: product

        INTEGER :: i

        product = array(1)
        DO i = 2, size(array)
          product = times(product, array(i))
        END DO
      END FUNCTION
    END TEMPLATE
  CONTAINS
    PURE FUNCTION sum(array)
      TYPE(T), intent(in) :: array(:)
      TYPE(T) :: sum

      INTEGER :: i

      sum = array(1)
      DO i = 2, size(array)
        sum = plus(sum, array(i))
      END DO
    END FUNCTION
  END TEMPLATE

The important contents of the outer template, A, appear after the contents
of the inner template B, which causes it to be difficult to read and
understand the purpose of the outer template.