Give a submodule as an argument in Fortran code

main.f90

program main

  use ops

  implicit none

  call exec(add)
  call exec(sub)

end program main

subroutine exec(routine)
  implicit none

  interface
    subroutine routine(ans, a, b)
      implicit none
      integer, intent(out) :: ans
      integer, intent(in)  :: a, b
    end subroutine routine
  end interface

  integer :: ans, a, b

  a = 7
  b = 3

  call routine(ans, a, b)
  write(*, "('routine(',i3,',',i3,') = ',i3)") a, b, ans

end subroutine exec

Makefile

FC = gfortran

%.mod: %.f90 %.o
	@:

%.o: %.f90
	$(COMPILE.f) $<


### PROJECT UNIQUE ###

TARGET = main

.PHONY: all
all: $(TARGET)

.PHONY: clean
clean:
	rm -f *.o *.mod $(TARGET)

$(TARGET): main.o ops.o
	$(LINK.f) -o $@ $^

main.o: ops.mod

ops.f90

module ops
  implicit none

contains

  subroutine add(ans, lhs, rhs)
    implicit none

    integer, intent(out) :: ans
    integer, intent(in)  :: lhs, rhs

    ans = lhs + rhs

    return
  end subroutine add

  subroutine sub(ans, lhs, rhs)
    implicit none

    integer, intent(out) :: ans
    integer, intent(in)  :: lhs, rhs

    ans = lhs - rhs

    return
  end subroutine sub

end module ops