Calling procedures across sibling submodules

They can invoke one another after two adjustments:

• Put interfaces for the child routines into the parent module

• You don't need use, as every child shares that parent module

A full working example follows:

demo.f90:

  module root_mod
  implicit none
  interface
  module subroutine routine_c()
  end subroutine routine_c
  module subroutine routine_d()
  end subroutine routine_d
  end interface
  end module root_mod

  program demo
  use root_mod
  implicit none
  call routine_d()
  end program demo

c.f90:

  submodule(root_mod) c
  implicit none
  contains
  module subroutine routine_c
  print *, "hello from routine_c"
  end subroutine routine_c
  end submodule c

d.f90:

  submodule(root_mod) d
  implicit none
  contains
  module subroutine routine_d
  print *, "hello from routine_d"
  call routine_c()
  end subroutine routine_d
  end submodule d

Putting taskC in the parent module makes it accessible from every submodule, and that isn't what I want.

Sibling Fortran submodules don't know that the others exist. The hierarchy of submodules can, however, be extended as far as you need:

  module root1
  implicit none
  ...
  end module root1

  submodule (root1) layer1
  implicit none
  interface
  module subroutine task1() ! visible to descendants of layer1 but not to root1
  end subroutine
  end interface
  end submodule layer1

  submodule (root1:layer1) layer2
  implicit none
  contains
  subroutine perform_task()
  call task1()
  end subroutine
  end submodule layer2

  submodule (root1:layer1) layer3
  implicit none
  contains
  subroutine perform_task()
  call task1()
  end subroutine
  end submodule layer3

Observe that both layer2 and layer3 define perform_task, because as siblings they cannot see one another's declarations.