Preprocessor conditional datatype best practice

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If this is the case, I think that the approach you proposed is the best way to go. However, since you want to add complex input functionality, I would suggest creating routines for real and complex types separately in a module and then use them in the main program:

module defs
  implicit none
  private
  public :: work
  interface work
    module procedure :: work_r
    module procedure :: work_c
  end interface work
contains
  subroutine work_r(x)
    real, intent(out) :: x
    x = (2.5, 3.1)
  end subroutine
  subroutine work_c(x)
    complex, intent(out) :: x
    x = (2.5, 3.1)
  end subroutine
end module defs

program main
  use defs, only: work
  implicit none
  real :: x
  complex :: y
  call work(x)
  call work(y)
  print*, x
  print*, y
end program main

To generate this module from the program that you already have, I would suggest you take a look at Fypp, a thread was opened here recently.

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