Is creating nested subroutines/functions considered good practice in Fortran?

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@aerosayan , you may want to keep “Modern Fortran Explained” handy as that can provide a ready handbook reference with a fair amount of prose to provide the background on the semantics and syntax of the ISO IEC language standard for Fortran which online forum comments cannot quite provide.

With that in mind, you can review this example:

module solver_m
   abstract interface
      function Ifunc( x ) result(r)
         ! Argument list
         real, intent(in) :: x
         ! Function result
         real :: r
      end function 
   end interface
contains
   subroutine eval( x, func )
      ! Argument list
      real, intent(in) :: x
      procedure(Ifunc) :: func
      print *, "In eval: f(x) = ", func(x)
   end subroutine 
end module 
module calc_m
   use solver_m, only : eval
contains
   subroutine sub( x )
      ! Argument list
      real, intent(in) :: x
      call eval( x, myfunc )   !<-- NOTE: `myfunc` as procedure argument 
   contains
      function myfunc( x ) result(r)
         ! Argument list
         real, intent(in) :: x
         ! Function result
         real :: r
         real, parameter :: a = 1.0, b = 2.0, c = 1.0
         r = a*x**2 + b*x + c
      end function 
   end subroutine 
end module
   use calc_m, only : sub
   call sub( x=99.0 )
end
  • The response you can expect by a processor as confirmed by gfortran on Windows is as follows:
C:\temp>gfortran -ffree-form p.f -o p.exe

C:\temp>p.exe
 In eval: f(x) =    10000.0000

Now, note my strong recommendation to Fortranners will be to strive for IMPORT, NONE if they decide to pursue such internal subprograms. However, the gfortran users will have to become developers or support development to enhance the GCC/gfortran to support this Fortran 2018 feature.

..
   contains
      function myfunc( x ) result(r)
         import, none
         ! Argument list
         real, intent(in) :: x
..
  • response using Intel Fortran:
C:\temp>ifort /free /standard-semantics p.f
Intel(R) Fortran Intel(R) 64 Compiler Classic for applications running on Intel(R) 64, Version 2021.10.0 Build 20230609_000000
Copyright (C) 1985-2023 Intel Corporation.  All rights reserved.

Microsoft (R) Incremental Linker Version 14.36.32537.0
Copyright (C) Microsoft Corporation.  All rights reserved.

-out:p.exe
-subsystem:console
p.obj

C:\temp>p.exe
 In eval: f(x) =  10000.00

C:\temp>ifx /free /standard-semantics p.f
Intel(R) Fortran Compiler for applications running on Intel(R) 64, Version 2023.2.0 Build 20230627
Copyright (C) 1985-2023 Intel Corporation. All rights reserved.

Microsoft (R) Incremental Linker Version 14.36.32537.0
Copyright (C) Microsoft Corporation.  All rights reserved.

-out:p.exe
-subsystem:console
p.obj

C:\temp>p.exe
 In eval: f(x) =  10000.00
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