Fortran 90 pass subroutine as an argument to the other subroutine

In fortran 90 it is not enough to just declare(unless the passed function has parameters):

	subroutine caller(arg, called)
	integer :: arg
	external :: called
	call called(arg)
	end subroutine caller

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as it is in fortran 77. It actually compiles by pgf90 but does not run (worked for the function which did not take any arguments).
You should actually include interface description for it to work in f90:

	subroutine caller(arg, called)
	integer :: arg
	interface
	subroutine called(arg)
	integer :: arg
	end subroutine called
	end interface
	call called(arg)
	end subroutine caller

view raw passing_subroutine_as_par.f90 hosted with ❤ by GitHub

Update: This appeared also to be the case when passing to a subrouting allocatable array which is supposed to be initialized inside the subroutine. And also, as I understood from the posts of smart people, it is not necessary to describe the interface of the called subroutine if it is declared inside a module.
Below is the example of a subroutine parameter which has an allocatable array as one of the arguments:

	program main
	implicit none
	interface
	subroutine to_be_called(arg, nx, ny, arr)
	integer :: arg, nx, ny
	integer, allocatable, intent(out) :: arr(:, :)
	end subroutine to_be_called
	end interface
	print *, "Hello"
	call caller(5, to_be_called)
	end program main
	subroutine to_be_called(iarg, nx, ny, arr)
	integer :: iarg, nx, ny
	integer, allocatable, intent(out) :: arr(:, :)
	allocate (arr(nx, ny))
	arr = iarg
	end subroutine to_be_called
	subroutine caller(arg, called)
	integer :: arg
	interface
	subroutine called(arg, nx, ny, arr)
	integer :: arg, nx, ny
	integer, allocatable, intent(out) :: arr(:, :)
	end subroutine called
	end interface
	integer :: nx, ny
	integer, allocatable :: arr(:, :)
	nx = 5;
	ny = 5;
	call called(arg, nx, ny, arr)
	print *, size(arr)
	print *, arr
	end subroutine caller

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