Pointers in pure functions

Question

In order to traverse a linked list in Fortran, I use a pointer to the current element that is moved to the next one inside a loop. Trying to apply this inside a pure function that operates on said linked list results in an error.

Example:

module list
  implicit none

  ! Node
  type n_list
    integer               :: val
    type(n_list),pointer  :: next => NULL()
  end type

  ! Linked list
  type t_list
    type(n_list),pointer  :: head
  end type

contains

  pure function in_list( list, val ) result(res)
    implicit none
    class(t_list),intent(in)  :: list
    integer,intent(in)        :: val
    logical                   :: res
    type(n_list),pointer      :: cur

    res = .true.
    ! Traverse the list
    cur => list%head
    do while ( associated(cur) )
      if ( cur%val == val ) return 
      cur => cur%next
    enddo

    ! Not found
    res = .false.
  end function
end module

Results in

    cur => list%head
         1
Error: Bad target in pointer assignment in PURE procedure at (1)

I am aware of the rationale behind the error/warning, and that it is difficult to ensure that the arguments of the function are not changed when using pointers (Fortran 2008, ch. 12.7 "Pure procedures", esp. C1283). In this case, though, list is never changed.

Is it possible to tell the compiler (ifort and gfortran) that intent(in) is not violated?

Answer 1

OK, I found a solution using the transfer intrinsic. The main idea is to clone the list struct (without the data, I checked), and use the pointer to the first node (unchanged) as a start value. Yeah, it is a loop-hole, but both ifort and gfortran accept this without warnings.

module list_mod
  implicit none

  ! Node
  type n_list
    integer               :: val
    type(n_list),pointer  :: next => NULL()
  end type

  ! Linked list
  type t_list
    type(n_list),pointer  :: head
  end type

contains

  pure function getHead(list) result(res)
    implicit none
    class(t_list),intent(in)  :: list
    type(n_list),pointer      :: res
    type(t_list),pointer      :: listPtr

    ! Create a copy of pointer to the list struct
    allocate( listPtr )
    listPtr = transfer( list, listPtr )

    ! Set the pointer
    res => listPtr%head

    ! Free memory
    deallocate( listPtr )
  end function

  pure function in_list( list, val ) result(res)
    implicit none
    class(t_list),intent(in)  :: list
    integer,intent(in)        :: val
    logical                   :: res
    type(n_list),pointer      :: cur

    res = .true.

    ! Traverse the list
    cur => getHead(list)
    do while ( associated(cur) )
      if ( cur%val == val ) return
      cur => cur%next
    enddo

    ! Not found
    res = .false.
  end function

end module

program test
  use list_mod
  implicit none
  integer,parameter     :: MAXELEM = 10000000
  integer               :: i
  type(t_list)          :: list
  type(n_list),pointer  :: cur

  ! Fill list
  list%head => NULL()
  allocate( list%head )
  list%head%val = 1

  cur => list%head
  do i=2,MAXELEM
    allocate( cur%next )
    cur%next%val = i
    cur => cur%next
  enddo !i

  print *,'is MAXELEM/2 in list? ', in_list( list, MAXELEM/2 )
  print *,'is MAXELEM+1 in list? ', in_list( list, MAXELEM+1 )
end program