Fortran Wiki
recursion

Recursion

Fortran 77 did not explicitly allow recursive functions and subroutines, although there were several ways around this eg Fortran Examples. From Fortran 90 onwards, recursive functions and subroutines could be declared and used freely.

A well known recursion benchmark:

 program ackermann
   integer :: ack
   write(*,*) ack(3, 12)
 end program ackermann

 recursive function ack(m, n) result(a)
   integer, intent(in) :: m,n
   integer :: a
   if (m == 0) then
     a=n+1
   else if (n == 0) then
     a=ack(m-1,1)
   else
     a=ack(m-1, ack(m, n-1))
   end if
 end function ack