Recursion – Fortran finally gets it!

Fortran gave recursion a wide berth until Fortran 90. More than 25 years after ALGOL first allowed recursion, Fortran was finally allowing. Modern Fortran is one of the few languages where the function must be explicitly defined recursive. The prefix recursive must be used before the keyword function.

recursive function fibonacci (n) result (fib_result)
   integer, intent (in) :: n
   integer :: fib_result
   if (n <= 2) then
      fib_result = 1
      fib_result = fibonacci (n-1) + fibonacci (n-2)
   end if
end function fibonacci

In Fortran 2018, the specs changed again. Here procedures without an explicit recursive attribute behave as if recursive is specified. F2018 provides a new attribute non_recursive, which can be used to mark a procedure that may not be called recursively.

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