Question

Why is it that Fortran will promote a scalar expression to an array, in an expression, but not as an argument to a procedure? In particular, why did the standards body make this design decision? Is it solely because of ambiguity, should the procedure be overloaded? Could an error message in that situation be an alternative approach?

For example, In the code below, the last statement, x = foo(7), produces the GFortran error: Error: Rank mismatch in argument 'a' at (1) (1 and 0).

module m

  public :: foo

  contains

  function foo(a) result(b)
    integer, dimension(:)       :: a
    integer, dimension(size(a)) :: b
    b = a+1
  end function foo

end module m

program p

  use m

  integer, dimension(4) :: x
  integer, parameter, dimension(4) :: y = (/1,2,3,4/)

  x = 7
  x = foo(x)
  x = foo(y)
  x = foo(x + 7)
  x = foo(7)

end program p

This question should have asked about why an array assignment will promote a scalar value source to an array target; unlike an array function. I expect that’s simply a convenient special case though. Any comments gratefully received in the begging caps below.

Answer 1

If you want the function to handle scaler and array arguments, declare it as “elemental” and with scaler dummy arguments. Then it will be able to handle both scaler and array actual arguments, including scaler expressions. Will that meet your need?

The change:

  elemental function foo(a) result(b)
    integer, intent (in)      :: a
    integer :: b
    b = a+1
  end function foo

Perhaps they provided a way to do what you want, and one way was enough?