Examples for the new @ rank agnostic array notation

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This feature will be more useful if rank is allowed to be allocatable. For example, it can be used for generalizing ‘reduce/fold’ functions as shown below. (I have left some details for brevity.)

pure function general_reduce(binary_op,x,dim) result (r)
  real   ,intent(in) :: x(..)
  integer,intent(in) :: dim
  real,allocatable  :: r(..)
  integer :: i, x_shape(RANK(x)), idx(RANK(x)), idr(RANK(x)-1)
  interface
    real pure function binary_op(x,y)
      real,intent(in) :: x, y
    end function binary_op
  end interface

  x_shape = SHAPE(x)
  r = ALLOCATE(r([x_shape(:dim-1),x_shape(dim+1:)]))
  do i = 1,PRODUCT(x_shape)
    idx = counter_to_tuple(i)
    idr = [idx(:dim-1),idx(dim+1:)]
    if(idx(dim)==1) then
      r(@idr) = x(@idx)
    else
      r(@idr) = binary_op(r(@idr),x(@idx))
    end if
  end do
contains
  pure function counter_to_tuple(n) result (t)
  ! find index tuple from 1D counter. e.g. 1->(1,1,1), 2->(2,1,1) etc
    integer,intent(in) :: n
    integer :: t(RANK(x))
    ...
  end function counter_to_tuple
end function general_reduce

One can still achieve this Fortran by:
a) Using PACK and RESHAPE. But, this will require memory duplication.
b) Defining an array type. This can be as close to the proposed syntax as x.AT.[1,2,1] etc.

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