Title: Pointer function references as lhs in assignment

Submitted by: Aleksandar Donev

Status: For Consideration

References: Interp 000074

Basic Functionality:
Allow references to functions returning an associated pointer to appear as the lhs of 
assignment statement, where the value of the target is changed. This is parallel to the 
way we treat regular pointers.

Example:

FUNCTION storage(key) RESULT(loc)
    INTEGER, INTENT(IN) :: key
    REAL, POINTER :: loc
    loc=>...
END FUNCTION

! Make the following legal
storage(5)=0.5

Rationale:
The proposed change makes the language more symmetrical. It arose while looking at an 
interpretation request, number 000074, titled "TARGET dummy arguments and POINTER 
expressions".

The proposed change also brings in a very useful functionality: It allows one to change 
between different data-structures with little to no change in the source code. Consider 
an example of a dictionary data-structure, where the keys are INTEGER and the data 
values are REAL. 
A simple implementation might use an array to store the values:
TYPE :: Dictionary_t
    REAL, DIMENSION(:), ALLOCATABLE :: storage
    ...
END TYPE

Later however, a more sophisticated data-structure (say a hash-table) that conserves 
memory may be implemented, substituting the array with a pointer-valued function which 
returns a pointer to the storage location corresponding to the key:
TYPE :: Dictionary_t
    REAL, DIMENSION(:), ALLOCATABLE :: storage
CONTAINS
    PROCEDURE :: storage ! See previous function
END TYPE

FUNCTION storage(dictionary, key) RESULT(loc)
    CLASS(Dictionary_t), INTENT(INOUT) :: dictionary
    INTEGER, INTENT(IN) :: key
    REAL, POINTER :: loc
    loc =>...
END FUNCTION
The text of the code will need minor changes:

TYPE(Dictionary_t) :: dictionary
...
dictionary%storage(5)=0.5

Another example in numerical problems includes switching from full to sparse storage 
for a matrix without having to change all the references.

Estimated Impact:
The proposed change requires introducing new syntax rules, however, it is simple in 
nature and so we should easily get it right. The cost to implementations will be low.

Detailed Specification:
Add a second form of <assignment-smtm> in 7.4.1.1 in R734:
    <assignment-smtm> is <lhs> = <expr>
    <lhs> is <variable> or <expr>
    where <expr> is a <function-reference> and is a pointer

History:

Comments:
Malcolm Cohen, Nihon NAG, Tokyo:
(1) I have been sent two related Fortran 2003+ 
suggestions, which I would like the U.K. panel to consider.  They are both from 
Aleksandar Donev.

The second one (pointer_lhs) I have yet to form a definite opinion on, except that I 
think we should consider it.

P.S. If we do decide to put them forward I think the text and examples need a little 
work. I'm willing to help with that.

John Reid, JKR Associates, Oxford: I will advocate that we go for both of these. It 
seems to me to be illogical to go for one and not the other.

Malcolm Cohen, Nihon NAG, Tokyo:
(2) I don't like "pointer lhs" so much, but I don't really object.