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[REBOL] Re: On mutability and sameness

From: joel:neely:fedex at: 13-Jun-2001 9:19

Hi, Ladislav, Have read it. Ladislav Mecir wrote:
> I have got nothing against Occam's razor. But don't forget > the way how modern microprocessors manipulate bits in the > main memory, although they normally can read and write only > bigger chunks of information. >
I haven't (I just haven't wanted to descend to that level of detail in this thread). But since you bring it up... ;-) Don't forget that modern processors can also manipulate bits in registers, despite the fact that there's no way (for many CPU architectures) to have a pointer to a register in the same way that one can have a pointer to a memory address. By my use of the term, such a register can still be mutable. By your usage, it could not be. I truly believe that the concept of whether a value (all by itself!) is mutable is a *different* issue from whether one can have a pointer/reference to that value. Your definition and test cases all hinge on entangling those two issues. Since we're down at the level of hardware implementation, I can give examples of all combinations of mutability and (for lack of a better term) "referrability" (capable of being accessed via a pointer/handle/reference). "Referrable" "Non-referrable" Immutable ROM data, output CPU status data (hardware registers of ID on some CPUs), overflow some memory-mapped bits, etc. controllers Mutable RAM data, control CPU registers, I/O ports registers of some on some CPUs memory-mapped controllers
> Yes, our points of view differ here. The reason is simple: > noone of us succeeded to give a simple definition of > Mutability until now. >
It seems to me that "capable of having one part changed without affecting the remaining parts" is a pretty simple definition. That's the one I use, and it seems to describe
>> a: now == 13-Jun-2001/8:03:15-5:00 >> a/day: 15 == 15 >> a == 15-Jun-2001/8:03:15-5:00
quite adequately. I have yet to see any discussion of why that wouldn't be the case, that didn't drag in the separate issue of whether the value of A is sharable (e.g., by some sort of reference).
> You may dislike it ... >
Not dislike, just disagree, for the reasons stated above. -jn- ------------------------------------------------------------ Programming languages: compact, powerful, simple ... Pick any two! joel'dot'neely'at'fedex'dot'com