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[REBOL] Re: On mutability and sameness

From: joel:neely:fedex at: 7-Jun-2001 8:23

Hello, Volker, Volker Nitsch wrote:
> Am 07.06.01, 00:07:53, schrieb "Larry Palmiter" > <[larry--ecotope--com]> zum > Thema [REBOL] Re: On mutability and sameness: > > > Hi Joel > > > > But it is equally inaccurate to assume that they are > > > fixed-size > > > scalar values, isn't it? > > > > > > >> foo: > > > == > > > >> repeat i 10 [print foo/:i] > > > >> foo: > > ** Syntax Error: Invalid tuple -- >
You're right. Experiments confirm that ten is the maximum number of elements in a tuple.
> > The "scalar" types could be tagged pointers (references) > > as suggested by Volker. A few bits in the pointer could > > designate the type and whether the value is contained > > within the pointer. As you probably know this is a common > > way of implementing "scalar" types in Scheme.
> Ok, correction: max slot size 10 byte. Not much different > to ~8 (around 8). ;-) > i was speculationg a usual floating point number needs 8 byte, > a pair! 2* 4-byte integer, so 8 sounded good. > > Well, with 16 bytes there is space for > 1 native pointer/handle(4 bytes) > + 10 bytes data > + 2 byte something. > some kind of type-description + tuple-length? > i think this would be most simple to implement >
Yes. This was what I referred to earlier as a "self-relative descriptor". That technique has been around since the late 60's. A data value is represented by a descriptor, which contains type information and a "pointer" to the data. To save space and time, however, data which can fit into the pointer portion of the descriptor are placed there. There are some variations on this theme. The decision as to where to allocate the data storage may be determined solely by the data type (e.g., integer). In this case, such data values are not "sharable". OTOH, the decision may be deferred, in which case there is a need for a separate bit in the descriptor -- the "self-relative" bit -- that indicates whether the data is inside the descriptor or simply referenced by the descriptor. In this case, even "small" data values may be shared. The behavior we've been exploring is consistent with the idea that REBOL uses a type-dependent descriptor, and implements the refinements on DATE! values as special-purpose mutations on the self-relative value. -jn- ------------------------------------------------------------ Programming languages: compact, powerful, simple ... Pick any two! joel'dot'neely'at'fedex'dot'com