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bit or byte?
This page is part of the web mail archives of SRFI 74 from before July 7th, 2015. The new archives for SRFI 74 contain all messages, not just those from before July 7th, 2015.
It is a controversial thing, but it
is also important:
Why is the smallest
addressable unit in a blob an octet and not a bit?
We should think about the implications
first, but I think it will be a great
benefit if this SRFI would be bit-accurate
instead of byte-accurate.
The benefits of bit-accurate blobs are
1. They are occasionally useful as a
data type themselves, e.g. for representing
subsets of a fixed finite set.
2. Many binary format specifications
(e.g. in communication systems, networking,
image compression, ...) are specified
in terms of not necessarily byte-aligned bit
fields. Implementing bit-accurate access
on top of bytes is error prone and tedious.
(I know everything about that from communication
The cost of being bit-accurate are as
* Implementations might be tempted to
represent the length as a FIXNUM. If
this is design is chosen, and immediate
integers are 30-bits, the maximal blob is
just 64 MByte. This will very likely
turn out to be a problem one day.
* The implementation is substantially
complicated. In particular the COPY!
operation is not an easy thing to write
if it is to be efficient. (I did do that for Ocaml.)
* Users might forget that they have
to multiply the length by 8 to get bytes.
Note that the actual execution speed
is unaffected since blob-operations that are
byte- or word-aligned can (and should)
be recognized as special cases.