Jason Orendorff wrote:
Thomas Lord wrote:
You miss my point. We have overwhelming empirical evidence
that in some situations it is useful for string-ref to be O(1) and
in other situations for it to be O(n). If an implementation is
to be Scheme, is its choice between those to be mandated by the
report?
If there are two use cases, there should be two APIs. That way,
people know what to expect when they use each one. They can write
portable programs. They can even handle when both kinds of situations
arise in the same program.
...Or was this a metaphor for something else? I guess I'm missing
your point too.
-j
How about an analogy, then? And then I'll apply it.
The analogy is to, say, the handling of local bindings in a
classic lisp -- the choice between shallow and deep binding.
They have very different performance characteristics, at
least if implemented using simple techniques. One may be
a fantastic choice for a dinky little lisp designed to run in
an embedded system, the other a fantastic choice for an
optimizing compiler. (Don't mistake this for a comprehensive
account of the trade-offs between shallow and deep binding
but, hopefully you take my point or can google around to
get the gist of it.)
Now, one lisp dialect can admit both techniques. And
the author of a portable library can write for that dialect,
rather than for either kind of implementation, and their
code can be useful in both.
So, applying the analogy:
It is useful to have a semantic definition for the dialect
that is agnostic about the performance characteristics of
various operations. It can be useful to write code against
that dialect.
-t
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