Oh boy do I owe Constantine an apology! Just goes to prove that too much
coffee and too little sleep can make you really really stupid! Also
obnoxious. :-)
So first, Constantine is right. We need something like what he is
calling "facets." As I thought about I/O today, I realized (a) why we
need them and (b) that we already have them (sort-of).
For all externally observable purposes, a facet is simply a structure
all of whose member fields are procedures. For example:
(defstruct trivstream
get : (fn () char)
put : (fn (char) ()))
We can instantiate this using something like the following:
(local ((define repr-state (make-stream-state))
(trivstream (lambda () body-returning char)
(lambda (char) body-consuming-char)))
If this corresponds behaviorally to what Constantine means by facets
(does it?) then I see two remaining issues:
1. Should there be a convenience syntax for this? [I am coming to
dislike LOCAL, but that is a side issue].
2. Should facets differ from structures by creating a vtable-like data
structure, on the theory that the method table might grow large and can
be shared? That is, where the actual object was a struct of the form
(defstruct (facet-rep 'a)
state : 'a
methods : (struct-containing-methods 'a))
the only difference between this and the structure form would be
implicitly performing the insertion of the arguments.
For the moment, I think that the right thing to do is do this using
structures until we have a better sense of what we need.
shap
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