building other EQUIV?-like predicates

[Sebastian Egner <sebastian.egner@xxxxxxxxxxx>]

Will,

Good thing! Fills a long standing gap
in Scheme,
and whenever I actually needed EQUIV?
there was
no time to implement it reliably (so
I always
ended up with a hack for the specific
situation).

There is one thing I was asking myself
when I
saw the SRFI:

In many cases I need an application-specific
equivalence which is not among the predefined
ones. For example, it should deal with
cyclic
structures the way EQUIV? does, but
floats are
compared only up to a certain tolerance
(not
that this results in an actual 'equivalence,'
but it is often needed as a predicate
anyhow).
Another example is the presence of user-defined
record types; equivalence predicates
that are
meaningful for the application can ignore
some
fields while recursing only on others.

In these situations, I would still end
up implementing a 
lot of code similar to the reference
implementation you 
give but with little modifications in
the recursion, or
for the atomic types.

So I asked myself if it would be feasible
to make
a 'toolkit' for defining recursive equivalence
relations. Without having thought thoroughly
about
this, I think the toolkit can take the
form of a single macro,
say SELECT-EQUIV?.

In SRFI 67 (compare procedures) there
is a macro
SELECT-COMPARE which makes it a convenient
thing
to define a (partially) recursive comparison
procedure:

    http://srfi.schemers.org/srfi-67/srfi-67.html#node_toc_node_sec_4.5

An example is DEFAULT-COMPARE:

    http://srfi.schemers.org/srfi-67/srfi-67.html#node_toc_node_sec_4.4

For SRFI 67 we did not want to go into
cyclic structures at all,
mainly because the SRFI was already
becoming too complex
for its overall purpose.

So my concrete question/proposal:

Do you think a 'SELECT-EQUIV?' macro
would make
sense which dispatches on the types
of its two arguments
to control recursion, but can handle
cyclic structures by 
maintaining a stack of subtasks?

Sebastian.