Re: [Scheme-reports] Seeking review of sets and hash tables proposals

[Kevin Wortman <kwortman@x>]

- It is harder to explain and understand.  It requires a new helper
   procedure that returns a weird object.

+1. By the principle of minimal surprise, I would prefer to either make do with using two conventional procedures, or create a new type that behaves like a conventional record.

 

That to me suggests another idea: Perhaps we can introduce
a "comparator" type which is a record with (optionally):
   - an equality predicate
   - an ordering operator (either like < or which returns -1/0/1)
     (If the ordering operator returns -1/0/1 then it can provide
     a default for equality predicate.)
   - a hash operation
One or more of these can be "don't care".

A comparator object can be passed to a hash-table constructor, to
a sort routines, to a binary search routine, etc.

I like this idea, especially the part about reusing one representation of orderedness ini all those places.

Actually a comparator can be defined by just a well-behaved < predicate and a hash procedure, since

(> a b)
is equivalent to

(< b a)

and

(= a b)
is equivalent to

(and (not (< a b))

       (not (< b a)))

Kevin Wortman

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