[plt-dev] scheme/set

I agree with Matthew here, also based on Java experiences.


On Feb 23, 2010, at 3:20 PM, Matthew Flatt wrote:

> At Tue, 23 Feb 2010 14:09:02 -0500, Carl Eastlund wrote:
>> The one real issue I see is the name make-set.  I would much prefer
>> make-hash-set (with *eq and *eqv versions), which indicates  
>> explicitly
>> what kind of set it is, and leaves us open to future kinds of sets.
>
> The flip side is that `make-set' doesn't commit to a particular
> implementation --- just a particular equivalence predicate.
>
>
> I think Java uses the `make-hash-set' strategy a lot, and I'm pretty
> sure the following has happened to me on more than one occasion:
>
> * I've wanted a Foo.
>
> * I discover that Foo is an interface with specific implementations
>   XFoo, YFoo, and ZFoo.
>
> * It turns out that any of X, Y, or Z would work for my simple
>   purposes, but I have to learn about them and pick one.
>
> * I'd really rather just say `new Foo' and let someone else pick a
>   good default implementation for me.
>
> In contrast to Java, nothing prevents us from having `make-set' (the
> equivalent of `new Foo') while later making `set?' an interface-like
> predicate, should we eventually decide that the generality is
> worthwhile.
>
> Along similar lines, we could embed "equal" in the name `make-set',  
> but
> `equal?' is such a good default for the equivalence relation that it's
> nicer in practice to just call it `make-set'.
>
>
>> Just like we don't call make-hash "make-dict", because there are many
>> kinds of dicts,
>
> Does anyone use dictionaries other than hash tables? So far,
> `scheme/dict' doesn't seem to be worth even the day or two I spent on
> it.
>
> The sequence generalization, in contrast, has definitely paid off. I
> think it's because we so often need to combine different kinds of
> sequences, such as iterating over a list and natural numbers in
> parallel; otherwise, we'd just use `map', `vector-map', etc. I don't
> see lots of applications that need to deal with two different
> representations of sets at the same time, though.
>
>
>> Similarly for set-eq? and set-eqv?; we should have hash-set-eq? and
>> hash-set-eqv?, because these predicates don't mean anything for  
>> "sets"
>> in general.
>
> It seems like the notion of equality used by a set is relevant to
> operations on the set. What does it mean to union two sets that have
> different notions of equivalent elements?
>
>
>
> Again: I'm not going to stand in the way of anyone who wants to make
> the `scheme/set' library more generalizable and extensible. But please
> don't take away `make-set'.
>
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