On Fri, Jan 18, 2013 at 3:37 PM, Stephen Compall
<stephen.comp...@gmail.com> wrote:
> On Jan 18, 2013 2:36 PM, "Ben Wolfson" <wolf...@gmail.com> wrote:
>> >> I could just extend ILookup, which contains only valAt, but I'd also
>> >> like to be able to implement containsKey and entryAt, which AFAICT are
>> >> only found in Associative.
>
> Associative also contains
>
> Associative assoc(Object key, Object val);
>
> I would propose, as a law, that anAssociative.assoc(k, v).valAt(k) = v. Do
> you satisfy that? (An enumeration of Associative laws would be helpful
> here.)
No, I don't: assoc is a is a no-op (as is without). Both assoc and
without are, in fact, only there to prevent set/rename-keys and a few
other functions from blowing up (set/rename-keys is *also* a no-op
with those definitions of assoc and without---but given the definition
of valAt, rename-keys being a no-op is correct).
However, it would certainly be possible to support that behavior while
retaining the nonsensicality of calling seq on the structures:
(defn constmap [o & [override]]
(let [override (or override {})]
(reify
clojure.lang.Associative
(containsKey [this _] true)
(entryAt [this k] (if (.containsKey override k)
(.entryAt override k)
(clojure.lang.MapEntry. k o)))
(valAt [this k] (second (.entryAt this k)))
(assoc [this k v] (constmap o (.assoc override k v))))))
> Anyway, anyone is free to assume that, given an IPersistentMap (or what have
> you), all of the laws of its superclasses are also implied.
Yes, that is probably so.
> So I think
> inheriting Seqable is a positive statement of this law:
>
> anAssociative.entryAt(k) returning entry E,
> implies that (seq anAssociative) contains E.
>
> But I cannot be sure. Without this law, though, you are basically left with
> (pure) function.
Right: I basically want (constantly 'foo), but supporting the map
interface. Or, rather, a subset of the map interface as it's currently
defined: it seems odd to me that being able to answer the question "do
I contain x?" implies "I can enumerate my contents".
--
Ben Wolfson
"Human kind has used its intelligence to vary the flavour of drinks,
which may be sweet, aromatic, fermented or spirit-based. ... Family
and social life also offer numerous other occasions to consume drinks
for pleasure." [Larousse, "Drink" entry]
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