On Thu, Jan 13, 2011 at 4:05 PM, Eduardo Julian <eduardo...@gmail.com> wrote:
> I noticed that although you can use assoc with sequences and vectors
> to modify them
I get an exception trying to "assoc" a sequence in 1.2.
> you could not use dissoc to eliminate elements from
> them. Why is this so?
Deletion in the middle is not easy with lists or vectors. (Interesting
that (dissoc '(1 2 3) 0) and (dissoc [1 2 3] 2) don't work, though;
(assoc [1 2 3] 3 4) works to extend by one so why not allow dissoc to
contract by one? Although it would just be synonymous with pop. And
(assoc '(1 2 3) 0 7) doesn't work, though (cons 7 '(1 2 3)) would do
the same thing anyway, and (rest '(1 2 3)) the same as (dissoc '(1 2
3) 0).)
Treating a vector as a map from integers to objects, perhaps dissoc
could quasi-work on vectors by having (dissoc v n) act like (assoc v n
nil); then again, it wouldn't cause (contains? v n) to go from true to
false, which people might want from dissoc.
It's probably best to leave things as they are. Need to shorten by
one? Use pop (vectors) or rest (lists). Need to really dissoc? Use a
"sparse vector", that is, an actual map with integer keys, like {0 1,
1 2, 2 3} instead of [1 2 3].
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