It would probably be better to convert to/from normal Clojure maps at the
edges of the Clojure code. If the input is '((k1 v1) (k2 v2) (k3 v3) (k1
v4)) and we want the earliest occurrence of k1 to "win", then that suggests
(into {} (reverse alist)). If the input's flattened that would be (into {}
(reverse (partition 2 alist))). The reverse makes sure that into assoces
{k1 v4} first and then overwrites it with {k1 v1}, so the earliest
occurrence of k1 "wins". If conj onto a map rejects two-element lists (as
opposed to vectors; I don't have the docs or a repl in front of me where I
am right now) then you'll need a (map vec) in there or something as well.
Output is simpler; (seq amap) alone might suffice, but (map seq amap)
should give you the '((k1 v1) (k2 v2) ...) form from prn, and (mapcat seq
amap) '(k1 v1 k2 v2 ...).
Of course, if you want the key-shadowing behavior not as a cheap overwrite
but as a kind of "stack", so that (drop 1) on the original example would
result in (k1 v4) being a mapping, then you need something beyond a normal
Clojure map -- possibly a map of keys to stacks of values (maintained in
vectors, say), or a stack of maps, depending on the exact use case.
On Mon, Nov 18, 2013 at 10:54 AM, Tassilo Horn <t...@gnu.org> wrote:
> Justin Smith <noisesm...@gmail.com> writes:
>
> Hi Justin & Hans-Peter,
>
> > Typically in clojure we use hash-maps (represented literally as {})
> > where other lisps would use an alist.
>
> One difference between alists and maps is that in alists a "key" can
> occur multiple times, and then the first entry with that key shadows all
> following entries with that key (with respect to retrieval with
> `assoc`).
>
> Well, that feature is probably not used very often, but if you can't
> rule out its usage, you also can't convert alists to maps and expect
> they're equivalent.
>
> > Regarding reading assoc list literals, I wouldn't be surprised if
> > someone had written this function already, but I doubt it is in the
> > core language.
>
> This should do the trick:
>
> (defn alist-assoc [key alist]
> (first (filter #(= key (first %)) alist)))
>
> And to add a list to an alist one would use `cons` in Clojure just like
> one would use `cons` in CL, Scheme, or Elisp, too.
>
> Bye,
> Tassilo
>
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