On Mon, Sep 13, 2010 at 12:37 PM, Tom Hicks <hickstoh...@gmail.com> wrote:
>
> On Sep 12, 10:44 pm, Meikel Brandmeyer <m...@kotka.de> wrote:
>>
>> The default map specifies the default for the symbols which are bound,
>> not the source of the values.
>>
>> (let [{foo :some-key bar "some-other-key" :or {foo 1}} ....] ...)
>
> Sorry Meikel, I don't understand your answer. My question was why does
> the
> default map not lookup the values using the same lookup method as the
> original map (:keys, :strs, or :syms)? Why does it always assume
> symbols for keys?
What he's saying is that the map after the :or is not doing a lookup
in the original map you're destructuring, so it's not a question of
doing the same kind of lookup or not. The map after the :or is a map
of local symbols to default values (as opposed to being a map of
keys-found-in-the-original-map to default values.
(let [foo bar] ...)
Assigns the value of bar to the local symbol foo
(let [{:keys [foo]} {:foo "bar"}] ...)
Assigns the string "bar" to the local symbol foo by looking up the
keyword :foo in the supplied map
(let[{:keys [foo baz] :or {baz "quux"}} {:foo "bar"}]...)
The tricky bit: it looks up the key :foo in the supplied map, and
creates a local symbol "foo" with the value "bar". Next, it looks up
the keyword :baz (which is nil) and creates a local symbol named "baz"
that is also nil. At this point, it's done looking things up in the
supplied map. Now we're up to the :or part. For each local symbol with
a nil value, we look in the :or map to see if there's a key with the
same symbol. If there is, we use that as the value of the local symbol
instead of nil. But the point is we're looking up local symbols now,
not anything in the original map.
Caveat: that's a high-level overview from a fairly new Clojure user
(me) who is only attempting to rephrase what he thinks he's understood
from reading the words of people who know what they're talking about.
The source code for :or destructuring may use a different process
completely, or I may have been more or less correct. I don't really
know, but if I've mangled anything I'm hopeful someone will correct
me.
Cheers.
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