On Mon, Aug 25, 2008 at 9:16 AM, Chouser <[EMAIL PROTECTED]> wrote:
> On Mon, Aug 25, 2008 at 8:45 AM, Stuart Halloway
> <[EMAIL PROTECTED]> wrote:
>>
>> (defn keys-with-value-matching [map test-fn]
>> (for [pair (map identity map) :when (test-fn (last pair))]
>> (first pair)))
>>
>> (defn basic-colors-in [color]
>> (keys-with-value-matching color (comp not zero?)))
>
> I'm not sure why you used "(map identity map)" instead of just "map",
Sorry to reply to myself, but immediately after sending I realized
(map identity map) isn't doing what I had thought it was, and since
others may be similarly confused...
You've already shadowed the builtin clojure/map function with your
function argument named map. So inside that function, it doesn't
matter whether you use the word "map" in the initial position or in
the last position -- both uses of the word "map" in (map identity map)
refer to your argument. In order to get to the builtin function you'd
have to say "clojure/map".
When I first read your code I assumed, probably partially because of
the use of "identity", that that expression was calling clojure/map.
But since it's not, how is it working? Let's say the hash-map {:foo
:bar} is passed in for "map". That means the expression in question
is the same as ({:foo :bar} identity {:foo :bar}). A hash-map in the
function position like that looks up it's first parameter ("identity"
in this case) in itself. Chances are the identity function wasn't
used as a key the hash-map, so the lookup will fail. Therefore the
lookup falls back to the default expression given in the next
argument, which in this case is the hash-map itself {:foo :bar}. And
thus we get the return value we were expecting, but for entirely
different reasons.
This also means there's a subtle bug lurking in the code. Not likely
to be a problem, I admit, but it would nonetheless be surprising:
user=> (color-string (assoc (struct color 20 30 10) identity {:red 99}))
"Red: 20"
By supplying a color struct that has an "identity" key, color-string
uses the key list from identity's value (that is, just :red), but the
value number from the original struct.
I don't know if that made any sense to anyone, but anyway...
--Chouser
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