I got it, Think you very much!
在 2014年2月4日星期二UTC+8上午2时01分50秒，Justin Smith写道：
>
> if pred is false or nil (the two cases "when" would rule out), it would be
> an error to apply it to an argument
>
> #{\a \b} is a literal set syntax, containing keys \a and \b. {\a \b} is a
> literal hash-map syntax, with one key \a mapped to the value \b. As far as
> index-filter is concerned, it only selects \a, since that is the only key
> you can match.
>
> On Monday, February 3, 2014 7:57:55 AM UTC-8, Jim foo.bar wrote:
>>
>> a vector is a function of its indices
>> a map is a function of its keys
>> a set is a function of its elements
>>
>> does this help at all?
>>
>> Jim
>>
>>
>>
>> On 03/02/14 15:29, action wrote:
>> > (defn index-filter [pred coll]
>> > (when pred
>> > (for [[idx elt] (indexed coll) :when (pred elt)] idx)))
>> >
>> > (index-filter #{\a \b} "abcdef")
>> > -> (0 1)
>> >
>> > but I don't know why use "when pred" in the code,
>> > and why "(index-filter {\a \b} "abcdef")" doesn't work?
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