I would have written the fn like this, if I was following what you've been
doing:
(def m {:planet {:country {:state {:city {:borough 4}}}}})
(let [mm (update-in m [:planet :country :state :city :borough ] (fnil inc
-1))]
(get-in mm [:planet :country :state :city :borough ]))
However, when I run into this pattern I usually
(let [v ((fnil inc -1) (get-in m [:planet :country :state :city :borough
]))]
(assoc-in m [:planet :country :state :city :borough ] v))
The update in does a get, and I do a get-in, so I'm guessing the
performance is pretty similar, though I haven't tested it.
Cheers, Jay
On Thu, Apr 26, 2012 at 12:00 PM, blais <goo...@furius.ca> wrote:
> Hi,
> I have this use-case for (update-in) which keeps showing up in my code
> which I don't have a solution for and I'm wondering if there isn't a
> solution.
> How do I _efficiently_ obtain the new value created by an invocation of
> (update-in), that is, without having to get the modified value by lookups
> in the new map?
>
> Simplified example:
>
> (def m {:planet {:country {:state {:city {:borough 4}}}}})
>
> (let [mm (update-in m
> [:planet :country :state :city :borough]
> (fn [old] (if (nil? old) 0 (inc old))))]
> (get-in mm [:planet :country :state :city :borough]))
>
> (update-in) returns the new/modified map, but what I want is to obtain the
> new count returned by the anonymous function.
> Having to call (get-in) right after is inefficient.
>
> This is obviously a contrived example for this question, but I have a lot
> of real use cases for this (where at the top level of my app I have a ref
> for a deep structure which changes as a response to network events).
>
> Is there an idiomatic way to do this without having to resort to
> mutability?
> (The only solution that comes to my mind is to wrap up update-in and the
> modifier function with a special version that updates a local mutable and
> return both the new map and the new object.)
> Nasty?
>
> Thank you,
>
>
>
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