Mutation is not a bad performance optimization, and is super useful when the
algorithm in question just works better with it.
--Ashton
Sent from my iPhone
> On Nov 10, 2014, at 11:11 AM, Jacob Goodson <submissionfight...@gmx.com>
> wrote:
>
> I like the map suggestion but there are still those times when mutation makes
> for a cleaner hack(imo of course).
>
>> On Monday, November 10, 2014 5:44:25 AM UTC-5, Thomas Heller wrote:
>> @Jacob: If you get too many arguments in a loop I found it best to use a map.
>>
>> (loop [{:keys [a b c] :as state} a-map]
>> (cond
>> (and (= a 1) (= b 2))
>> (recur (update state :a inc)) ;; 1.7+ only, otherwise use update-in
>> ...))
>>
>> Working with named arguments (vs. positional) is a lot more user-friendly
>> since you don't have to repeat everything all the time.
>>
>> HTH,
>> /thomas
>>
>>> On Monday, November 10, 2014 8:21:57 AM UTC+1, Jacob Goodson wrote:
>>> Sometimes, when writing code that loops with a good bit of branching, it
>>> can be quite annoying to stay immutable.
>>>
>>> (loop [way 1
>>> too 2
>>> many 3
>>> args 4
>>> makes 5
>>> things 6
>>> annoying 7]
>>> (cond (and (= way 3) (= too 4)) (recur (inc way).... you get the point.
>>>
>>> Imagine about 14 different conditions under cond and this thing starts
>>> looking like crap. I got around this with macros and pattern matching,
>>> however, I do not think that this happens too often for many clojurians.
>>>
>>>> On Saturday, November 8, 2014 11:49:42 PM UTC-5, Fluid Dynamics wrote:
>>>> I wonder if the OP is aware that you can rebind the same name multiple
>>>> times in a let. For instance
>>>>
>>>> (let [x something
>>>> y otherthing
>>>> x (if (pred? x y) x (some-func x y))
>>>> x (further (complex (calculations x)))
>>>> ...]
>>>> (do-something-with x))
>>>>
>>>> No actual mutability, but most of the times that suffices for whatever you
>>>> might use a mutable local for in another language.
>>>>
>>>> Then there's loop/recur. I'd consider let rebinding and loop/recur long
>>>> before resorting to any sort of mutable. The most significant pain point
>>>> in my experience has been wanting to "smuggle" a side calculation out of
>>>> some closure that has to return something else. The most recent case I ran
>>>> into like that involved (swap! some-atom conj thingy) where the atom held
>>>> a vector, I also wanted to know the new length of the vector, I didn't
>>>> want any race conditions (following up with a (count @some-atom) allowed
>>>> the possibility of the vector changing again in between the swap and the
>>>> deref, but I wanted to know the position of the item just conjed on), and
>>>> dosync/ref seemed like overkill (only the one isolated mutable). I *could*
>>>> have done something like
>>>>
>>>> (let [c (int-array 1)]
>>>> (swap! some-atom (fn [x] (let [x (conj x thingy)] (aset c 0 (count x))
>>>> x)))
>>>> (let [c (aget c 0)]
>>>> ; work with c
>>>> ...))
>>>>
>>>> but it was unnecessary to use this kluge, for swap! returns not the atom
>>>> itself but the new value that was returned by the passed-in function. So
>>>> all I actually needed was
>>>>
>>>> (let [c (count (swap! some-atom conj thingy))]
>>>> ...)
>>>>
>>>> with no mutability besides the atom itself (and in particular no local
>>>> mutability). I've since needed swap!'s return value on another occasion,
>>>> when it was a map, resulting in (get-in (swap! m update-in [k1 k2] f arg1
>>>> arg2) [k1 k2]) to both update the map and have the exact value for the
>>>> sub-key that was updated, as of that update. With maps, it may also be
>>>> possible to store some extra information in the map with a
>>>> ::module-local-keyword without this interfering with anything else, which
>>>> can be pulled out of swap!'s return value, and with several kinds of
>>>> objects you can smuggle extra information out of a closure by adding a
>>>> ::module-local-keyword to the object's *metadata* (in particular, this
>>>> won't perturb the equality semantics of the object, as well as working
>>>> with vectors and several other non-map-like things as well as with records
>>>> and maps. And if you're wanting to return extra information out of an
>>>> ordinary function or a loop where you control how the return value is
>>>> interpreted, you can bind and destructure the return value after making
>>>> that a short vector or a map with several thingys in it.
>>>>
>>>> Lately I hardly ever find myself feeling the need for any kind of local
>>>> mutables, and only small amounts of global state (often nothing, or just
>>>> one atom wrapping a map handled with nesting, update-in, assoc-in, and
>>>> get-in, though refs and dosync will put in an appearance if a high degree
>>>> of concurrency is required).
>
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