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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