@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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