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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