Assume we want to memoize a function that is part of a protocol extended
to some user-defined record, like so:
(defprotocol SomeOps (someop [_]))
(defrecord AImp [x] SomeOps (someop [me] (comment magic here)))
one quite heavy-weight way of doing is includes a couple of indirections:
(defn ^:private mimp-someop [me]
(println "working with" me)
(:x me))
(def ^:private memoized-mimp-someop (memoize mimp-someop))
(defrecord MImp [x] SomeOps (someop [me] (memoized-mimp-someop me)))
(someop (MImp. 23))
; working with #user.MImp{:x 23}
; 23
(someop (MImp. 23))
; 23
a cleaner? way is to use one defn less and one lambda more..
(def ^:private limp-someop (memoize (fn [me] (println "working with" me)
(:x me))))
(defrecord LImp [x] SomeOps (someop [me] (limp-someop me)))
(someop (LImp. 17))
; working with #user.LImp{:x 17}
; 17
(someop (LImp. 17))
; 17
of course we can inline the def, can't we?
(defrecord IImp [x]
SomeOps
(someop [me] ((memoize (fn [myself] (println "working with" myself) (:x
myself))) me)))
Hmm seems we can't:
(someop (IImp. 11))
; working with #user.IImp{:x 11}
; 11
(someop (IImp. 11))
; working with #user.IImp{:x 11}
; 11
what's wrong? we're recreating the memoized version each time. let/fn to
the rescue:
(defrecord HImp [x]
SomeOps
(someop [me]
; could as well go (memoize (fn [_] (println "working with" me) x)) here..
(let [yay (memoize (fn [myself] (println "working with" myself) (:x
myself)))]
(yay me))))
but wait, the let (or letfn) will still be re-executed each time. We can
move the let outside the defrecord, of course, but then we could as well
go with (def ^:private), no?
So is there a way to do it cleaner without resorting to using the
map/implementation (detail) form of protocol extension?
TIA,
-Martin
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