You're right. The following includes code for handling this case via setin
and getin. I've also ditched macros, because that code couldn't support new
lexical scopes in the setter/getter definition. setin getin support works by
dynamically resolving getters and setters, thus this is slower than direct
access via assoc-in and get-in.
Personally I have to say now that this is written, I prefer the terseness
of:
(set-x some-map v)
over
(assoc some-map :x v)
This
(get-x some-map)
isn't as nice as
(:x map)
But it's also more explicit about how the map is being used- that a level
indirection is required because the implementation might change.
;; ==========================================
(comment
(accessors x y z)
; {:x 4}
(set-x {} 4)
(defset x [m v]
(assoc (assoc m :new "new") :x v))
;; {:new "new", :x {:y {:z 1}}}
(setin {:x {:y {:z 0}}} '(x y z) 1)
;; 0
(getin {:x {:y {:z 0}}} '(x y z))
; {:x 4, :new "new", :foo 9}
(let [z {:foo 9}]
(set-x z 4))
; 10
(let [y {:z 10}]
(get-z y))
; [0 1 2]
(map get-x [{:x 0}, {:x 1}, {:x 2}]))
(defn setter* [sym]
`(defn ~(symbol (str "set-" sym)) [~'m ~'v]
(assoc ~'m ~(keyword (str sym)) ~'v)))
(defn getter* [sym]
`(defn ~(symbol (str "get-" sym)) [~'m]
(~(keyword (str sym)) ~'m)))
(defmacro defset [sym args & forms]
(let [set-sym (symbol (str "set-" sym))]
`(defn ~set-sym [...@args]
~...@forms)))
(defmacro defget [sym args & forms]
(let [get-sym (symbol (str "get-" sym))]
`(defn ~get-sym [...@args]
~...@forms)))
(defn find-accessor [sym acc-type]
(let [ns (or (namespace sym)
(str *ns*))
sym-name (name sym)]
(find-var (symbol ns (str (name acc-type) "-" sym-name)))))
(defn setin [m [sym & syms] v]
(let [setter (find-accessor sym :set)
getter (find-accessor sym :get)]
(if syms
(setter m (set-in (getter m) syms v))
(setter m v))))
(defn getin [m [sym & syms]]
(let [getter (find-accessor sym :get)]
(if syms
(getin (getter m) syms)
(getter m))))
(defmacro accessors [& syms]
`(do
~@(map setter* syms)
~@(map getter* syms)))
On Sat, Apr 25, 2009 at 4:42 AM, MattH <[email protected]> wrote:
>
> It's worth considering how *nested* accessors would work in the
> context of immutability.
>
> The nested maps approach works really nicely, due in part to functions
> like assoc-in:
>
> ; From Mark Volkmann's tutorial
> (assoc-in person [:employer :address :city] "Clayton")
>
> What would the above update look like if 'address' was accessed using
> functions like get-address and set-address?
>
> Functions like assoc-in clearly rely on a uniform way of getting/
> setting fields (i.e. maps).
>
> My *hunch* is that the right avenue is to extend/implement clojure's
> map classes if injecting behaviour is ever necessary. (A standard/
> supported way to do this would be nice.)
>
> I'd be happy using constructor functions like (make-complex-number)
> and (make-person) which can hide detail of their implementations, but
> I'd also like to benefit from all that goes with the idiomatic use of
> maps.
>
> (My 2c)
> >
>
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