On Aug 26, 8:25 pm, Parth Malwankar <[EMAIL PROTECTED]> wrote:
> Hello,
>
> In order to update fields in nested structures/maps easily
> I have created a macro 'field-write'.
> (defmacro field-write [st k v access-spec]
> ; st=data, k=key to update, v=val to put, access-spec=access
> vector
> ; ... code listing after interaction
>
> user=> nx
> {:a {:b {:c {:content 10}}}}
> user=> (field-write nx :content 1000 [:a :b :c])
> {:a {:b {:c {:content 1000}}}}
> user=> (macroexpand '(field-write nx :content 1000 [:a :b :c]))
> (clojure/assoc
> nx :a
> (clojure/assoc
> (clojure/-> nx :a) :b
> (clojure/assoc
> (clojure/-> nx :a :b) :c
> (clojure/assoc
> (clojure/-> nx :a :b :c):content 1000))))
>
> [formatting added above for readability]
>
> It seems to be working fine but I thought it may be
> good to get inputs from the Clojure experts here to
> see if there is a better way to do this in Clojure.
>
> (def nx {:a {:b {:c {:content 10}}}})
>
> (def field-write)
> (defmacro field-write [st k v access-spec]
> ; st=data, k=key to update, v=val to put, access-spec=access
> vector
> (if (pred/empty? access-spec)
> `(assoc ~st ~k ~v)
> (field-write st (last access-spec)
> `(assoc (-> ~st [EMAIL PROTECTED]) ~k ~v)
> (butlast access-spec))))
>
After some more experimentation I found that the field-write
macro didn't work with access-specs like (vector :a :b :c)
... I should have thought of that before.
So I reimplemented field-read and field-write as functions
which seem to work in all scenarios.
(defn field-read [st as]
"user=> nx
{:a {:b {:c {:data 10}}}}
user=> (field-read nx [:a :b :c])
{:data 10}"
(eval (reduce (comp reverse list) st as)))
(defn field-write [structure kwd value access-spec]
"user=> nx
{:a {:b {:c {:data 10}}}}
user=> (field-write nx :data 20 [:a :b :c])
{:a {:b {:c {:data 20}}}}
user=> (field-write nx :data (+ 2 2 2) (vector :a :b :c))
{:a {:b {:c {:data 6}}}}"
(loop [st structure
k kwd
v value
as access-spec]
(if (pred/empty? as)
(assoc st k v)
(recur st (last as)
(assoc (field-read st as) k v)
(butlast as)))))
I suspect using eval in field-read is a bad idea (preformance?) but I
couldn't come up with a better way to do it. access-spec
needs to be a vector as its something thats returned by
another function in my use case (otherwise I could have
used it with ->).
Can I do something better here? Comments?
Thanks very much.
Parth
> Thanks.
> Parth
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