On Aug 27, 4:39 am, Parth Malwankar <[EMAIL PROTECTED]> wrote:
> 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?
>
I posted a variant here:
http://paste.lisp.org/display/65964
Rich
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