That pattern will be a great addition to the toolbox, thank you :)
On Sep 16, 9:58 am, Laurent PETIT <laurent.pe...@gmail.com> wrote:
> And note that the pattern works at any level, and is easily readable, since
> update-in "flattens" the arguments of the modifying function :
>
> If you want to "touch" the path [:a :b :c :d] and provide specific default
> values at each level if the key is not found, it's as "simple" as :
>
> (update-in coll
> [:a] (fnil update-in default-for-first-level)
> [:b] (fnil update-in default-for-second-level)
> [:c] (fnil update-in default-for-third-level)
> [:d] (fnil update-in default-for-fourth-level)
> the-function-you-wanted-to-use-in-the-first-place
> arg2-for-the-function
> arg3-for-the-function ...)
>
> 2010/9/16 Laurent PETIT <laurent.pe...@gmail.com>
>
>
>
> > So nested calls to update-in are needed in order to be able to provide a
> > specific default value, everytime just having an empty map created isn't
> > sufficient.
>
> > So instead of (update-in {} [:a :b] identity) which returns {:a {:b nil}} ,
> > you can break the key path at :a so that the default value if no :a is
> > specified by (fnil XXX default-value). And by specifying that XXX is
> > update-in, you can continue your "key path" where you stopped (after :a),
> > either using the found value at :a, either the provided default value :
>
> > (update-in {} [:a] (fnil update-in {:label "great label"}) [:b] identity)
> > => {:a {:b nil, :label "great label"}}
>
> > or you can also decide that it's not a map which should be the default
> > value, imagine you want to modify a path [:a 2] :
>
> > (update-in {} [:a] (fnil update-in [:foo :bar :baz]) [2] str) => {:a [:foo
> > :bar ":baz"]}
>
> > 2010/9/16 Btsai <benny.t...@gmail.com>
>
> > My poor brain can't handle nested calls to update-in, so this is what
> >> I came up with:
>
> >> (defn add-meetings [data k meetings]
> >> (cond
> >> (nil? (data k)) (assoc data k {:title "title" :meetings meetings})
> >> :else (update-in data [k :meetings] concat meetings)))
>
> >> On Sep 16, 8:53 am, Laurent PETIT <laurent.pe...@gmail.com> wrote:
> >> > 2010/9/16 Meikel Brandmeyer <m...@kotka.de>
>
> >> > > Hi Laurent,
>
> >> > > On 16 Sep., 15:54, Laurent PETIT <laurent.pe...@gmail.com> wrote:
>
> >> > > > you don't like my one-liner ? :-)
>
> >> > > I saw your message only after I sent mine. :)
>
> >> > > > (update-in coll [k] (fnil update-in *default-value*) [:meetings]
> >> (comp
> >> > > vec
> >> > > > concat) data)
>
> >> > > Hmm... (comp vec concat) == into?
>
> >> > Yep.
> >> > so this is the killer one : :-)
>
> >> > (update-in coll [k] (fnil update-in *default-value*) [:meetings] into
> >> data)
>
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