On Fri, Jul 31, 2009 at 2:34 PM, Sean Devlin<[email protected]> wrote:
>
> I've been using Meikel's defnk macro a fair bit in some of my code. I
> have several functions that have sensible defaults that only need to
> be overridden occasionally, so keyword arguments fit the bill nicely.
>
> Let's consider two (contrived) keyword fns.
>
> (use 'clojure.contrib.def)
>
> (defnk add
> [x :y 10 :z 20]
> (+ x y z))
>
> (defnk nil-free?
> [a :b 1 :c 2]
> (every? (comp not nil?) [a b c]))
>
> There are several occasions I would like to use apply, and supply a
> map for the keyword arguments. For those of you with a Python
> background, it would work just like the ** operator. Let's call this
> function apply-map (a definition can be found here:
> http://gist.github.com/159324
> ). You can use apply-map just like you would apply.
>
> (def useful-vec [:y 0])
> (def useful-map {:y 0})
>
> user=> (apply add 1 usefull-vec)
> 21
>
> ;This could be used to apply setting from a user...
> user=>(apply-map add 1 useful-map)
> 21
>
> Some more use cases:
>
> user=> (def test-map {:a "A" :b "B" :c "C"})
> #'user/test-map
> user=> (vector test-map)
> [{:a "A", :b "B", :c "C"}]
> user=> (apply vector test-map)
> [[:a "A"] [:b "B"] [:c "C"]]
> user=> (apply-map vector test-map)
> [:a "A" :b "B" :c "C"]
>
> This helps with higher order functions
>
> ;This worked before, because Meikel is just awesome.
> user=>(map add [1 2 3])
> (31 32 33)
>
> ;Slightly ugly, but it works...
> user=>(map
> #(apply-map add (first %) (second %))
> [[1 {:z 0}] [2 {:y 0}] [3]])
> (11 22 33)
>
> Similarly, this works for filter/remove
>
> ;Again, somewhat ugly...
> user=>(filter
> #(apply-map nil-free? (first %) (second %))
> [[nil {}] [1 {:b nil}] [2]])
> ([2])
>
> This does NOT attempt to solve the problem of mixing keyword and
> variable length arguments. However, I think it is a step forward in
> making keyword arguments more useful in Clojure.
>
> Feedback?
I think your 'uglies' are a hint - perhaps better to make a function
that takes a keyword fn and returns a fn that works the way you want:
(fixfnk keywordfn) -> mapargfn
((fixfnk add) 4 {:y 101}) -> 125
Then you can map, filter etc.
But the whole thing is somewhat an exercise in the non-composability
of keyword fns.
Rich
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