Supplied-p parameter in clojure similar to lisp lambda lists

[Dave Tenny]

What is the commonly accepted technique for declaring/using 'supplied-p' type lambda list functionality in clojure?

http://www.ai.mit.edu/projects/iiip/doc/CommonLISP/HyperSpec/Body/sec_3-4-1.html


I have some clojure functions with a large number of keywords and various defaults, I want to know if a keyword was specified by the caller (rather than defaulted) in some cases.

Certainly I could implement my own destructuring macros that did this, but I'd like to avoid reinventing a wheel here if I can, and also to know the idiomatic clojure way to do it.

Thanks for any tips.

[Jason Felice]

If you destructure the parameters like this:
(defn f [& {:as a-map}] ...)

You can use map primitives on a-map.  But you can also supply defaults here.

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[Dave Tenny]

Well, it took me a while to perhaps get what you were telling me here.

In my case I I had something like

(defn foo [ & {:keys [bar ... more keys ...] :or {bar 1}} ] ...)

and I wanted to know whether the user had explicilty invoked foo with :bar.

What wasn't clear to me was that :as solved this problem.  

Reading http://clojure.org/special_forms#Special Forms--Binding Forms (Destructuring)-Map binding destructuring

I guess I can see that it's telling me :as shows things that weren't in the init-form, but that's with hindsight.

So, to emulated common lisp 'supplied-p' semantics, you can check the :as form, which will **not**

contain :or values for keywords.

E.g.

user> (defn bar [ & {:keys [baz] :or {baz 'baz} :as all-keys} ] (println baz all-keys))

#'user/bar

user> (bar :bof 1)

baz {:bof 1}

nil

And not that the all-keys form does not show a binding for baz, and that's what I wanted.

Just fyi in case anybody searches topics for 'supplied-p' again.

[dennis zhuang]

I think that adding a :p option to destructuring would be great:

(let [ {:keys [a b c] :p {a a-p}} params]

    (if a-p

        (println a)

        (println "a is not exists.")))

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[dennis zhuang]

I created a ticket http://dev.clojure.org/jira/browse/CLJ-1508

[Dave Tenny]

I don't think that a :p feature is necessary, since all you need to do to emulate it is a

(:baz all-keys) to know if the user explicitly specified it.  I.e. I think the capability is already present in adequate form but the documentation on map destructuring could be improved.

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[dennis zhuang]

Yep, it's an optional syntax sugar.

Indeed, you should use (contains? all-keys :baz) to check if :baz is present in options. If :baz is present but it's value is nil,then (:baz all-keys) returns nil too.