There is no protocol for "truthiness", which is what you want. nil and false
are false, everything else is true.
Even if there were a protocol for truthiness, you could not use :yes and :no
(keywords), you would have to create your own custom type and implement the
protocol on it. (Think about it: what if the truthiness of a *specific* keyword
depended on application code you didn't write? E.g. if some library globally
redefined the truthiness protocol on keywords so that :no is false?)
Clojure is philosophically opposed to making truthiness user-definable. Many
other languages (including lisp and scheme) have not-very-simple truthiness
rules that Clojure deliberately simplified. Allowing user-definable truthiness
would bring all that complexity back *and then some*.
You have a problem-domain-specific use of :yes and :no and a
problem-domain-specific notion of truthiness. You can either use functions
which implement that domain notion (which you have done, and which is the best
approach I think), or you can write your *own* protocol for this and extend it
either to custom YES NO types or keywords.
You could also create a boundary in your application where you convert from a
:yes-:no representation to a more native true-false.
It smells like a bad design that you have the *same* function and code
accepting both :yes and :no *and* true and false. Since this is a
domain-specific problem and type, it seems like the same code shouldn't be used
in both cases. I.e., your "not" function should be:
(defn a-not [answer]
(case answer
:yes :no
:no :yes
(throw (ex-info (str "Not a valid questionnaire answer: " (pr-str answer))
{}))))
On Wednesday, January 14, 2015 at 2:50:21 PM UTC-6, Yehonathan Sharvit wrote:
> In my app, I am using :yes and :no sometimes instead of true and false.
>
>
>
>
> I’d like to know what is the best way to extends the `not` function to be
> able to handle :yes and :no properly.
>
>
>
>
> I was expecting some kind of Boolean protocol but I wasn’t able to find it.
>
>
>
>
> I could write this:
>
>
>
>
>
>
> (defn not [x]
>
> (case x
>
> :yes :no
>
> :no :yes
>
> (cljs.core.not x)))
>
>
>
>
> (map not [false true :yes :no]); (true false :no :yes)
>
>
>
>
> But I’m not sure it is the most idiomatic solution.
>
> Any ideas?
>
>
>
> —
> Sent from Mailbox
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