Ok - ClojureScript has an undefined? function that works like:
ClojureScript:cljs.user> (def v)
ClojureScript:cljs.user> (undefined? v)
true
ClojureScript:cljs.user> (def w nil)
nil
ClojureScript:cljs.user> (undefined? w)
false
ClojureScript:cljs.user> (defn f [v] (if (undefined? v) :no-value v))
#<function f(v){
if((void 0 === v))
{return "\uFDD0:no-value";
} else
{return v;
}
}>
ClojureScript:cljs.user> (f z)
:no-value
ClojureScript:cljs.user> (f 1)
1
ClojureScript:cljs.user> (f y)
nil
which indicates that I can use that scheme to pass and detect "no-value"
arguments in cljs.
Isn't it great to answer your own Qs ;-)
-FS.
On Jan 14, 2013, at 7:47 PM, Frank Siebenlist <frank.siebenl...@gmail.com>
wrote:
> For clojure I came up with the following alternative using an unbound var:
>
> user=> (def v)
> #'user/v
> user=> (defn f [x] (if (var? x) (if (bound? x) x :no-value) x))
> #'user/f
> user=> (f v)
> #<Unbound Unbound: #'user/v>
> user=> (f #'v)
> :no-value
> user=> (f #'map)
> #'clojure.core/map
> user=> (type (f v))
> clojure.lang.Var$Unbound
> user=>
>
> which would indicate that I could pass an unbound var as the argument to
> indicate a "no-value" value, which I can either detect thru the type or the
> bound? test.
>
> However, next issue is how to do this in ClojureScript as we do not have any
> vars and therefor no unbound vars…?
>
> -FS.
>
>
> On Jan 14, 2013, at 7:07 PM, Frank Siebenlist <frank.siebenl...@gmail.com>
> wrote:
>
>> Understood.
>>
>> … but shouldn't it be a standardized constant for the whole community to use
>> to avoid any interoperability issues and many reinvented wheels?
>>
>> The concept of no-value versus nil is a pretty basic one - there must be
>> better solutions (re)invented many times over ;-)
>>
>> -FS.
>>
>>
>> On Jan 14, 2013, at 5:23 PM, Timothy Baldridge <tbaldri...@gmail.com> wrote:
>>
>>> It's fairly common in situations like this to use a namespaced keyword. For
>>> instance ::unknown (which is short for my.namespace/unknown).
>>>
>>>
>>> On Mon, Jan 14, 2013 at 6:06 PM, Frank Siebenlist
>>> <frank.siebenl...@gmail.com> wrote:
>>> I'm using those watcher-fns that get called when the watched ref changes,
>>> and the watcher-fn gets passed the old-value and the new-value.
>>>
>>> Now, nil is a proper value for a key-value and well as a val-value in a
>>> map, so passing nil does not give you the info whether or not an old-value
>>> existed or not, or whether a new-value is nil or no-value.
>>>
>>> With the getter fn: (get m k no-value-here), you have the option to pass
>>> this no-value-here argument which gets returned if there was no value -
>>> this allows you to distinguish nil from "no-value". However, in the
>>> watcher-fn you cannot pass such a no-value argument.
>>>
>>> One possible solution would be to define a well-know URI for the value of
>>> "no-value" - something like for example:
>>> "uri:http://clojure.org/uri/no-value";, or some other standardized constant.
>>>
>>> Any other/better suggestions?
>>>
>>> Thanks, FrankS.
>>>
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