Hello Brent,
The use case I had in mind was to keep a map readable during development.
Take a simple map: {:type QTDIR :path (hash "a string")}. It's easier to
play with this data if evaluation of certain symbols and functions is
delayed.
Thanks you both for your answer,
kind regards,
Dieter
On Mon, Nov 22, 2021 at 3:39 AM Brent Millare <brent.mill...@gmail.com>
wrote:
> I'm curious why you are saving hashmaps that have clojure code within it
> with the intention of evaluating this code as embedded in the hashmap? What
> is the use case? Are you trying to delay evaluation? Regardless, eval
> always incurs a cost and should generally be avoided if you can use
> "runtime" techniques instead. Is the embedded code trusted?
>
> Best,
> Brent
>
> On Sunday, November 21, 2021 at 9:22:47 AM UTC-5 dieter.v...@gmail.com
> wrote:
>
>> Hello,
>>
>> It seems to be a design decision that 0-arity invoke of a composite data
>> type gives an ArityException: the composite data type does not implement
>> the IFn when no arguments are given.
>> Is there a certain design decision behind this behavior? (or a certain
>> use-case)
>>
>>
>> repl> ;composite data type evaluates to itself
>> repl> {:a 1 :b (hash "word")}
>> {:a 1 :b -304538205}
>> repl> '{:a 1 :b (hash "word")}
>> {:a 1 :b (hash "word")}
>> repl> (def mydata '{:a 1 :b (hash "word")})
>> repl> mydata
>> {:a 1 :b (hash "word")}
>> repl> ;composite data type implements IFn for its keys
>> repl> (mydata :b)
>> (hash "word")
>> repl> ; there is no '0-arity' implementation of IFn for composite data
>> type
>> repl> ({})
>> ... (ArityException)...
>> repl> (mydata)
>> ... (ArityException)...
>> repl> ; instead i have to type eval
>> repl> ((eval mymap) :b)
>> -304538205
>>
>> I know its only 4 letters and a space extra, but software composition is
>> supposed to avoid code duplication and perhaps the idea makes sense that
>> invoking a map without arguments evaluates it... Hence the question about
>> the choice made for the current behavior.
>>
>> A possible small workaround
>> (defrecord qid [qid]
>> clojure.lang.IFn
>> (invoke [this] (eval qid))
>> But expect this to throw alot of bugs: this record is not the same simple
>> map.
>> (issues with other protocols, reducers, transducers and much more I don't
>> know of.)
>>
>> I hope this is the right google group to ask this question.
>> kind regards,
>> Dieter
>>
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