RE: First post: how to mimic a Java List with types?

[Sam Hahn]

Jack - I'll call you :) 

Cheers - Sam

-------- Original Message --------
Subject: Re: First post: how to mimic a Java List with types?
From: Jack Park <jackp...@topicquests.org>
Date: Fri, August 14, 2020 11:34 am
To: clojure@googlegroups.com

Mapping and transducers seems appropriate, though I'm still wrapping my head around how to make this work.

From the "class with functions" mindset, I need an ArrayList into which I can:

a) add members to the list from time to time

b) run eval() on it; a conjunctive list will exit false on first false firing of any member, or default exit true otherwise; a disjunctive list will exit true on first true firing of a member, and default false.

When I read the code here, I am not yet clear (sorry, it's  on me, not the code) how to make such a "class-like" object.

I am still exploring this avenue, as well as looking at java-interop or something about a proxy.

Many thanks.

Jack

On Fri, Aug 14, 2020 at 1:25 AM Oleksandr Shulgin <oleksandr.shul...@zalando.de> wrote:

On Fri, Aug 14, 2020 at 9:05 AM Oleksandr Shulgin <oleksandr.shul...@zalando.de> wrote:

On Fri, Aug 14, 2020 at 7:51 AM Alexandre Almosni <alexandre.almo...@gmail.com> wrote:

Maybe your objects could be defined by a map containing functions and objects.

So exampleAndList = {:fn and, :coll [a b c]}

And then eval goes through your objects recursively, using

(def eval [o]

(apply (:fn o) (map eval (:coll o)))) 

- with a special case that if o is not a map but say a Boolean you just return itself

A slight problem with that approach is that clojure.core/and is not a function, but a macro: the reason being is that it stops evaluating forms as soon as it hits a falsey value (similar for "or").

A smart implementation of And/OrList would like to short-circuit as well I guess, so you'll need to express that somehow.

Of the clojure.core library that property is satisfied by transducers: https://clojure.org/reference/transducers#_early_termination

I came up with the following:

;; and-transducer with early termination:

 (defn xand [rf]
   (fn
     ([] (rf))
     ([res] (rf res))
     ([res inp] (if inp
                  (rf res inp)
                  (reduced inp)))))

;; reducing function implementing and:

  (defn andr

   ([] true)
   ([i] i)
   ([r i] (and r i)))

;; noisy variant of get:

 (defn noisy-get
  ([k]
   (fn [m]
     (noisy-get m k)))
  ([m k]
   (println "noisy-get" m k)
   (get m k)))

;; all in action:

user> (def xf (comp (map (noisy-get :data))

                    xand))

#'user/xf

user> (transduce xf andr [{:data 1} {:data false} {:no-data 3}])
noisy-get {:data 1} :data
noisy-get {:data false} :data
false
user> (transduce xf andr [{:data 1} {:data 2} {:no-data 3} {:data 4}])
noisy-get {:data 1} :data
noisy-get {:data 2} :data
noisy-get {:no-data 3} :data
nil

user> (transduce xf andr [{:data 1} {:data 2} {:data 3} {:data 4}])
noisy-get {:data 1} :data
noisy-get {:data 2} :data
noisy-get {:data 3} :data
noisy-get {:data 4} :data
4

It does feel a bit redundant to have the reducing function on top of a transducer that accomplishes mostly the same, but I haven't found a way to make it shorter.

Cheers,

--

Alex

On 14 Aug 2020, at 02:24, Jack Park <jackp...@topicquests.org> wrote:



The problem:

In Java, I have an interface IInferrable which is basically  boolean eval();

Any Java object which extends IInferrable, no matter what it is, will answer to eval() and return a boolean.

The idea lies at the heart of an inference engine.

But, I also define a class AndList which implements IInferrable and extends java.util.ArrayList<Inferrable>

So, AndList, and its sibling OrList behave just like a List object, but also will answer to eval() by running the collection and dealing with what each element returns when it, too, is eval()'d.

I'd really love to discover how to pull that off in Clojure.

 

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