[erlang-questions] PropEr way to deal with recursive types

[James Aimonetti]

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I'm wondering how to handle testing with recursive types, especially
with regards to handling the depth at which those structures are created
in the generators.

Take, for instance, a decoded json object ({struct, [{K,V},...]} in this
case). V can obviously be a json object itself or even a list of json
objects.

I'd like to write a function is_json_object/1 that will deeply inspect a
passed json object and verify it is, in fact, a json object.

Assuming simplified types similar to:

- -type json_prop_key() :: binary().
- -type json_prop_value() :: binary() | json_object().
- -type json_object() :: {struct, [{json_prop_key(), json_prop_value()}]}.

and a function definition
- -spec is_json_object/1 :: (term()) -> boolean().
is_json_object(MaybeJObj) ->
 % Implementation here
 ...
 % return a boolean

Now, I'd like to create the PropEr tests for valid json objects. Here's
my first crack:

prop_is_json_object() ->
    ?FORALL(JObj, json_object_builder(),
      ?WHENFAIL(io:format("Failed prop_is_json_object ~p~n", [JObj]),
      ?MODULE:is_json_object(JObj)
    )).

%% GENERATORS
%% create json objects
json_object_builder() ->
    ?LET(Prop, a_proplist(), {struct, Prop}).

a_proplist() ->
    ?SUCHTHAT(L, list( ?LET({K,V}, {binary(), proplist_value()}, {K, V})
), length(L) < 5).

proplist_value() ->
    ?LAZY(weighted_union([{15, binary()}, {1, json_object_builder()}])).

I tried limiting the proplist to 5 tuples, heavily weighting the
proplist_value union towards binary(), etc. When I run
"proper:module(json_tester).", I see a string of periods (varies how far
it gets), then the VM hangs and I have to get into the JCL to interrupt
the process. My assumption is that the generator is getting stuck
creating a big data structure, as CPU and memory increase rapidly.

I've looked at three examples[1][2][3] that Fred Herbert shared but
haven't found the mental bridge between his examples and my attempt. I
don't doubt that I'm misusing or ignorant of the right functions or
macros in PropEr to make this easier.

Any hints? :)

James

[1]
https://bitbucket.org/ferd/zippers/src/9548780f0261/test/prop_zipper_lists.erl#cl-92
[2]
https://bitbucket.org/ferd/zippers/src/9548780f0261/test/prop_zipper_bintrees.erl#cl-88
[3]
https://bitbucket.org/ferd/zippers/src/9548780f0261/test/prop_zipper_forests.erl#cl-198

- -- 
James Aimonetti
Distributed Systems Engineer / DJ MC_

2600hz | http://2600hz.com
sip:james@REDACTED
tel: 415.886.7905
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