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

[Joseph Norton]

James -

See the SIZED macro and resize command.

Here is an example that decrements the size for each recursive call:

type(Gen,Type) ->
    ?SIZED(Size,resize(if Size > 0 -> Size-1; true -> 0 end,type1(Gen,Type))).

https://github.com/norton/ubf/blob/master/test/qc/qc_ubf_types.erl

Depending on your needs, you may want to divide by 2, take the square root, etc.

thanks,

Joseph Norton


p.s. Take note - there is an open issue related to PropEr's resize implementation:

https://github.com/manopapad/proper/issues/10


On Sep 10, 2011, at 9:10 AM, James Aimonetti wrote:

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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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