# Property-Based Testing with PropEr, Erlang, and Elixir: Finding a simplified case of inconsistency (page 146)

**URL:** <https://forum.devtalk.com/t/property-based-testing-with-proper-erlang-and-elixir-finding-a-simplified-case-of-inconsistency-page-146/10947>\
**Category:** PragProg Customers\
**Tags:** book-property-based-testing-with-proper-erlang-and-elixir, suggestion\
**Created:** [29 June 2021 15:32 UTC](https://forum.devtalk.com/t/property-based-testing-with-proper-erlang-and-elixir-finding-a-simplified-case-of-inconsistency-page-146/10947 "2021-06-29T15:32:18Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![rustkas](https://forum.devtalk.com/user_avatar/forum.devtalk.com/rustkas/32/2933_2.png) [@rustkas](https://forum.devtalk.com/u/rustkas)\
**Post date:** [29 June 2021 15:32 UTC](https://forum.devtalk.com/t/property-based-testing-with-proper-erlang-and-elixir-finding-a-simplified-case-of-inconsistency-page-146/10947/1 "2021-06-29T15:32:19Z")

</div>

I noticed that the necessary inconsistence, which describes in an example [PropEr](http://proper.softlab.ntua.gr/index.html) framework does not find a discrepancy in the [RFC 4180](https://tools.ietf.org/html/rfc4180) specification, is far from at once. I was surprised when I ran the property several times and didn’t get the error I was looking for.

```auto
$ rebar3 proper
===> Testing prop_csv:prop_roundtrip()
....................................................................................................
OK: Passed 100 test(s).
===>
1/1 properties passed

```

When I increased the number of tests to 1000, the framework reproduced the necessary use case.

```auto
$ rebar3 proper -n 1000
===> Testing prop_csv:prop_roundtrip()
..........................!
Failed: After 27 test(s).
[#{[40,36] => []},#{[40,36] => []}]

Shrinking .......(7 time(s))
[#{[66,65] => []},#{[66,65] => []}]
===>
0/1 properties passed, 1 failed
===> Failed test cases:
prop_csv:prop_roundtrip() -> false

```

[PropEr](http://proper.softlab.ntua.gr/index.html) can help us in it better. As we know about a possible problem we can narrow `Size` to `1`. For this task we can add simplified generator and property-based test:

```auto
%%%%%%%%%%%%%%%%%%
%%% Properties %%%
%%%%%%%%%%%%%%%%%%
prop_roundtrip_01() ->
    ?FORALL(Maps,
            csv_source_01(),
            Maps
            =:= bday_csv:decode(
                    bday_csv:encode(Maps))).

%%%%%%%%%%%%%%%%%%
%%% Generators %%%
%%%%%%%%%%%%%%%%%%

csv_source_01() ->
    ?LET(Keys, header(1), list(entry(1, Keys))).

```

Launching that property we get the expected error has gotten every time:

```auto
$ rebar3 proper -d apps/bday/test -p prop_roundtrip_01
===> Linking _build/default/plugins/rebar3_format to _build/test/plugins/rebar3_format
===> Linking _build/default/plugins/katana_code to _build/test/plugins/katana_code
===> Verifying dependencies...
===> Linking _build/default/plugins/covertool to _build/test/plugins/covertool
===> Compiling bday
===> Testing prop_csv:prop_roundtrip_01()
.......!
Failed: After 8 test(s).
[#{[125] => [75,85,50]},#{[125] => []},#{[125] => []}]

Shrinking ......(6 time(s))
[#{[65] => []},#{[65] => []}]
===>
0/1 properties passed, 1 failed
===> Failed test cases:
prop_csv:prop_roundtrip_01() -> false

```