[erlang-questions] proper char() type

[Jesper Louis Andersen]

I don't think you are missing anything.

Some QuickCheck systems will use a size parameter like 1-100 and then they
"scale" the range over that size. If you ask for, say, 30 test cases, they
usually scale 1-100 over the first 15, and then they run 15 with the full
range.

The idea, roughly, is that while you eventually want to check a full range,
but many errors occur when a range is small. And small cases are often
easier to debug.

This notion goes hand in hand with shrinking: if you _do_ find a large test
case, pick a strategy for finding a failure, but for a simpler generated
input.

As for integers, I often use something like:

%% @doc pow_2_int/0 generates integers close to a power of two
%% It turns out that integers around powers of two are often able to mess
up stuff
%% because of their bitwise representation. This generator generates
integers close
%% to a power of two deliberately.
%% @end
pow_2_int() ->
    ?LET({Sign, Exponent, Perturb}, {sign(), choose(0, 128), choose(-3, 3)},
        Sign * pow(2, Exponent) + Perturb).

sign() -> elements([1, -1]).

pow(0, 0) -> 0;
pow(_Base, 0) -> 1;
pow(Base, N) -> Base * pow(Base, N-1).

(This is from [0])

It doesn't generate any integer, but since a lot of systems cannot handle
corner case integers well, this is an excellent way to check them.
Especially if they have two-complement representations and have one more
negative value than positive. The above code sequence has found bugs in
Erlangs bignum handling btw.

[0] https://github.com/jlouis/eqc_lib/blob/master/eqc_lib.erl#L14-L27

On Thu, May 17, 2018 at 12:35 PM Ryan Maclear <ryanm@REDACTED>
wrote:

> Hi All,
>
> I've gone through the code for the integer() type and have found where my
> issue arises from.
>
> the proper_gen:pick/1 function has a default Size of 10, but the
> proper_gen:pick/2 function provides for a size to be passed.
>
> If I call:
>
> proper_gen:pick(proper_types:char(), 1114111), I get results as expected.
>
> Since the definition of char() is equivalent to integer(0,16#10ffff), I
> would have expected the proper_gen:pick_type(proper_types:char()) call to
> result in the arity 2 version of this function being called, or at least
> have the Size set to the upper bound of the integer range in the
> integer_gen function call, either in proper_types:integer_gen/2 function
> call.
>
> Does this make sense or am I missing something?
>
> Regards,
> Ryan Maclear
>
>
>
>
> On 17 May 2018 at 11:08, Ryan Maclear <ryanm@REDACTED> wrote:
>
>> Appologies, I hit send before finisinh the email. Here is the complete
>> mail:
>>
>> I'm busy learning to use proper and have the following issue. When
>> calling the function
>>
>> From the API docs, the char() type has a range of 0 to (1114111)
>> 16#10ffff.
>>
>> When calling the function
>>
>> proper_gen:pick(proper_types:term()).
>>
>> manually many times (I've tried in around 3000 times) and on different
>> VMs, the function always seems to return only one of the following values:
>>
>> {ok,0},
>> {ok,2},
>> {ok,3},
>> {ok,6}
>> {ok,12} and
>> {ok,13}
>>
>> I see the same behaviour for
>>
>> proper_gen:pick(proper_types:range(0,16#10ffff)).
>>
>> which I believe is equivalent.
>>
>> I have tried on erlang 19.3.6.9, erlang 20.3.6 on a Mac as well as on a
>> ubuntu trusty docker image with erlang 19.3.
>>
>> In all three vms I see the same set of results being returned.
>>
>> Is there something else I need to do before calling this function?
>>
>> Thanks,
>> Ryan
>>
>>
>> On 17 May 2018 at 11:07, Ryan Maclear <ryanm@REDACTED> wrote:
>>
>>> Good Day,
>>>
>>> I'm busy learning to use proper and have the following issue. When
>>> calling the function
>>>
>>> From the API docs, the char() type has a range of 0 to (1114111)
>>> 16#10ffff.
>>>
>>> When calling the function
>>>
>>> proper_gen:pick(proper_types:term()).
>>>
>>> manually many times (I've tried in around 3000 times) and on different
>>> VMs, the function always seems to return only one of the following values:
>>>
>>> {ok,0},
>>> {ok,2},
>>> {ok,3},
>>> {ok,6}
>>> {ok,12} and
>>> {ok,13}
>>>
>>> I see the same behaviour for
>>>
>>>
>>> I have tried on erlang 19.3.6.9, erlang 20.3.6 on a Mac as well as on a
>>> ubuntu trusty docker image with erlang 19.3.
>>>
>>> In all three vms I see the same set of results being returned.
>>>
>>> Is there something else I need to do before calling this function?
>>>
>>> Thanks,
>>> Ryan
>>>
>>
>>
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