[erlang-questions] PropER generator for list of records with a sequence number

[Jesper Louis Andersen]

On Mon, Dec 10, 2018 at 1:03 PM Frans Schneider <fchschneider@REDACTED>
wrote:

>  I don't want to make changes to the application's code base to
> facilitate testing
> because that would give to much clutter.
>

I'd beg you reconsider this stance.

In my experience, adding tooling around an application in order to query
its internal state in a well-defined way is often beneficial when there is
a bug and you need to introspect the code in question. Debugging is often
helped by having a canonical way to inspect and analyze the application.
Also, if you need to run through hoops before you can test your code,
chances are you are looking at the wrong API, so it should at least be
entertained a better one might exist.

In linear logic, there are two kinds of choice called "additive
conjunction" and "additive disjunction". One type, conjunction, is
controlled by either the PropER model or by the caller into the
application/SUT. The other kind of choice, disjunction, is made by the
application/SUT internally when it executes. Additive disjunctions are hard
because they make the model unable to predict what will happen. So it has
to guard against all types of outcome from the SUT/application, and this
makes the models quite weak. The Wikipedia page has a nice example with a
vendoring machine interpretation of the model[0], but the idea of a linear
dialogue between a buyer and a vendoring machine is exactly mapped into an
API user and the process behind the API, implementing it.

One important point about PropER testing is that you must try to shift as
much choice to be controlled by the model. This often requires an API where
one can inject parameters from the outside to control the setup of certain
states. You can always have a sane default which initializes if not
overriden, and this is what I've often done. Important examples are that
randomness and time must be controlled from the outside in some way.
Otherwise, the system tend to become impossible to test. And it also hides
important details to the user of the API to boot.
[0] https://en.wikipedia.org/wiki/Linear_logic
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