rustkas

rustkas

Property-Based Testing with PropEr, Erlang, and Elixir: the location of the function under test (page 411)

In the 5th Question of the third chapter, it was proposed to create a function, as well as a testing property to test its work. In your answer on page 411, author suggested to write in a function in a properties file. Interesting. Why did you decide to write the function under test here? May be the module solutions in the source code folder src would be better place for it? It is very interesting to know the course of your thoughts on this issue.

Source code of the module in the src folder would be like this:

-module(solutions).

-export([word_count/1]).

word_count(String) ->
% ...

Property based test source code in the test folder would be like this:

-module(prop_solutions).

-include_lib("proper/include/proper.hrl").

%%%%%%%%%%%%%%%%%%
%%% Properties %%%
%%%%%%%%%%%%%%%%%%
prop_word_count() ->
    ?FORALL(String, (non_empty(string())),
            (word_count(String) =:= alt_word_count(String))).
% ...

I also noticed that Fred placed the call to the function under test on the left in the comparison expression. Why? He already explained this to me earlier.

Full project.

Marked As Solved

ferd

ferd

Author of Property-Based Testing with PropEr, LYSE, & Erlang in Anger

In normal production code you’d expect to ship it would be correct to put all implementation code on src/ and keep tests in test/.

However, since this is a book, I put them the way they are so that I could have one test file and one solutions file, and keep them separate from the project code for chapters when I was working on them. It reduces some amount of scattering and made it easier for me to just focus on the exercises, while also simplifying the annotations internal to source files when selecting subsets that get to be included in the book.

A reader downloading the code archive would also be able to look at the project without getting its code interspersed with exercises, which they legitimately my want to skip over, which is something I wanted to avoid as a potential confusion here.

One of the things not seen by the reader (by design), for example, is that I wanted all code to be testable for the book, including failing cases. So there are chapters there are 3 or 4 copies of a module existing. In chapter 3 it’s possible I had src/thinking.erl (final version), src/thinking.erl.a (first iteration shown), src/thinking.erl.b (second iteration), and so on.

This is something I could do so that if PropEr got upgrades while writing the book, I could go back in and re-generate the snippets for result sets in a way that wouldn’t require me to re-write everything every single time, while still leaving an artifact on disk that the PragProg editing system can bring into the page.

What I’m saying here is that the choice of where to put the code in this case is one of code organization related to the book and its authoring process, and specifically here, keeping exercises and solutions separate from the chapter’s project both for my sake, but also for the sake of readers who might want to look ahead in the code bundle that ships with the book without caring for the exercises.

The code has a distinct purpose from shipping software, and its organization may reflect that.


A note on the last comment there: there is far less thought going into which order to put functions in the assertions than you are implying here. My other response mentions a possible approach I could suggest in general, but at the time of writing it the book, I didn’t ask myself that question at all, and I don’t think it really matters ultimately due to being in comparison. I think I can suggest swapping the order from what I typically write, but I would never call it out in a code review, if that helps qualify the strength of how I feel about this.

Also Liked

rustkas

rustkas

Fred, thanks a lot for the detailed answer. Your invaluable experience and the order of thought is very interesting, as it helps to better understand your plan and priorities.

The tool (rebar3), thanks to which it is easy and pleasant to learn Erlang, helps a lot.

Where Next?

Popular Pragmatic Bookshelf topics Top

abtin
page 20: … protoc command… I had to additionally run the following go get commands in order to be able to compile protobuf code using go...
New
mikecargal
Title: Hands-on Rust: question about get_component (page 295) (feel free to respond. “You dug you’re own hole… good luck”) I have somet...
New
raul
Hi Travis! Thank you for the cool book! :slight_smile: I made a list of issues and thought I could post them chapter by chapter. I’m rev...
New
herminiotorres
Hi! I know not the intentions behind this narrative when called, on page XI: mount() |> handle_event() |> render() but the correc...
New
AleksandrKudashkin
On the page xv there is an instruction to run bin/setup from the main folder. I downloaded the source code today (12/03/21) and can’t see...
New
joepstender
The generated iex result below should list products instead of product for the metadata. (page 67) iex> product = %Product{} %Pento....
New
jskubick
I found an issue in Chapter 7 regarding android:backgroundTint vs app:backgroundTint. How to replicate: load chapter-7 from zipfile i...
New
brunogirin
When running tox for the first time, I got the following error: ERROR: InterpreterNotFound: python3.10 I realised that I was running ...
New
jwandekoken
Book: Programming Phoenix LiveView, page 142 (157/378), file lib/pento_web/live/product_live/form_component.ex, in the function below: d...
New
tkhobbes
After some hassle, I was able to finally run bin/setup, now I have started the rails server but I get this error message right when I vis...
New

Other popular topics Top

Exadra37
I am thinking in building or buy a desktop computer for programing, both professionally and on my free time, and my choice of OS is Linux...
New
New
AstonJ
Inspired by this post from @Carter, which languages, frameworks or other tech or tools do you think is killing it right now? :upside_down...
New
Exadra37
I am asking for any distro that only has the bare-bones to be able to get a shell in the server and then just install the packages as we ...
New
PragmaticBookshelf
Learn different ways of writing concurrent code in Elixir and increase your application's performance, without sacrificing scalability or...
New
PragmaticBookshelf
Author Spotlight Rebecca Skinner @RebeccaSkinner Welcome to our latest author spotlight, where we sit down with Rebecca Skinner, auth...
New
AstonJ
If you want a quick and easy way to block any website on your Mac using Little Snitch simply… File > New Rule: And select Deny, O...
New
New
First poster: bot
Large Language Models like ChatGPT say The Darnedest Things. The Errors They MakeWhy We Need to Document Them, and What We Have Decided ...
New
AstonJ
If you’re getting errors like this: psql: error: connection to server on socket “/tmp/.s.PGSQL.5432” failed: No such file or directory ...
New

Sub Categories: