# Developing a unit in Dusk

Let's imagine that you want to create a new unit called "lib/foo", how would
you go about that? This document tells you how you'd do it on a system running
[app/gcon], that is, the canonical Dusk console.

Let's begin by creating the unit:

  pf"lib/foo.fs"

See [fs/core] for details. You now have an empty "lib/foo.fs" file. You'll write
automated tests for it, right? so let's create it too:

  pf"tests/lib/foo.fs"

We can now add the boilerplate, which in Dusk is minimal. Let's open
[lib/foo.fs] by pressing CTRL+E on this very tag. Don't forget that you can
press CTRL+[ to come back here.

That will take you to a blank screen because the file is empty. You
can type "i" followed by "unit lib/foo", then press Return. You have your
boilerplate set, but the file is not saved yet. Press Shift+W to save.

Tests now. Open [tests/lib/foo.fs]. If you get mixed up in ed buffers, you can
press CTRL+B to get a list of them.

So, we have an empty buffer again.

Press "i" then "needs tests/harness lib/foo" then Return to add the first line
of the test unit, then press "o" then "testbegin" then Return to add a line
underneath, then again with "testend". You now have a complete boilerplate.
Don't forget to save.

Test files can be ran individually with f<<, in this instance:

  f<< tests/lib/foo.fs

You can also press CTRL+R on the file tag to run it. Units in the "tests"
directory are also automatically picked up when you run all tests:

  f<< tests/all.fs

We can verify that our test unit isn't mistakenly ignored by adding a line like
"0 #true" in it. Running the file will raise an assertion error, running the
full suite with "f<< tests/all.fs" will error out at some point.

So! boilerplate's done. Can we start coding? If you want, but if there's
significant work involved, chances are you'll trigger a footgun during your
experimentations and crash the machine. After a while, you'll get tired to
typing the same "setup" commands all over again. That's not the Dusk way.

The Dusk way is to configure your machine to "boot into" the work at hand so
that crashing is painless. The best way to do so is to amend your [init.txt]
welcome file. Go add references to the files you've just created. This means
that upon reboot, you can quickly re-open them.

Even when testing doesn't crash your machine, you'll want to forcibly reload
the unit you're working on, then run the tests. Therefore, you'll likely want
to add a line like this in your init.txt:

  f<< lib/foo.fs f<< tests/lib/foo.fs

You'll run that line every time you want to test a new change you've made to
either your tests or your unit.

Alright, now you're ready to write your code!