== Modifying the flake

Let’s make a simple modification to the script. This will give you an
opportunity to check your understanding of flakes.

The first step is to enter a development shell.

....
$ cd ~/tutorial-practice/hello-flake
$ nix develop
....

The `flake.nix` file specifies all of the tools that are needed during
development of the package. The `nix develop` command puts us in a shell
with those tools. As it turns out, we didn’t need any extra tools
(beyond the standard environment) for development yet, but that’s
usually not the case. Also, we will soon need another tool.

A development environment only allows you to _develop_ the package.
Don’t expect the package _outputs_ (e.g. executables) to be available
until you build them. However, our script doesn’t need to be compiled,
so can’t we just run it?

....
$ hello-flake
....

That worked before; why isn’t it working now? Earlier we used
`nix shell` to enter a _runtime_ environment where `hello-flake` was
available and on the `$PATH`. This time we entered a _development_
environment using the `nix develop` command. Since the flake hasn’t been
built yet, the executable won’t be on the `$PATH`. We can, however, run
it by specifying the path to the script.

....
$ ./hello-flake
....

We can also build the flake using the `nix build` command, which places
the build outputs in a directory called `result`.

....
$ nix build
$ result/bin/hello-flake
....

Rather than typing the full path to the executable, it’s more convenient
to use `nix run`.

....
$ nix run
....

Here’s a summary of the more common Nix commands.

[width="100%",cols="<17%,<83%",options="header",]
|===
|command |Action
|`nix develop` |Enters a _development_ shell with all the required
development tools (e.g. compilers and linkers) available (as specified
by `flake.nix`).

|`nix shell` |Enters a _runtime_ shell where the flake’s executables are
available on the `$PATH`.

|`nix build` |Builds the flake and puts the output in a directory called
`result`.

|`nix run` |Runs the flake’s default executable, rebuilding the package
first if needed. Specifically, it runs the version in the Nix store, not
the version in `result`.
|===

Now we’re ready to make the flake a little more interesting. Instead of
using the `echo` command in the script, we can use the Linux `cowsay`
command. The `sed` command below will make the necessary changes.

....
$ sed -i 's/echo/cowsay/' hello-flake
$ cat hello-flake
....

Let’s test the modified script.

....
$ ./hello-flake
....

What went wrong? Remember that we are in a _development_ shell. Since
`flake.nix` didn’t define the `devShells` variable, the development
shell only includes the Nix standard environment. In particular, the
`cowsay` command is not available.

To fix the problem, we can modify `flake.nix`. We don’t need to add
`cowsay` to the `inputs` section because it’s included in `nixpkgs`,
which is already an input. However, we do need to indicate that we want
it available in a develoment shell. Add the following lines before the
`packages = rec {` line.

....
        devShells = rec {
          default = pkgs.mkShell {
            packages = [ pkgs.cowsay ];
          };
        };
....

Here is a ``diff'' showing the changes in `flake.nix`.

....
$# sed -i '15i\\ \ \ \ \ \ \ \ devShells = rec {\n\ \ \ \ \ \ \ \ \ \ default = pkgs.mkShell {\n\ \ \ \ \ \ \ \ \ \ \ \ packages = [ pkgs.cowsay ];\n\ \ \ \ \ \ \ \ \ \ };\n\ \ \ \ \ \ \ \ };\n' flake.nix
$ git diff flake.nix
....

We restart the development shell and see that the `cowsay` command is
now available and the script works. Because we’ve updated source files
but haven’t `git commit`ed the new version, we get a warning message
about it being ``dirty''. It’s just a warning, though; the script runs
correctly.

....
$# echo '$ nix develop'
$# nix develop --command sh
$ which cowsay # is it available now?
$ ./hello-flake
....

Alternatively, we could use `nix run`.

....
$ nix run
....

Note, however, that `nix run` rebuilt the package in the Nix store and
ran _that_. It did not alter the copy in the `result` directory, as
we’ll see next.

....
$ cat result/bin/hello-flake
....

If we want to update the version in `result`, we need `nix build` again.

....
$ nix build
$ cat result/bin/hello-flake
....

Let’s `git commit` the changes and verify that the warning goes away. We
don’t need to `git push` the changes until we’re ready to share them.

....
$ git commit hello-flake flake.nix -m 'added bovine feature'
$ nix run
....

=== Development workflows

If you’re getting confused about when to use the different commands,
it’s because there’s more than one way to use Nix. I tend to think of it
as two different development workflows.

My usual, _high-level workflow_ is quite simple.

[arabic]
. `nix run` to re-build (if necessary) and run the executable.
. Fix any problems in `flake.nix` or the source code.
. Repeat until the package works properly.

In the high-level workflow, I don’t use a development shell because I
don’t need to directly invoke development tools such as compilers and
linkers. Nix invokes them for me according to the output definition in
`flake.nix`.

Occasionally I want to work at a lower level, and invoke compiler,
linkers, etc. directly. Perhaps want to work on one component without
rebuilding the entire package. Or perhaps I’m confused by some error
message, so I want to temporarily bypass Nix and ``talk'' directly to
the compiler. In this case I temporarily switch to a _low-level
workflow_.

[arabic]
. `nix develop` to enter a development shell with any development tools
I need (e.g. compilers, linkers, documentation generators).
. Directly invoke tools such as compilers.
. Fix any problems in `flake.nix` or the source code.
. Directly invoke the executable. Note that the location of the
executable depends on the development tools – It probably isn’t
`result`!
. Repeat until the package works properly.

I generally only use `nix build` if I just want to build the package but
not execute anything (perhaps it’s just a library).

=== This all seems like a hassle!

It is a bit annoying to modify `flake.nix` and ether rebuild or reload
the development environment every time you need another tool. However,
this Nix way of doing things ensures that all of your dependencies, down
to the exact versions, are captured in `flake.lock`, and that anyone
else will be able to reproduce the development environment.
