Dissecting Guix -- blog post series

[(]

Heya!

Some of you may have seen on IRC that I've been writing a post for the Guix
blog that I hope will form the first part of a series.  This series aims to
dissect the internals of Guix, from bottom to top.  Perhaps they could go in
the cookbook once the series is done?  My aim is to write the following posts
(complete with cheesy titles :P), hopefully one per week:

* Dissecting Guix, Part 1: Derivations

Discusses derivations, the bottom layer of the Guix compilation tower, and
dissects some example derivations.

A draft of this post may be found below. Please feel free to critique! :)

* Dissecting Guix, Part 2: The Humble G-Expression

Talks about g-expressions and why they're necessary, explains file-like objects,
and provides demonstrations of each kind of file-like.

* Dissecting Guix, Part 3: Packages

A walkthrough for package creation.

* Dissecting Guix, Part 4: Monads

``mlet'', ``>>='', ``define-monad'', ``return'', and all that.

* Dissecting Guix, Part 5: Profiles and Search Paths

Explores profiles and search paths.

* Dissecting Guix, Part 6: Goings-On in the Build Container

Explains build systems, examines a build script, and talks about what exactly
happens when a package is built.  Also demonstrates some of the
``(guix build utils)'' APIs.

* Dissecting Guix, Part 7: Record Types

Demonstrates the ``(guix records)'' API in all its glory.

* Dissecting Guix, Part 8: Substitutes and Grafts

Discusses substitutes, and that persistent thorn in our sides, grafting.

* Dissecting Guix, Part 9: Cross-Compilation

Building packages on architecture X for architecture Y, and how that all
works.

* Dissecting Guix, Part 10: Services

Walks you through the process of creating a service, and thouroughly explains
system configuration.

* Dissecting Guix, Part 11: Home Services

Similar to Part 9, except it's about ``guix home'', of course.

* Dissecting Guix, Part 12: Writing a Subcommand

Guides you through the process of adding a new command to Guix with the
extensions feature, demonstrating several utility APIs in the process.

* Dissecting Guix, Part 13: Lending a Hand

How to edit the Guix source code and submit patches to be reviewed by the
lovely Guix community!

    -- (

title: Dissecting Guix, Part 1: Derivations and Derivation
date: TBC
author: (
tags: Dissecting Guix, Functional package management, Programming interfaces, 
Scheme API
---
To a new user, Guix's functional architecture can seem quite alien, and
possibly offputting.  With a combination of extensive `#guix`-querying,
determined manual-reading, and plenty of source-perusing, they may
eventually figure out how everything fits together by themselves, but this
can be frustrating and often takes a fairly long time.

However, once you peel back the layers, the "Nix way" is actually rather
elegant, if perhaps not as simple as the mutable, imperative style
implemented by the likes of [`dpkg`](https://wiki.debian.org/dpkg) and, 
[`pacman`](https://wiki.archlinux.org/title/pacman).  This series of blog
posts will cover basic Guix concepts, taking a "ground-up" approach by
dealing with lower-level concepts first, and hopefully make those months of
information-gathering unnecessary.

Before we dig in to Guix-specific concepts, we'll need to learn about one
inherited from [Nix](https://nixos.org), the original functional package
manager and the inspiration for Guix; the idea of a _derivation_ and its
corresponding _store items_.

These concepts were originally described by Eelco Dolstra, the author of Nix,
in their [PhD thesis](https://edolstra.github.io/pubs/phd-thesis.pdf); see
_§ 2.1 The Nix store_ and _§ 2.4 Store Derivations_.

# Store Items

As you almost certainly know, everything that Guix builds is stored in the
_store_, which is almost always the `/gnu/store` directory.  It's the job of
the 
[`guix-daemon`](https://guix.gnu.org/manual/en/html_node/Invoking-guix_002ddaemon.html)
to manage the store and build things.  If you run
[`guix build 
PKG`](https://guix.gnu.org/manual/en/html_node/Invoking-guix-build.html),
`PKG` will be built or downloaded from a substitute server if available, and
a path to an item in the store will be displayed.

```
$ guix build irssi
/gnu/store/v5pd69j3hjs1fck4b5p9hd91wc8yf5qx-irssi-1.4.3
```

This item contains the final result of building [`irssi`](https://irssi.org).
Let's peek inside:

```
$ ls $(guix build irssi)
bin/  etc/  include/  lib/  share/
$ ls $(guix build irssi)/bin
irssi*
```

`irssi` is quite a simple package.  What about a more complex one, like
[`glib`](https://docs.gtk.org/glib)?

```
$ guix build glib
/gnu/store/bx8qq76idlmjrlqf1faslsq6zjc6f426-glib-2.73.3-bin
/gnu/store/j65bhqwr7qq7l77nj0ahmk1f1ilnjr3a-glib-2.73.3-debug
/gnu/store/3pn4ll6qakgfvfpc4mw89qrrbsgj3jf3-glib-2.73.3-doc
/gnu/store/dvsk6x7d26nmwsqhnzws4iirb6dhhr1d-glib-2.73.3
/gnu/store/4c8ycz501n2d0xdi4blahvnbjhd5hpa8-glib-2.73.3-static
```

`glib` produces five `/gnu/store` items, because it's possible for a package
to produce multiple 
[outputs](https://guix.gnu.org/manual/en/html_node/Packages-with-Multiple-Outputs.html).
Each output can be referred to separately, by prefixing a package's name with
`:OUTPUT` where supported.  For example, this
[`guix 
install`](https://guix.gnu.org/manual/en/html_node/Invoking-guix-package.html)
invocation will add `glib`'s `bin` output to your profile:

```
$ guix install glib:bin
```

The default output is `out`, so when you pass `glib` by itself to that
command, it will actually install `glib:out` to the profile.

`guix build` also provides the `--source` flag, which produces the store
item corresponding to the given package's downloaded source code.

```
$ guix build --source irssi
/gnu/store/cflbi4nbak0v9xbyc43lamzl4a539hhb-irssi-1.4.3.tar.xz
$ guix build --source glib
/gnu/store/d22wzjq3xm3q8hwnhbgk2xd3ph7lb6ay-glib-2.73.3.tar.xz
```

But how does Guix know how to build these store outputs in the first place?
That's where derivations come in.

# `.drv` Files

You've probably seen these being printed by the Guix CLI now and again.
Derivations, represented in the daemon's eyes by `.drv` files, contain
instructions for building store items.  We can retrieve the paths of
these `.drv` files with the `guix build --derivations` command:

```
$ guix build --derivations irssi
/gnu/store/zcgmhac8r4kdj2s6bcvcmhh4k35qvihx-irssi-1.4.3.drv
```

`guix build` can actually also accept derivation paths as an argument, in
lieu of a package, like so:

```
$ guix build /gnu/store/zcgmhac8r4kdj2s6bcvcmhh4k35qvihx-irssi-1.4.3.drv
/gnu/store/v5pd69j3hjs1fck4b5p9hd91wc8yf5qx-irssi-1.4.3
```

Let's look inside this derivation file.

```
Derive([("out","/gnu/store/v5pd69j3hjs1fck4b5p9hd91wc8yf5qx-irssi-1.4.3","","")],[("/gnu/store/9mv9xg4kyj4h1cvsgrw7b9x34y8yppph-glib-2.70.2.drv",["out"]),("/gnu/store/baqpbl4wck7nkxrbyc9nlhma7kq5dyfl-guile-2.0.14.drv",["out"]),("/gnu/store/bfirgq65ndhf63nn4q6vlkbha9zd931q-openssl-1.1.1l.drv",["out"]),("/gnu/store/gjwpqzvfhz13shix6a6cs2hjc18pj7wy-module-import-compiled.drv",["out"]),("/gnu/store/ij8651x4yh53hhcn6qw2644nhh2s8kcn-glib-2.70.2.drv",["out"]),("/gnu/store/jg2vv6yc2yqzi3qzs82dxvqmi5k21lhy-irssi-1.4.3.drv",["out"]),("/gnu/store/qggpjl9g6ic3cq09qrwkm0dfsdjf7pyr-glibc-utf8-locales-2.33.drv",["out"]),("/gnu/store/zafabw13yyhz93jwrcz7axak1kn1f2cx-openssl-1.1.1s.drv",["out"])],["/gnu/store/af18nrrsk98c5a71h3fifnxg1zi5mx7y-module-import","/gnu/store/qnrwmby5cwqdqxyiv1ga6azvakmdvgl7-irssi-1.4.3-builder"],"x86_64-linux","/gnu/store/hnr4r2d0h0xarx52i6jq9gvsrlc3q81a-guile-2.0.14/bin/guile",["--no-auto-compile","-L","/gnu/store/af18nrrsk98c5a71h3fifnxg1zi5mx7y-module-import","-C","/gnu/store/6rkkvvb7pl1l9ng8vvywvwf357vhm3va-module-import-compiled","/gnu/store/qnrwmby5cwqdqxyiv1ga6azvakmdvgl7-irssi-1.4.3-builder"],[("allowSubstitutes","0"),("guix
 properties","((type . graft) (graft (count . 
2)))"),("out","/gnu/store/v5pd69j3hjs1fck4b5p9hd91wc8yf5qx-irssi-1.4.3"),("preferLocalBuild","1")])
```

It's... not exactly human-readable.  We could try to format it and break
it down, but it'd still be pretty hard to understand, since `.drv` files
contain no labels for the "arguments" or any other human-readable indicators.
Instead, we're going to explore derivations in a Guile REPL.

# Exploring Guix Interactively

Before we continue, we'll want to start a REPL, so that we can try out
the Guix Guile API interactively.  To run a REPL in the terminal, simply
[call `guix 
repl`](https://guix.gnu.org/manual/devel/en/html_node/Using-Guix-Interactively.html).

If you're using Emacs, you can instead install 
[Geiser](https://nongnu.org/geiser),
which provides a comfortable Emacs UI for various Lisp REPLs, invoke
`guix repl --listen=tcp:37146 &`, and type `M-x geiser-connect RET RET RET` to
connect to the running Guile instance.

There are a few Guix modules we'll need.  Run this Scheme code to import
them:

```scheme
(use-modules (guix derivations)
             (guix gexp)
             (guix packages)
             (guix store)
             (gnu packages glib)
             (gnu packages irc))
```

We now have access to the store, G-expression, package, and derivation
APIs, along with the `irssi` and `glib` `<package>` objects.

# Creating a `<derivation>`

The Guix API for derivations revolves around the `<derivation>` record,
which is the Scheme representation of that whole block of text surrounded by
`Derive(...)`.  If we look in `guix/derivations.scm`, we can see that it's
defined like this:

```scheme
(define-immutable-record-type <derivation>
  (make-derivation outputs inputs sources system builder args env-vars
                   file-name)
  derivation?
  (outputs  derivation-outputs)      ; list of name/<derivation-output> pairs
  (inputs   derivation-inputs)       ; list of <derivation-input>
  (sources  derivation-sources)      ; list of store paths
  (system   derivation-system)       ; string
  (builder  derivation-builder)      ; store path
  (args     derivation-builder-arguments)         ; list of strings
  (env-vars derivation-builder-environment-vars)  ; list of name/value pairs
  (file-name derivation-file-name))               ; the .drv file name
```

With the exception of `file-name`, each of those fields corresponds to
an "argument" in the `Derive(...)` form.  Before we can examine them,
though, we need to figure out how to _lower_ that `irssi` `<package>`
object into a derivation.

The procedure we use to turn a high-level object like `<package>` into a
derivation is called `lower-object`; more on that in a future post.
However, this doesn't produce a derivation:

```scheme
(pk (lower-object irssi))
;;; (#<procedure 7fe17c7af540 at guix/store.scm:1994:2 (state)>)
```

`pk` is an abbreviation for the procedure `peek`, which takes the given
object, writes a representation of it to the output, and returns it.
It's especially handy when you want to view an intermediate value in a
complex expression.

The returned object is a procedure that needs to be evaluated in the
context of a store connection.  We do this by first using `with-store`
to connect to the store and bind the connection to a name, then wrapping
the `lower-object` call with `run-with-store`:

```scheme
(define irssi-drv
  (pk (with-store %store
        (run-with-store %store
          (lower-object irssi)))))
;;; (#<derivation /gnu/store/zcgmhac8r4kdj2s6bcvcmhh4k35qvihx-irssi-1.4.3.drv 
=> /gnu/store/v5pd69j3hjs1fck4b5p9hd91wc8yf5qx-irssi-1.4.3 7fe1902b6140>)

(define glib-drv
  (pk (with-store %store
        (run-with-store %store
          (lower-object glib)))))
;;; (#<derivation /gnu/store/81qqs7xah2ln39znrji4r6xj85zi15bi-glib-2.70.2.drv 
=> /gnu/store/lp7k9ygvpwxgxjvmf8bix8d2aar0azr7-glib-2.70.2-bin 
/gnu/store/22mkp8cr6rxg6w8br9q8dbymf51b44m8-glib-2.70.2-debug 
/gnu/store/a6qb5arvir4vm1zlkp4chnl7d8qzzd7x-glib-2.70.2 
/gnu/store/y4ak268dcdwkc6lmqfk9g1dgk2jr9i34-glib-2.70.2-static 7fe17ca13b90>)
```

And we have liftoff!  Now we've got two `<derivation>` records to play
with.

# Exploring `<derivation>`

The first "argument" in the `.drv` file is `outputs`, which tells the
Guix daemon about the outputs that this build can produce:

```scheme
(define irssi-outputs
  (pk (derivation-outputs irssi-drv)))
;;; ((("out" . #<<derivation-output> path: 
"/gnu/store/v5pd69j3hjs1fck4b5p9hd91wc8yf5qx-irssi-1.4.3" hash-algo: #f hash: 
#f recursive?: #f>)))

(pk (assoc-ref irssi-outputs "out"))

(define glib-outputs
  (pk (derivation-outputs glib-drv)))
;;; ((("bin" . #<<derivation-output> path: 
"/gnu/store/lp7k9ygvpwxgxjvmf8bix8d2aar0azr7-glib-2.70.2-bin" hash-algo: #f 
hash: #f recursive?: #f>) ("debug" . #<<derivation-output> path: 
"/gnu/store/22mkp8cr6rxg6w8br9q8dbymf51b44m8-glib-2.70.2-debug" hash-algo: #f 
hash: #f recursive?: #f>) ("out" . #<<derivation-output> path: 
"/gnu/store/a6qb5arvir4vm1zlkp4chnl7d8qzzd7x-glib-2.70.2" hash-algo: #f hash: 
#f recursive?: #f>) ("static" . #<<derivation-output> path: 
"/gnu/store/y4ak268dcdwkc6lmqfk9g1dgk2jr9i34-glib-2.70.2-static" hash-algo: #f 
hash: #f recursive?: #f>)))

(pk (assoc-ref glib-outputs "bin"))
;;; (#<<derivation-output> path: 
"/gnu/store/lp7k9ygvpwxgxjvmf8bix8d2aar0azr7-glib-2.70.2-bin" hash-algo: #f 
hash: #f recursive?: #f>)
```

It's a simple association list mapping output names to `<derivation-output>`
records, and it's equivalent to the first "argument" in the `.drv` file:

```
[ ("out", "/gnu/store/v5pd69j3hjs1fck4b5p9hd91wc8yf5qx-irssi-1.4.3", "", "")
]
```

The `hash-algo` and `hash` fields are for storing the content hash and the
algorithm used with that hash for what we term a _fixed-output derivation_,
which is essentially a derivation where we know what the hash of the content
will be in advance.  For instance, `origin`s produce fixed-output derivations:

```scheme
(define irssi-src-drv
  (pk (with-store %store
        (run-with-store %store
          (lower-object (package-source irssi))))))
;;; (#<derivation 
/gnu/store/mcz3vzq7lwwaqjb8dy7cd69lvmi6d241-irssi-1.4.3.tar.xz.drv => 
/gnu/store/cflbi4nbak0v9xbyc43lamzl4a539hhb-irssi-1.4.3.tar.xz 7fe17b3c8d70>)

(define irssi-src-outputs
  (pk (derivation-outputs irssi-src-drv)))
;;; ((("out" . #<<derivation-output> path: 
"/gnu/store/cflbi4nbak0v9xbyc43lamzl4a539hhb-irssi-1.4.3.tar.xz" hash-algo: 
sha256 hash: #vu8(185 63 113 82 35 163 34 230 127 66 182 26 8 165 18 174 41 227 
75 212 165 61 127 34 55 102 102 10 170 90 4 52) recursive?: #f>)))
  
(pk (assoc-ref irssi-src-outputs "out"))
;;; (#<<derivation-output> path: 
"/gnu/store/cflbi4nbak0v9xbyc43lamzl4a539hhb-irssi-1.4.3.tar.xz" hash-algo: 
sha256 hash: #vu8(185 63 113 82 35 163 34 230 127 66 182 26 8 165 18 174 41 227 
75 212 165 61 127 34 55 102 102 10 170 90 4 52) recursive?: #f>)
```

Note how the `hash` and `hash-algo` now have values.

Perceptive readers may note that the `<derivation-output>` has four fields,
whereas the tuple in the `.drv` file only has three (minus the label).  If
we read the source for `write-derivation`, we can see that the `recursive?`
field is serialised by prefixing the `hash-algo` with `r:` if it's true:

```scheme
;;; guix/derivations.scm:630:2

(define (write-output output port)
  (match output
    ((name . ($ <derivation-output> path hash-algo hash recursive?))
     (write-tuple (list name path
                        (if hash-algo
                            (string-append (if recursive? "r:" "")
                                           (symbol->string hash-algo))
                            "")
                        (or (and=> hash bytevector->base16-string)
                            ""))
                  write-escaped-string
                  port))))
```

The purpose of `recursive?` is difficult to explain, and is out of scope for
this post.

The next field is `inputs`, which corresponds to, you guessed it, the
second pseudo-"argument" in the `.drv` file format:

```
[ ("/gnu/store/9mv9xg4kyj4h1cvsgrw7b9x34y8yppph-glib-2.70.2.drv", ["out"]),
  ("/gnu/store/baqpbl4wck7nkxrbyc9nlhma7kq5dyfl-guile-2.0.14.drv", ["out"]),
  ("/gnu/store/bfirgq65ndhf63nn4q6vlkbha9zd931q-openssl-1.1.1l.drv", ["out"]),
  ("/gnu/store/gjwpqzvfhz13shix6a6cs2hjc18pj7wy-module-import-compiled.drv", 
["out"]),
  ("/gnu/store/ij8651x4yh53hhcn6qw2644nhh2s8kcn-glib-2.70.2.drv", ["out"]),
  ("/gnu/store/jg2vv6yc2yqzi3qzs82dxvqmi5k21lhy-irssi-1.4.3.drv", ["out"]),
  ("/gnu/store/qggpjl9g6ic3cq09qrwkm0dfsdjf7pyr-glibc-utf8-locales-2.33.drv", 
["out"]),
  ("/gnu/store/zafabw13yyhz93jwrcz7axak1kn1f2cx-openssl-1.1.1s.drv", ["out"])
]
```

Here, each tuple specifies a derivation that needs to be built before this
derivation can be built, and the outputs of the derivation that the build
process of this derivation uses.  Let's grab us the Scheme equivalent:

```scheme
(define irssi-inputs
  (pk (derivation-inputs irssi-drv)))
;;; [a fairly large amount of output]

(pk (car irssi-inputs))
;;; (#<<derivation-input> drv: #<derivation 
/gnu/store/9mv9xg4kyj4h1cvsgrw7b9x34y8yppph-glib-2.70.2.drv => 
/gnu/store/2jj2mxn6wfrcw7i85nywk71mmqbnhzps-glib-2.70.2 7fe1902b6640> 
sub-derivations: ("out")>)
```

Unlike `derivation-outputs`, `derivation-inputs` maps 1:1 to the `.drv`
form; the `drv` field is a `<derivation>` to be built, and the
`sub-derivations` field is a list of outputs.

The other pseudo-"arguments" are pretty simple; none of them involve new
records.  The third is `derivation-sources`, which contains a list of all
store items used in the build which aren't themselves built using
derivations, whereas `derivation-inputs` contains the dependencies which
are.

This list usually just contains the path to the Guile _build script_ that
realises the store items when run, which we'll examine in a later post, and
the path to a directory containing extra modules to add to the build script's
`%load-path`, called `/gnu/store/...-module-import`.

The next "argument" is self-explanatory: `derivation-system`, which specifies
the Nix system we're building for.  Next is `derivation-builder`, pointing to
the `guile` executable that runs the script; and the second-to-last is
`derivation-args`, which is a list of arguments to pass to `derivation-builder`.
Note how we use `-L` and `-C` to extend the Guile `%load-path` and
`%load-compiled-path` to include the `module-import` and 
`module-import-compiled`
directories:

```scheme
(pk (derivation-system irssi-drv))
;;; ("x86_64-linux")

(pk (derivation-builder irrsi-drv))
;;; ("/gnu/store/hnr4r2d0h0xarx52i6jq9gvsrlc3q81a-guile-2.0.14/bin/guile")

(pk (derivation-builder-arguments irrsi-drv))
;;; (("--no-auto-compile" "-L" 
"/gnu/store/af18nrrsk98c5a71h3fifnxg1zi5mx7y-module-import" "-C" 
"/gnu/store/6rkkvvb7pl1l9ng8vvywvwf357vhm3va-module-import-compiled" 
"/gnu/store/qnrwmby5cwqdqxyiv1ga6azvakmdvgl7-irssi-1.4.3-builder"))
```

The final "argument" contains a list of environment variables to set before
we start the build process:

```scheme
(pk (derivation-builder-environment-vars irssi-drv))
;;; ((("allowSubstitutes" . "0") ("guix properties" . "((type . graft) (graft 
(count . 2)))") ("out" . 
"/gnu/store/v5pd69j3hjs1fck4b5p9hd91wc8yf5qx-irssi-1.4.3") ("preferLocalBuild" 
. "1")))
```

Obviously, the last record field, `derivation-file-name`, simply allows you to
retrieve the path to the `.drv` file in Scheme, and so isn't represented
in a serialised derivation.  Speaking of serialisation, to convert between the
`.drv` text format and the Scheme `<derivation>` record, you can use
`write-derivation`, `read-derivation`, and `read-derivation-from-file`.

# Conclusion

Derivations are one of Guix's most important concepts, but are fairly easy to
understand once you get past the obtuse `.drv` file format.  They provide the
Guix daemon with the initial instructions that it uses to build store items
like packages, origins, and other file-likes such as `computed-file` and
`local-file`, which will be discussed in a future post!

#### About GNU Guix

[GNU Guix](https://guix.gnu.org) is a transactional package manager and
an advanced distribution of the GNU system that [respects user
freedom](https://www.gnu.org/distros/free-system-distribution-guidelines.html).
Guix can be used on top of any system running the Hurd or the Linux
kernel, or it can be used as a standalone operating system distribution
for i686, x86_64, ARMv7, AArch64 and POWER9 machines.

In addition to standard package management features, Guix supports
transactional upgrades and roll-backs, unprivileged package management,
per-user profiles, and garbage collection.  When used as a standalone
GNU/Linux distribution, Guix offers a declarative, stateless approach to
operating system configuration management.  Guix is highly customizable
and hackable through [Guile](https://www.gnu.org/software/guile)
programming interfaces and extensions to the
[Scheme](http://schemers.org) language.

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