Re: Dissecting Guix -- blog post series

[Mekeor Melire <mekeor@posteo.de>]

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2022-12-08 18:24 paren@disroot.org:

> 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.

Great, I'm looking forward to read it! Especially, personally, I'm eager for the rather fundamental concepts of Guix (and Nix).

> Perhaps they could go in the cookbook once the series is done?

Yes, I personally think the content should be published at two places at the same time: OTOH, it should either be incorporated into either the manual or the cookbook; and OTOH, it should be published as a blog-post. The blog-post should also link to the respective section of the manual or cookbook in a preamble.

> * Dissecting Guix, Part 1: Derivations
> * Dissecting Guix, Part 2: The Humble G-Expression
> * Dissecting Guix, Part 3: Packages
> * Dissecting Guix, Part 4: Monads
> * Dissecting Guix, Part 5: Profiles and Search Paths
> * Dissecting Guix, Part 6: Goings-On in the Build Container
> * Dissecting Guix, Part 7: Record Types
> * Dissecting Guix, Part 8: Substitutes and Grafts
> * Dissecting Guix, Part 9: Cross-Compilation

I think it'd also be fine to come up with the titles during the process of writing as sometimes that process itself is insightful. E.g. I could imagine parts 2 and 4 to collapse. Maybe, maybe not, you'll see.

> * Dissecting Guix, Part 10: Services
>
> Walks you through the process of creating a service, and thouroughly explains system configuration.

How'd this part differ from section "12.18 Defining Services" of the manual?

> * Dissecting Guix, Part 11: Home Services
> * Dissecting Guix, Part 12: Writing a Subcommand
> * 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!

How'd this part differ from sections 22 and "22.6 Submitting Patches" from the manual?

Now, as for the actual article. Firstly, I added some comments below. Secondly, I created a "patch" suggesting some changes.

By the way, the text does not seem to strictly fill columns at a certain line width. So I did not bother to fill columns in the "patch".

> 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.

This approach did not work nicely for me. In particular, the last command made Emacs freeze (even with and 'emacs -Q'). And then, when evaluating the first code block below, i.e. the module-import, I got the warning "Unknown meta command: geiser-no-values". That's why, in the "patch", I suggest to simply run 'M-x geiser RET' which works just fine.

(I guess this is due to the fact that "Geiser invokes geiser-guile-binary with certain flags": <https://gitlab.com/emacs-guix/emacs-guix/-/issues/16>.)

> 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>`

I suggest to use subheadings within this chapter because otherwise readers loose the overview.

> 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.

If the purpose is out of scope, then we should not dive in that deeply. Especially, I'd suggest skip the code snippet.

> 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

Here you write "pretty simple". Later you also write "self-explanatory" and "obviously". I suggest to let the reader decide what's simple.

> 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

Before this point, the record fields have been called '"argument"'s all the time. I think it's not nice style to carry on a quoted term through many paragraphs like this. Let's simply write "record field" all the time, instead.

> 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`.

I think an example for invoking read-derivation-from-file would round up this tutorial really nicely because it'd close the circle between .drv-files and <derivation>-objects.

> # 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!

Here, in the conclusion, IMHO, there could be another brief listing of all fields of a derivation.

> #### 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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From 2abcfaff2f3203d5e568044beffbb964684b99a0 Mon Sep 17 00:00:00 2001
From: user <>
Date: Sun, 11 Dec 2022 00:44:07 +0100
Subject: [PATCH] many different, rather small changes

---
 one.md | 52 +++++++++++++++++++++++++++++++++-------------------
 1 file changed, 33 insertions(+), 19 deletions(-)

diff --git a/one.md b/one.md
index cb2c9d9..ccd0397 100644
--- a/one.md
+++ b/one.md
@@ -4,7 +4,7 @@ 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,
+possibly be off-putting.  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.
@@ -118,7 +118,7 @@ Derive([("out","/gnu/store/v5pd69j3hjs1fck4b5p9hd91wc8yf5qx-irssi-1.4.3","","")]
 
 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.
+contain no labels for their fields or any other human-readable indicators.
 Instead, we're going to explore derivations in a Guile REPL.
 
 # Exploring Guix Interactively
@@ -128,9 +128,8 @@ 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.
+which provides a comfortable Emacs UI for various Lisp REPLs, run
+`M-x geiser RET`.
 
 There are a few Guix modules we'll need.  Run this Scheme code to import
 them:
@@ -149,7 +148,7 @@ APIs, along with the `irssi` and `glib` `<package>` objects.
 
 # Creating a `<derivation>`
 
-The Guix API for derivations revolves around the `<derivation>` record,
+The Guix API for derivations revolves around the [`<derivation>` record](https://guix.gnu.org/en/manual/en/html_node/Derivations.html),
 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:
@@ -170,7 +169,7 @@ defined like this:
 ```
 
 With the exception of `file-name`, each of those fields corresponds to
-an "argument" in the `Derive(...)` form.  Before we can examine them,
+a field 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.
 
@@ -183,7 +182,7 @@ However, this doesn't produce a derivation:
 ;;; (#<procedure 7fe17c7af540 at guix/store.scm:1994:2 (state)>)
 ```
 
-`pk` is an abbreviation for the procedure `peek`, which takes the given
+`pk` is an abbreviation for the Guile 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.
@@ -212,7 +211,9 @@ with.
 
 # Exploring `<derivation>`
 
-The first "argument" in the `.drv` file is `outputs`, which tells the
+## Outputs
+
+The first derivation record field in the `.drv` file is `outputs`, which tells the
 Guix daemon about the outputs that this build can produce:
 
 ```scheme
@@ -221,6 +222,7 @@ Guix daemon about the outputs that this build can produce:
 ;;; ((("out" . #<<derivation-output> path: "/gnu/store/v5pd69j3hjs1fck4b5p9hd91wc8yf5qx-irssi-1.4.3" hash-algo: #f hash: #f recursive?: #f>)))
 
 (pk (assoc-ref irssi-outputs "out"))
+;;; (#<<derivation-output> path: "/gnu/store/v5pd69j3hjs1fck4b5p9hd91wc8yf5qx-irssi-1.4.1" hash-algo: #f hash: #f recursive?: #f>)
 
 (define glib-outputs
   (pk (derivation-outputs glib-drv)))
@@ -231,7 +233,7 @@ Guix daemon about the outputs that this build can produce:
 ```
 
 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:
+records, and it's equivalent to the first field in the `.drv` file:
 
 ```
 [ ("out", "/gnu/store/v5pd69j3hjs1fck4b5p9hd91wc8yf5qx-irssi-1.4.3", "", "")
@@ -260,7 +262,7 @@ will be in advance.  For instance, `origin`s produce fixed-output derivations:
 
 Note how the `hash` and `hash-algo` now have values.
 
-Perceptive readers may note that the `<derivation-output>` has four fields,
+You might have noted 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:
@@ -285,8 +287,10 @@ field is serialised by prefixing the `hash-algo` with `r:` if it's true:
 The purpose of `recursive?` is difficult to explain, and is out of scope for
 this post.
 
+## Inputs
+
 The next field is `inputs`, which corresponds to, you guessed it, the
-second pseudo-"argument" in the `.drv` file format:
+second field in the `.drv` file format:
 
 ```
 [ ("/gnu/store/9mv9xg4kyj4h1cvsgrw7b9x34y8yppph-glib-2.70.2.drv", ["out"]),
@@ -317,8 +321,9 @@ 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
+## Sources
+
+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.
@@ -328,8 +333,10 @@ 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
+## System, Builder and Arguments
+
+The next record field is `derivation-system`, which specifies
+the Nix system we're building for.  Then, there 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
@@ -347,7 +354,7 @@ directories:
 ;;; (("--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
+The final record field contains a list of environment variables to set before
 we start the build process:
 
 ```scheme
@@ -355,9 +362,16 @@ we start the build process:
 ;;; ((("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
+## File Name
+
+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
+in a serialised derivation. But how to do the inverse? How to read a
+derivation from a path?
+
+# Reading a derivation from file
+
+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`.
 
-- 
2.38.1


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