[RFC 0146] Meta.Categories, not Filesystem Directory Trees

rfcs/0146-meta-categories.md

@@ -0,0 +1,298 @@
+---
+feature: Decouple filesystem from categorization
+start-date: 2023-04-23
+author: Anderson Torres
+co-authors:
+shepherd-team: @7c6f434c @natsukium @fgaz @infinisil
+shepherd-leader: @7c6f434c
+related-issues: (will contain links to implementation PRs)
+---
+
+# Summary
+[summary]: #summary
+
+Deploy a new method of categorization for the packages maintained by Nixpkgs,
+not relying on filesystem idiosyncrasies.
+
+# Motivation
+[motivation]: #motivation
+
+Currently, Nixpkgs uses the filesystem, or more accurately, the directory tree
+layout in order to informally categorize the softwares it packages, as described
+in the [Hierarchy](https://nixos.org/manual/nixpkgs/stable/#sec-hierarchy)
+section of Nixpkgs manual.
+
+This is a simple, easy to understand and consecrated-by-use method of
+categorization, partially employed by many other package managers like GNU Guix
+and NetBSD pkgsrc.
+
+However this system of categorization has serious problems:
+
+1. It is bounded by the constraints imposed by the filesystem.
+
+   - Restrictions on filenames, subdirectory tree depth, permissions, inodes,
+     quotas, and many other things.
+     - Some of these restrictions are not well documented and are found simply
+       by "bumping" on them.
+     - The restrictions can vary on an implementation basis.
+       - Some filesystems have more restrictions or less features than others,
+         forcing an uncomfortable lowest common denominator.
+       - Some operating systems can impose additional constraints over otherwise
+         full-featured filesystems because of backwards compatibility (8 dot
+         3,anyone?).
+
+2. It requires a local checkout of the tree.
+
+   Certainly this checkout can be "cached" using some form of `find . >
+   /tmp/pkgs-listing.txt`, or more sophisticated solutions like `locate +
+   updatedb`. Nonetheless such solutions still require access to a fresh,
+   updated copy of the Nixpkgs tree.
+
+3. The creation of a new category - and more generally the manipulation of
+   categories - requires an unpleaseant task of renaming and eventually patching
+   many seemingly unrelated files.
+
+   - Moving files around Nixpkgs codebase requires updating their forward and
+     backward references.
+     - Especially in some auxiliary tools like editor plugins, testing suites,
+       autoupdate scripts and so on.
+   - Rewriting `all-packages.nix` can be error-prone (even using Metapad) and it
+     can generate huge, noisy patches.
+
+4. There is no convenient way to use multivalued categorization.
+
+   A piece of software can fulfill many categories; e.g.
+   - an educational game
+   - a console emulator (vs. a PC emulator)
+   - and a special-purpose programming language (say, a smart-contracts one).
+
+   The current one-size-fits-all restriction is artificial, imposes unreasonable
+   limitations and results in incomplete and confusing information.
+
+   - No, symlinks or hardlinks are not convenient for this purpose; not all
+     environments support them (falling on the "less features than others"
+     problem expressed before) and they convey nothing besides confusion - just
+     think about writing the corresponding entry in `all-packages.nix`.
+
+5. It puts over the (possibly human) package writer the mental load of where to
+   put the files on the filesystem hierarchy, deviating them from the job of
+   really writing them.
+
+   - Or just taking the shortest path and throw it on a folder under `misc`.
+
+6. It "locks" the filesystem, preventing its usage for other, more sensible
+   purposes.
+
+7. The most important: the categorization is not discoverable via Nix language
+   infrastructure.
+
+   Indeed there is no higher level way to query about such categories besides
+   the one described in the bullet 2 above.
+
+In light of such a bunch of problems, this RFC proposes a novel alternative to
+the above mess: new `meta` attributes.
+
+# Detailed design
+[design]: #detailed-design
+
+A new attribute, `meta.categories`, will be included for every Nix expression
+living inside Nixpkgs.
+
+This attribute will be a list, whose elements are one of the possible elements
+of the `lib.categories` set.
+
+A typical snippet of `lib.categories` will be similar to:
+
+```nix
+{
+  assembler = {
+    name = "Assembler";
+    description = ''
+      A program that converts text written in assembly language to binary code.
+    '';
+  };
+
+  compiler = {
+    name = "Compiler";
+    description = ''
+      A program that converts a source from a language to another, usually from
+      a higher, human-readable level to a lower, machine level.
+    '';
+  };
+
+  font = {
+    name = "Font";
+    description = ''
+      A set of files that defines a set of graphically-related glyphs.
+    '';
+  };
+
+  game = {
+    name = "Game";
+    description = ''
+      A program developed with entertainment in mind.
+    '';
+  };
+
+  interpreter = {
+    name = "Interpreter";
+    description = ''
+      A program that directly executes instructions written in a programming
+      language, without requiring compilation into the native machine language.
+    '';
+  };
+
+```
+
+# Examples and Interactions
+[examples-and-interactions]: #examples-and-interactions
+
+In file bochs/default.nix:
+
+```nix
+stdenv.mkDerivation {
+
+. . .
+
+  meta = {
+    . . .
+    categories = with lib.categories; [ emulator debugger ];
+    . . .
+    };
+  };
+}
+
+```
+
+In a `nix repl`:
+
+```
+nix-repl> :l <nixpkgs>
+Added XXXXXX variables.
+
+nix-repl> pkgs.bochs.meta.categories
+[ { ... } ]
+
+nix-repl> map (z: z.name) pkgs.bochs.meta.categories
+[ "debugger" "emulator" ]
+```
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+The most immediate drawbacks are:
+
+1. A huge treewide edit of Nixpkgs
+
+   On the other hand, this is easily sprintable and amenable to automation.
+
+2. Bikeshedding
+
+   How many and which categories we should create? Can we expand them later?
+
+   For start, we can follow/take inspiration from many of the already existing
+   categories sets and add extra ones when the needs arise. Indeed, it is way
+   easier to create such categories using Nix language when compared to other
+   software collections.
+
+3. Superfluous
+
+   It can be argued that there are other ways to discover similar or related
+   package sets, like Repology.
+
+   However, this argument is a bit circular, because e.g. the classification
+   shown by Repology effectively replicates the classification done by the many
+   software collections in its catalog. Therefore, relying in Repology merely
+   transfers the question to external sources.
+
+   Further it becomes more pronounced when we take into account the fact Nixpkgs
+   is top 1 of most Repology statistics. The expected outcome, therefore, should
+   be precisely the opposite: Nixpkgs being _the_ source of structured metainfo
+   for other software collections.
+
+# Alternatives
+[alternatives]: #alternatives
+
+1. Do nothing
+
+   This will exacerbate the problems already listed.
+
+2. Ignore/nuke the categorization completely
+
+   This is an alternative worthy of some consideration. After all,
+   categorization is not without its problems, as shown above. Removing or
+   ignoring classification removes all problems.
+
+   However, there are good reasons to keep the categorization:
+
+   - The complete removal of categorization is too harsh. A solution that keeps
+     and enhances the categorization is way more preferrable than one that nukes
+     it completely.
+
+   - As said before, the categorization is already present; this RFC proposes to
+     expose it to a higher level, in a structured, more discoverable format.
+
+   - Categorization is very traditional among software collections. Many of them
+     are doing this just fine for years on end, and Nixpkgs can imitate them
+     easily - and even surpass them, given the benefits of Nix language
+     machinery.
+
+   - Categorization is useful in many scenarios and use cases - indeed they
+     are ubiquitous in software world:
+     - specialized search engines (from Repology to MELPA)
+     - code forges, from Sourceforge to Gitlab
+     - as said above, software collections from pkgsrc to slackbuilds
+     - to organization and preservation (as Software Heritage)
+
+# Prior art
+[prior-art]: #prior-art
+
+As said above, categorization is very traditional among software collections. It
+is not hard to cite examples in this arena; the most interesting ones I have
+found are listed above (linked at [references section](#references))
+
+- FreeBSD Ports;
+- Debtags;
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+Still unsolved is what data structure is better suited to represent a category.
+
+- For now we stick to a set `{ name, description }`.
+- Given the redundancy of the option above, another possibility is something
+  like `nameOfCategory = { description = ""; . . . }`
+
+# Future work
+[future]: #future-work
+
+- Curate the categories.
+- Update documentation.
+- Update Continuous Integration.
+
+# References
+[references]: #references
+
+- [Desktop Menu
+  Specification](https://specifications.freedesktop.org/menu-spec/latest/);
+  specifically,
+  - [Main
+    categories](https://specifications.freedesktop.org/menu-spec/latest/apa.html)
+  - [Additional
+    categories](https://specifications.freedesktop.org/menu-spec/latest/apas02.html)
+  - [Reserved
+    categories](https://specifications.freedesktop.org/menu-spec/latest/apas03.html)
+
+- [Appstream](https://www.freedesktop.org/wiki/Distributions/AppStream/)
+
+- [Debtags](https://wiki.debian.org/Debtags)
+
+- [NetBSD pkgsrc guide](https://www.netbsd.org/docs/pkgsrc/)
+  - Especially, [Chapter 12, Section
+    1](https://www.netbsd.org/docs/pkgsrc/components.html#components.Makefile)
+    contains a short list of CATEGORIES.
+
+- [FreeBSD Porters
+  Handbook](https://docs.freebsd.org/en/books/porters-handbook/makefiles/#porting-categories)
+  - Especially
+    [Categories](https://docs.freebsd.org/en/books/porters-handbook/makefiles/#porting-categories)