format 2.0: sample writers over the abstraction, in D and in Gleam

Writers over sisudoc-ocda-lib: functions from the document model to
bytes, in five formats, one program, ocda-write, in each language.
Begun 2026-09-25; this is the writer as it stood at format 2.0.

  ocda-write --text|--md|--html|--epub|--typst <file> [lang]

They are samples. Spine is the output pipeline and nothing here
replaces it. What they show is that the abstraction is sufficient:
given a .ssp, a .ocda.db or a .sisupod and nothing else, a few hundred
lines per format make a complete document. A writer is never told
which form it was given, and every writer is run against every form
of every document.

Each format is held to an oracle that can say no:

  --text   the words equal the html writer's objects, as a multiset;
           every object number exactly once
  --md     CommonMark; every (#x) resolves to an <a id=x>, no id twice
  --html   it parses; every internal link resolves; the objects' text
           equals spine's scroll html
  --epub   epubcheck
  --typst  typst itself: a link to a missing label does not compile

and in every format the same bytes from every input form, and the D
and Gleam writers byte for byte alike. Over the markup document
samples every check holds for every document-language. The two
languages' writers were not written from each other, and their
agreement is the strongest check: twice it caught what nothing else
did (an ampersand in an attribute mangled by Erlang's replacement
string; typst label names built from bytes in one language and
graphemes in the other).

Byte identity with spine is not the aim, by decision. The text writer
began byte identical to spine --text, and was rewritten: a note is
written once, markup comments and dummy headings are not written, an
image keeps its caption, a table is padded to display width. The html
writer is valid html5 rather than a copy of spine's (h1..h6 where
spine writes <h0>; one charset; no placeholder metadata; an id for
every anchor tag; self-contained). Each difference is a better
presentation, and the code says why at the point of choice.

The typst writer puts the object number in the margin as the label
every citation points at, a note as a real footnote with a link back,
and a table of contents that locates by object number rather than by
page, as the markdown writer's does: a page number is a fact about one
typesetting, the object number is not. The markdown writer is
CommonMark with yaml front matter; a note is at its reference, with
the document's own number, and --image-prefix gives the image path,
which the model rightly does not know.

Holding the writers to these checks found defects in spine, several
since fixed there, and spine's typst and markdown writers were ported
from these.

Use: nix build, in dlang/ or in gleam/. Each flake takes the library
from git.sisudoc.org, and flake.lock pins the commit of it these were
built and checked against. gleam/ builds with nix-gleam's builder,
kept in gleam/nix-gleam/ (MIT); in a working tree, gleam.toml reads
the library through the symlink ./ocda-lib.

See README.md, dlang/README.md and gleam/README.md, and pipeline/
for building everything and running it over a directory of artefacts.

(assisted by Claude-Code)

1 files changed, 88 insertions, 0 deletions

diff --git a/gleam/flake.nix b/gleam/flake.nix
new file mode 100644
index 0000000..49cd3f1
--- /dev/null
+++ b/gleam/flake.nix
@@ -0,0 +1,88 @@
+{
+  description = "ocda-write: sample writers over the object-centric document abstraction, in Gleam";
+
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+
+  /* The library this is built on, as a source tree rather than a flake.
+     gleam.toml names it as the path dependency ./ocda-lib, which in a
+     working tree is a symlink to the library's gleam/ and is not carried
+     in git; the build is given the library from this input instead, and
+     nix-gleam puts it in place as a local package.
+
+     flake.lock pins the commit of the library this writer was built and
+     checked against. To build against a working copy of it instead:
+
+       nix build --override-input ocda-lib path:../../sisudoc-ocda-lib
+  */
+  inputs.ocda-lib = {
+    url = "git+git://git.sisudoc.org/tools/sisudoc-ocda-lib";
+    flake = false;
+  };
+
+  outputs = { self, nixpkgs, ocda-lib }:
+    let
+      system = "x86_64-linux"; # adjust for aarch64-linux / darwin
+      pkgs = import nixpkgs { inherit system; };
+
+      name = "ocda_write";
+
+      /* The builder, nix-gleam's, kept here in nix-gleam/ (see the README
+         there). It fetches each Hex dependency on its own, with the
+         checksum gleam has already written into manifest.toml, so there is
+         no hash in this file to keep up to date, and nothing a run of gleam
+         could make come out differently.
+      */
+      inherit (pkgs.callPackage ./nix-gleam { }) buildGleamApplication;
+
+      /* rebar3 with the port compiler already in it.
+
+         esqlite builds a NIF, and its rebar3 configuration asks for the
+         `pc` plugin, which rebar3 would otherwise fetch from Hex at build
+         time: impossible in a sandbox, and the build fails with
+         "missing_package pc". nixpkgs carries the plugin, so the answer is
+         a rebar3 that has it before it starts.
+      */
+      rebar3-pc = pkgs.beamPackages.rebar3WithPlugins {
+        plugins = [ pkgs.beamPackages.pc ];
+      };
+    in {
+      packages.${system} = {
+        default = self.packages.${system}.${name};
+
+        /* Shipped as an Erlang shipment rather than as an escript, because
+           sqlite on the BEAM is a NIF: esqlite compiles its own sqlite into
+           a shared object, and a NIF cannot be loaded from inside an
+           escript archive, the runtime needing a real file on disk. The
+           shipment keeps priv/ on disk, so it loads.
+        */
+        ${name} = buildGleamApplication {
+          pname = "ocda-write";
+          version = "2.0.0";
+          src = ./.;
+          localPackages = [ "${ocda-lib}/gleam" ];
+          rebar3Package = rebar3-pc;
+          nativeBuildInputs = [ pkgs.makeWrapper ];
+          preBuild = ''
+            export HOME=$TMPDIR
+          '';
+          installPhase = ''
+            runHook preInstall
+            mkdir -p $out/lib $out/bin
+            cp -r build/erlang-shipment $out/lib/${name}
+            makeWrapper $out/lib/${name}/entrypoint.sh $out/bin/ocda-write \
+              --add-flags run \
+              --prefix PATH : ${pkgs.lib.makeBinPath [ pkgs.beamPackages.erlang pkgs.coreutils ]}
+            runHook postInstall
+          '';
+          meta = {
+            description = "writes a document out from either serialisation of its abstraction";
+            mainProgram = "ocda-write";
+          };
+        };
+      };
+
+      devShells.${system}.default = pkgs.mkShell {
+        packages = [ pkgs.gleam pkgs.beamPackages.erlang pkgs.rebar3 pkgs.sqlite ];
+      };
+    };
+}