From 6f99abe9cf750d0e5339bb7ac50efcb94f05539e Mon Sep 17 00:00:00 2001
From: JJ <git@toki.la>
Date: Thu, 10 Jul 2025 16:16:28 -0700
Subject: [PATCH 1/2] Add support for forest syntax trees

---
 _extensions/diagram/diagram.lua | 67 +++++++++++++++++++++++++++++++++
 1 file changed, 67 insertions(+)

diff --git a/_extensions/diagram/diagram.lua b/_extensions/diagram/diagram.lua
index 37f4a62..68fb001 100644
--- a/_extensions/diagram/diagram.lua
+++ b/_extensions/diagram/diagram.lua
@@ -291,6 +291,72 @@ local cetz = {
   end,
 }
 
+--- LaTeX template used to compile forest syntax trees.
+local forest_template = pandoc.template.compile [[
+\documentclass{standalone}
+\usepackage{forest}
+$for(header-includes)$
+$it$
+$endfor$
+$additional-packages$
+\begin{document}
+$body$
+\end{document}
+]]
+
+--- The forest engine is adopted entirely from the TikZ template.
+local forest = {
+  line_comment_start = '%%',
+
+  mime_types = {
+    ['application/pdf'] = true,
+  },
+
+  --- Compile LaTeX with forest syntax trees to an image
+  compile = function (self, src, user_opts)
+    return with_temporary_directory("forest", function (tmpdir)
+      return with_working_directory(tmpdir, function ()
+        -- Define file names:
+        local file_template = "%s/forest-image.%s"
+        local forest_file = file_template:format(tmpdir, "tex")
+        local pdf_file = file_template:format(tmpdir, "pdf")
+
+        -- Treat string values as raw LaTeX
+        local meta = {
+          ['header-includes'] = user_opts['header-includes'],
+          ['additional-packages'] = {pandoc.RawInline(
+            'latex',
+            stringify(user_opts['additional-packages'] or '')
+          )},
+        }
+        local tex_code = pandoc.write(
+          pandoc.Pandoc({pandoc.RawBlock('latex', src)}, meta),
+          'latex',
+          {template = forest_template}
+        )
+        write_file(forest_file, tex_code)
+
+        -- Execute the LaTeX compiler:
+        local success, result = pcall(
+          pipe,
+          self.execpath or 'pdflatex',
+          { '-interaction=nonstopmode', '-output-directory', tmpdir, forest_file },
+          ''
+        )
+        if not success then
+          warn(string.format(
+                 "The call\n%s\nfailed with error code %s. Output:\n%s",
+                 result.command,
+                 result.error_code,
+                 result.output
+          ))
+        end
+        return read_file(pdf_file), 'application/pdf'
+      end)
+    end)
+  end
+}
+
 local default_engines = {
   asymptote = asymptote,
   dot       = graphviz,
@@ -298,6 +364,7 @@ local default_engines = {
   plantuml  = plantuml,
   tikz      = tikz,
   cetz      = cetz,
+  forest    = forest,
 }
 
 --

From e809919506d52b03350e5aa79aa3993b34bd96f7 Mon Sep 17 00:00:00 2001
From: JJ <git@toki.la>
Date: Sat, 12 Jul 2025 16:46:56 -0700
Subject: [PATCH 2/2] Remove need for explicit \begin{forest}\end{forest}

---
 _extensions/diagram/diagram.lua | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/_extensions/diagram/diagram.lua b/_extensions/diagram/diagram.lua
index 68fb001..8cbfd07 100644
--- a/_extensions/diagram/diagram.lua
+++ b/_extensions/diagram/diagram.lua
@@ -292,19 +292,22 @@ local cetz = {
 }
 
 --- LaTeX template used to compile forest syntax trees.
+--- We provide the \begin{forest} item, too, so that only the [] syntax is needed.
 local forest_template = pandoc.template.compile [[
 \documentclass{standalone}
-\usepackage{forest}
+\usepackage[linguistics]{forest}
 $for(header-includes)$
 $it$
 $endfor$
 $additional-packages$
 \begin{document}
+\begin{forest}
 $body$
+\end{forest}
 \end{document}
 ]]
 
---- The forest engine is adopted entirely from the TikZ template.
+--- The forest engine is adopted from the TikZ template.
 local forest = {
   line_comment_start = '%%',