LaTeX to HTML: 'align' environment is incorrectly converted to 'aligned'

[SunderB]

Explain the problem.
I'm attempting to convert some LaTeX files to HTML (using MathJax for maths rendering) and am trying to get AMS equation numbering to work. I noticed in the generated HTML files that pandoc had converted align environments to aligned, which seems to break the AMS numbering. In researching this issue I also found a Stack Exchange post about it: https://tex.stackexchange.com/questions/561133/how-to-prevent-pandoc-from-converting-align-environment-to-aligned-environment.

Example:
Taken from https://tex.stackexchange.com/questions/561133/how-to-prevent-pandoc-from-converting-align-environment-to-aligned-environment. Try it yourself on the pandoc website.

Input:

\documentclass{article}
\usepackage{amsmath}
\title{Demo}
\begin{document}

\begin{align}
1 + 0 & = 1, \label{eq1} \\
1 + 1 & = 2. \label{eq2}
\end{align}

Equation \( \eqref{eq1} \) and \( \eqref{eq2} \) describe eternal
truths.

\end{document}

Output:

<p><span class="math display">\[\begin{aligned}
1 + 0 &amp; = 1, \label{eq1} \\
1 + 1 &amp; = 2. \label{eq2}\end{aligned}\]</span></p>
<p>Equation <span class="math inline">\(\eqref{eq1}\)</span> and <span
class="math inline">\(\eqref{eq2}\)</span> describe eternal truths.</p>

Expected output:

<p><span class="math display">\[\begin{align}
1 + 0 &amp; = 1, \label{eq1} \\
1 + 1 &amp; = 2. \label{eq2}\end{align}\]</span></p>
<p>Equation <span class="math inline">\(\eqref{eq1}\)</span> and <span
class="math inline">\(\eqref{eq2}\)</span> describe eternal truths.</p>

Pandoc version?
pandoc 2.17.1.1 on Windows 10

[jgm]

Actually your expected output is INVALID LaTeX, even though MathJax seems to accept it:

\[\begin{align}
1 + 0 & = 1, \label{eq1} \\
1 + 1 & = 2. \label{eq2}\end{align}\]

You can't put the align environment inside math mode (\[..\]). That's why pandoc does this transformation when parsing math.

[jgm]

Hard to know how to handle this. We could change the LaTeX reader so that if the raw_tex extension is enabled, we parse align environments as raw tex rather than math; the HTML writer already knows how to render these for MathJax. But this has the drawback that raw_tex will cause the LaTeX reader to parse certain other things in a way that will make them invisible in HTML output (e.g., lettrine, hbox, mbox, input).

Maybe we need a new extension that tells the LaTeX reader to parse just math environments as raw LaTeX. But this is a bit unprincipled; it's an extension solely motivated by one particular conversion (latex -> html with mathjax).

[jgm]

Here's the diff for parsing these as raw latex when raw_tex is enabled:

% git diff
diff --git a/src/Text/Pandoc/Readers/LaTeX/Math.hs b/src/Text/Pandoc/Readers/LaTeX/Math.hs
index 9f3d6fe53..bdb8be1b6 100644
--- a/src/Text/Pandoc/Readers/LaTeX/Math.hs
+++ b/src/Text/Pandoc/Readers/LaTeX/Math.hs
@@ -25,6 +25,8 @@ import Control.Applicative ((<|>), optional)
 import Control.Monad (guard, mzero)
 import qualified Data.Map as M
 import Data.Text (Text)
+import Text.Pandoc.Extensions (extensionEnabled, Extension(Ext_raw_tex))
+import Text.Pandoc.Options (ReaderOptions(readerExtensions))
 
 dollarsMath :: PandocMonad m => LP m Inlines
 dollarsMath = do
@@ -76,9 +78,15 @@ mathEnv name = do
 
 inlineEnvironment :: PandocMonad m => LP m Inlines
 inlineEnvironment = try $ do
-  controlSeq "begin"
-  name <- untokenize <$> braced
-  M.findWithDefault mzero name inlineEnvironments
+  (name, rawstart) <- withRaw (controlSeq "begin" *> braced)
+  case M.lookup (untokenize name) inlineEnvironments of
+    Nothing -> mzero
+    Just parser -> do
+      parseRaw <- extensionEnabled Ext_raw_tex <$> getOption readerExtensions
+      if parseRaw
+         then rawInline "latex" . untokenize . (rawstart ++) . snd
+                <$> withRaw parser
+         else parser
 
 inlineEnvironments :: PandocMonad m => M.Map Text (LP m Inlines)

[jgm]

Maybe a good solution would be a Lua filter that matches Math elements beginning with \begin{aligned} and changes them to RawInline "latex" elements with aligned changed to align. A bit kludgy but not too hard.

[jgm]

Here you go!

% cat align.lua 
function Math(el)
  if el.text:match("\\begin{aligned}") then
    local raw = el.text:gsub("{aligned}","{align}")
    return pandoc.RawInline("latex", raw)
  end
end

% pandoc -L align.lua -f latex -t html --mathjax
 
\begin{align}
1 + 0 & = 1, \label{eq1} \\
1 + 1 & = 2. \label{eq2}
\end{align}
^D
<p><span class="math display">\[\begin{align}
1 + 0 &amp; = 1, \label{eq1} \\
1 + 1 &amp; = 2. \label{eq2}\end{align}\]</span></p>

[tarleb]

This appears to be a duplicate of #4104; thus closing.