From 0bbbdbcdb6282c7f63bdc9617455ec088fe7779b Mon Sep 17 00:00:00 2001
From: John MacFarlane <jgm@berkeley.edu>
Date: Mon, 11 Aug 2025 17:35:56 -0700
Subject: LaTeX reader/writer: change handling of math environments.

Certain environments in LaTeX will trigger math mode and can't
occur within math mode: e.g., `align` or `equation`.  Previously
we "downshifted" these, parsing an `align` environment as a
Math element with `aligned`, and an `equation` environment as a
regular display math element.  With this shift, we put these in
Math inlines but retain the original environments.

This works because:

1) texmath handles these environments just fine,
2) and so does MathJax;
3) when writing LaTeX we detect these environments in Math elements
   and emit them verbatim instead of putting them in `$..$` or
   `$$..$$`.

Closes #9711. Closes #9296.
---
 src/Text/Pandoc/Readers/LaTeX/Math.hs | 29 +++++++++++++++--------------
 src/Text/Pandoc/Writers/LaTeX.hs      | 24 ++++++++++++++++++++++++
 2 files changed, 39 insertions(+), 14 deletions(-)

(limited to 'src')

diff --git a/src/Text/Pandoc/Readers/LaTeX/Math.hs b/src/Text/Pandoc/Readers/LaTeX/Math.hs
index 9f82618e0..1f4dc4f8c 100644
--- a/src/Text/Pandoc/Readers/LaTeX/Math.hs
+++ b/src/Text/Pandoc/Readers/LaTeX/Math.hs
@@ -98,20 +98,21 @@ inlineEnvironments :: PandocMonad m => M.Map Text (LP m Inlines)
 inlineEnvironments = M.fromList [
     ("displaymath", mathEnvWith id Nothing "displaymath")
   , ("math", math <$> mathEnv "math")
-  , ("equation", mathEnvWith id Nothing "equation")
-  , ("equation*", mathEnvWith id Nothing "equation*")
-  , ("gather", mathEnvWith id (Just "gathered") "gather")
-  , ("gather*", mathEnvWith id (Just "gathered") "gather*")
-  , ("multline", mathEnvWith id (Just "gathered") "multline")
-  , ("multline*", mathEnvWith id (Just "gathered") "multline*")
-  , ("eqnarray", mathEnvWith id (Just "aligned") "eqnarray")
-  , ("eqnarray*", mathEnvWith id (Just "aligned") "eqnarray*")
-  , ("align", mathEnvWith id (Just "aligned") "align")
-  , ("align*", mathEnvWith id (Just "aligned") "align*")
-  , ("alignat", mathEnvWith id (Just "aligned") "alignat")
-  , ("alignat*", mathEnvWith id (Just "aligned") "alignat*")
-  , ("flalign", mathEnvWith id (Just "aligned") "flalign")
-  , ("flalign*", mathEnvWith id (Just "aligned") "flalign*")
+  , ("equation", mathEnvWith id (Just "equation") "equation")
+  , ("equation*", mathEnvWith id (Just "equation*") "equation*")
+  , ("gather", mathEnvWith id (Just "gather") "gather")
+  , ("gather*", mathEnvWith id (Just "gather*") "gather*")
+  , ("multline", mathEnvWith id (Just "multline") "multline")
+  , ("multline*", mathEnvWith id (Just "multline*") "multline*")
+  , ("eqnarray", mathEnvWith id (Just "eqnarray") "eqnarray")
+  , ("eqnarray*", mathEnvWith id (Just "eqnarray*") "eqnarray*")
+  , ("align", mathEnvWith id (Just "align") "align")
+  , ("align*", mathEnvWith id (Just "align*") "align*")
+  , ("alignat", mathEnvWith id (Just "alignat") "alignat")
+  , ("alignat*", mathEnvWith id (Just "alignat*") "alignat*")
+  , ("flalign", mathEnvWith id (Just "flalign") "flalign")
+  , ("flalign*", mathEnvWith id (Just "flalign*") "flalign*")
+  -- the following are not yet handled by texmath, so we use substitutes:
   , ("dmath", mathEnvWith id Nothing "dmath")
   , ("dmath*", mathEnvWith id Nothing "dmath*")
   , ("dgroup", mathEnvWith id (Just "aligned") "dgroup")
diff --git a/src/Text/Pandoc/Writers/LaTeX.hs b/src/Text/Pandoc/Writers/LaTeX.hs
index b3d545a6b..9f5f6ed69 100644
--- a/src/Text/Pandoc/Writers/LaTeX.hs
+++ b/src/Text/Pandoc/Writers/LaTeX.hs
@@ -1021,6 +1021,10 @@ inlineToLaTeX (Quoted qt lst) = do
 inlineToLaTeX (Str str) = do
   setEmptyLine False
   liftM literal $ stringToLaTeX TextString str
+inlineToLaTeX (Math _ str)
+  | isMathEnv str -- see #9711
+  = do setEmptyLine False
+       pure $ literal str
 inlineToLaTeX (Math InlineMath str) = do
   setEmptyLine False
   inSoul <- gets stInSoulCommand
@@ -1310,3 +1314,23 @@ ldfLanguages =
   , "galician"
   , "slovene"
   ]
+
+isMathEnv :: Text -> Bool
+isMathEnv t =
+  case T.stripPrefix "\\begin{" t of
+    Nothing -> False
+    Just t' -> T.takeWhile (/= '}') t' `elem`
+      [ "align", "align*"
+      , "flalign", "flalign*"
+      , "alignat", "alignat*"
+      , "dmath", "dmath*"
+      , "dgroup", "dgroup*"
+      , "darray", "darray*"
+      , "gather", "gather*"
+      , "multline", "multline*"
+      , "split"
+      , "subequations"
+      , "equation", "equation*"
+      , "eqnarray"
+      , "displaymath"
+      ]
-- 
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