From 2b21aeae89f2775ac08c8ea3bded3dc1ccc2989d Mon Sep 17 00:00:00 2001
From: Aadhavan Srinivasan <aadhavan@twomorecents.org>
Date: Tue, 22 Jul 2025 09:56:41 -0400
Subject: [PATCH] Added implementation and tests for subscript and superscript;
 fixed nested list parsing

---
 src/MdToHTML.hs     | 34 +++++++++++++++++++++++++++++++---
 src/MdToHtmlTest.hs | 11 ++++++++++-
 2 files changed, 41 insertions(+), 4 deletions(-)

diff --git a/src/MdToHTML.hs b/src/MdToHTML.hs
index d7ec5f0..c9712a7 100644
--- a/src/MdToHTML.hs
+++ b/src/MdToHTML.hs
@@ -48,6 +48,8 @@ data MdToken
   | Figure MdToken URL (Maybe [CssClass])
   | Bold MdToken
   | Italic MdToken
+  | Superscript MdToken
+  | Subscript MdToken
   | Strikethrough MdToken
   | Unit String
   deriving (Eq)
@@ -72,6 +74,8 @@ instance Show MdToken where
   show (Figure txt url cssClasses) = "<figure><img src=\"" ++ getUrl url ++ "\" alt=\"" ++ show txt ++ "\"" ++ maybe "" (\classes -> " class=\"" ++ unwords classes ++ "\"") cssClasses ++ "/><figcaption aria-hidden=\"true\">" ++ show txt ++ "</figcaption></figure>"
   show (Bold token) = "<b>" ++ show token ++ "</b>"
   show (Italic token) = "<i>" ++ show token ++ "</i>"
+  show (Superscript token) = "<sup>" ++ show token ++ "</sup>"
+  show (Subscript token) = "<sub>" ++ show token ++ "</sub>"
   show (Strikethrough token) = "<s>" ++ show token ++ "</s>"
   show (Unit unit) = printf "%s" unit
 
@@ -201,6 +205,20 @@ parseItalic = parseItalicWith '*' <|> parseItalicWith '_'
       inside <- someTill parseLineToken (char delim)
       return (Italic (Line inside))
 
+-- Parse subscript
+parseSubscript :: Parser MdToken
+parseSubscript = do
+  char '~'
+  inside <- someTill parseLineToken (char '~')
+  return (Subscript (Line inside))
+
+-- Parse superscript
+parseSuperscript :: Parser MdToken
+parseSuperscript = do
+  char '^'
+  inside <- someTill parseLineToken (char '^')
+  return (Superscript (Line inside))
+
 -- Parse strikethrough text
 parseStrikethrough :: Parser MdToken
 parseStrikethrough = do
@@ -326,6 +344,8 @@ lineParsers =
     parseBold,
     parseItalic,
     parseStrikethrough,
+    parseSubscript,
+    parseSuperscript,
     parseLink,
     parseUnit
   ] -- A 'line' doesn't include a 'header'
@@ -338,6 +358,8 @@ lineParsersWithoutNewline =
     parseBold,
     parseItalic,
     parseStrikethrough,
+    parseSubscript,
+    parseSuperscript,
     parseLink,
     parseUnitExceptNewline
   ] -- A list line cannot contain newlines.
@@ -406,10 +428,16 @@ parseBlockquote = do
 -- Parse a nested list item.
 parseListNested :: Parser MdToken
 parseListNested = do
-  let firstCharParser = string (T.pack "    ") <|> string (T.pack "\t")
+  let firstCharParser = (<>) <$> (string (T.pack "  ") <|> string (T.pack "\t")) <*> (T.pack <$> many (char ' '))
   let restOfLineParser = manyTill anySingle (void (char '\n') <|> eof)
-  lines <- greedyParse1 (firstCharParser *> restOfLineParser)
-  let linesParsed = leftmostLongestParse (parseUnorderedList <|> parseOrderedList) (init $ unlines lines)
+  -- For the first line, I manually run firstCharParser and restOfLineParser. The
+  -- result of firstCharParser is saved. For every subsequent line, I parse exactly
+  -- the same string as firstCharParser.
+  firstLineSpaces <- firstCharParser
+  firstLine <- restOfLineParser
+  lines <- greedyParse (string firstLineSpaces *> restOfLineParser)
+  let allLines = firstLine : lines
+  let linesParsed = leftmostLongestParse (parseUnorderedList <|> parseOrderedList) (init $ unlines allLines)
   when (null (show linesParsed)) empty
   return linesParsed
 
diff --git a/src/MdToHtmlTest.hs b/src/MdToHtmlTest.hs
index 324afb0..5773ebe 100644
--- a/src/MdToHtmlTest.hs
+++ b/src/MdToHtmlTest.hs
@@ -31,7 +31,7 @@ boldTests =
 strikethroughTests =
   TestList
     [ check_equal "Should convert strikethrough" "<p><s>Hello</s></p>" (convert "~~Hello~~"),
-      check_equal "Should convert long sentence with tilde" "<p><s>The universe is ~7 days old</s>. The universe is 13 billion years old.</p>" (convert "~~The universe is ~7 days old~~. The universe is 13 billion years old.")
+      check_equal "Should convert long sentence with tilde" "<p><s>The universe is ~7 days old</s>. The universe is 13 billion years old.</p>" (convert "~~The universe is \\~7 days old~~. The universe is 13 billion years old.")
     ]
 
 linkTests =
@@ -74,6 +74,7 @@ unorderedListTests =
       check_equal "Paragraph before list" "<p>This is a list</p><ul><li>Item 1</li><li>Item 2</li></ul>" (convert "This is a list\n\n* Item 1\n* Item 2"),
       check_equal "Paragraph before list" "<h3>This is a list</h3><ul><li>Item 1</li><li>Item 2</li></ul>" (convert "### This is a list\n\n* Item 1\n* Item 2"),
       check_equal "Nested list then back" "<ul><li>Item 1</li><li>Item 2<ul><li>Item 3</li><li>Item 4</li></ul></li><li>Item 5</li></ul>" (convert "- Item 1\n- Item 2\n    - Item 3\n    - Item 4\n- Item 5"),
+      check_equal "Triply nested list" "<ul><li>Item 1</li><li>Item 2<ul><li>Item 3<ul><li>Item 4</li></ul></li></ul></li><li>Item 5</li></ul>" (convert "- Item 1\n- Item 2\n    - Item 3\n      - Item 4\n- Item 5"),
       check_equal "Blockquote in list" "<ul><li>Item 1</li><li><p>Item 2</p><blockquote><p>Quote</p></blockquote></li><li>Item 3</li></ul>" (convert "- Item 1\n- Item 2\n\n    > Quote\n\n- Item 3"),
       check_equal "Ordered list in unordered list" "<ul><li>Item 1</li><li>Item 2<ol><li>Item 1</li><li>Item 2</li></ol></li><li>Item 3</li></ul>" (convert "- Item 1\n- Item 2\n    1. Item 1\n    2. Item 2\n- Item 3")
     ]
@@ -120,6 +121,14 @@ horizontalRuleTests =
   TestList
     [check_equal "Horizontal Rule" "<p>a</p><hr><p>b</p>" (convert "a\n\n---\n\nb")]
 
+subscriptTests =
+  TestList
+    [check_equal "Should convert subscript" "A<sub>b</sub>" (convert "A~b~")]
+
+superscriptTests =
+  TestList
+    [check_equal "Should convert superscript" "A<sup>b</sup>" (convert "A^b^")]
+
 tableTests =
   TestList
     [ check_equal