Added expression eval; adjusted birdtrack, added sol combinator.

src/Lexer.hs

@@ -42,6 +42,7 @@ data Token
   | Escape
   | EmphasisOpen
   | EmphasisClose
+  | Expression
   | Header Level
   | MonospaceOpen
   | MonospaceClose
@@ -101,6 +102,7 @@ lexText = go
 
           , try module_
           , quotes
+          , try expression
           , birdTrack
 
             -- starts with "\"
@@ -158,16 +160,26 @@ delimited openP closeP openTok closeTok = asList <$> delimitedAsTuple (openTok <
   asList (a, tok, b) = [a, tok, b]
 
 delimitedNoTrailing :: Parser open -> Parser close -> Token -> Parser [LocatedToken]
-delimitedNoTrailing openP closeP openTok = asList <$> delimitedAsTuple (openTok <$ openP) (void closeP)
+delimitedNoTrailing openP closeP openTok =
+  asList <$> delimitedAsTuple (openTok <$ openP) (void closeP)
  where
   asList (a, tok, _) = [a, tok]
 
 delimitedSymmetric :: Parser a -> Token -> Token -> Parser [LocatedToken]
 delimitedSymmetric s = delimited s s
 
+--- End of line // end of file
 eol :: Parser ()
 eol = void "\n" <|> void "\r\n" <|> Parsec.eof
 
+-- Start of line // start of file
+sol :: Parser ()
+sol = do
+  p <- getPosition
+  if sourceLine p == 1 && sourceColumn p == 1
+    then pure ()
+    else fail "Not at start of line/document"
+
 header1 :: Lexer
 header1 = delimitedNoTrailing "= " eol (Header One)
 
@@ -256,7 +268,13 @@ mathInline = delimited "\\(" "\\)" MathInlineOpen MathInlineClose
 
 -- TODO: make sure this starts at column 0?
 birdTrack :: Lexer
-birdTrack = delimitedNoTrailing ">> " eol BirdTrack
+birdTrack = delimitedNoTrailing (sol <* "> ") eol BirdTrack
+
+-- TODO: also match following lines iff:
+-- they start with alphanum
+-- they're not empty
+expression :: Lexer
+expression = delimitedNoTrailing (sol <* ">>> ") eol Expression
 
 escape :: Lexer
 escape = delimitedNoTrailing "\\" eol Escape