Trying something to get rid of so much try/choice.

src/Lexer.hs

@@ -7,7 +7,7 @@ module Lexer (
 )
 where
 
-import Control.Monad (mfilter, void)
+import Control.Monad (guard, mfilter, void)
 import Data.Char (ord, toLower)
 import Data.Functor (($>))
 import Data.Text (Text, intercalate)
@@ -87,39 +87,39 @@ lexText = go
         rest <- go
         pure (toks <> rest)
 
-{- FOURMOLU_DISABLE -}
-  topLevel =
-    -- backtracking here so we always have a chance to try "other", the "catch-all-leave-to-parser-to-deal-with" choice
-    -- TODO: is this desirable? do we throw lexer error at all?
-    try
-      ( choice
+  topLevel = do
+    -- check for start-of-line markup first
+    lineStart <-
+      optionMaybe $
+        choice
+          [ try expression
+          , try birdTrack
+          , headers
+          ]
+
+    case lineStart of
+      Just toks -> pure toks
+      Nothing ->
+        choice $
           -- Sorted in
           -- - longest to shortest parse path
           -- - highest frequency to lowest frequency (for performance?)
           -- - more exact to more freeform (the latter can be the former but not vice versa)
           [ spaceToken
           , newlineToken
-
           , try module_
           , quotes
-          , try expression
-          , birdTrack
-
-            -- starts with "\"
-          , try mathMultiline
+          , -- starts with "\"
+            try mathMultiline
           , try mathInline
           , escape
-
-          , headers
           , labeledLink
           , link
           , anchor
           , numericEntity
           , textElement
+          , other
           ]
-      )
-      <|> other
-{- FOURMOLU_ENABLE -}
 
 -- Tokens
 
@@ -174,11 +174,7 @@ eol = void "\n" <|> void "\r\n" <|> Parsec.eof
 
 -- Start of line // start of file
 sol :: Parser ()
-sol = do
-  p <- getPosition
-  if sourceColumn p == 1
-    then pure ()
-    else fail "Not at start of line/document"
+sol = getPosition >>= guard . (== 1) . sourceColumn
 
 header1 :: Lexer
 header1 = delimitedNoTrailing "= " eol (Header One)