Minor improvements for new combinators

In particular, if input has no newline at the end, we need to treat it
specially, because otherwise we will get confusing “incorrect
indentation” message.

Text/Megaparsec/Lexer.hs

@@ -48,7 +48,7 @@ where
 import Control.Applicative ((<|>), some, optional)
 import Control.Monad (void)
 import Data.Char (readLitChar)
-import Data.Maybe (listToMaybe, fromMaybe)
+import Data.Maybe (listToMaybe, fromMaybe, isJust)
 import Prelude hiding (negate)
 import qualified Prelude
 
@@ -194,8 +194,8 @@ indentGuard spc p = do
   spc
   lvl <- indentLevel
   if p lvl
-  then return lvl
-  else fail ii
+    then return lvl
+    else fail ii
 
 -- | Parse non-indented construction. This ensures that there is no
 -- indentation before actual data. Useful, for example, as a wrapper for
@@ -232,8 +232,8 @@ data IndentOpt m a b
 -- information.
 --
 -- Tokens /must not/ consume newlines after them. On the other hand, the
--- first argument of this function /should/ consume newlines among other
--- white space characters.
+-- first argument of this function /must/ consume newlines among other white
+-- space characters.
 --
 -- @since 4.3.0
 
@@ -247,12 +247,12 @@ indentBlock sc r = do
   case a of
     IndentNone x -> return x
     IndentMany indent f p -> do
-      mlvl <- lookAhead . optional . try $ C.eol *> indentGuard sc (> ref)
+      mlvl <- optional . try $ C.eol *> indentGuard sc (> ref)
       case mlvl of
         Nothing  -> sc *> f []
         Just lvl -> indentedItems ref (fromMaybe lvl indent) sc p >>= f
     IndentSome indent f p -> do
-      lvl <- lookAhead . try $ C.eol *> indentGuard sc (> ref)
+      lvl <- C.eol *> indentGuard sc (> ref)
       indentedItems ref (fromMaybe lvl indent) sc p >>= f
 
 -- | Grab indented items. This is a helper for 'indentBlock', it's not a
@@ -270,7 +270,11 @@ indentedItems ref lvl sc p = go
     re pos
       | pos <= ref = return []
       | pos == lvl = (:) <$> p <*> go
-      | otherwise  = fail ii
+      | otherwise  = do
+          done <- isJust <$> optional eof
+          if done
+            then return []
+            else fail ii
 
 ii :: String
 ii = "incorrect indentation"

tests/Lexer.hs

@@ -94,12 +94,15 @@ mkSymbol :: Gen String
 mkSymbol = (++) <$> symbolName <*> whiteChars
 
 mkIndent :: String -> Int -> Gen String
-mkIndent x n = concat <$> sequence [spc, sym, tra, eol]
+mkIndent x n = (++) <$> mkIndent' x n <*> eol
+  where eol = frequency [(5, return "\n"), (1, listOf1 (return '\n'))]
+
+mkIndent' :: String -> Int -> Gen String
+mkIndent' x n = concat <$> sequence [spc, sym, tra]
   where spc = frequency [(5, vectorOf n itm), (1, listOf itm)]
         tra = listOf itm
         itm = elements " \t"
         sym = return x
-        eol = frequency [(5, return "\n"), (1, listOf1 (return '\n'))]
 
 whiteChars :: Gen String
 whiteChars = listOf (elements "\t\n ")
@@ -207,7 +210,7 @@ prop_indentBlock mn' = forAll mkBlock $ \(l0,l1,l2,l3,l4) ->
           l1 <- mkIndent sblb ib
           l2 <- mkIndent sblc (ib + 2)
           l3 <- mkIndent sblb ib
-          l4 <- mkIndent sblc (ib + 2)
+          l4 <- mkIndent' sblc (ib + 2)
           return (l0,l1,l2,l3,l4)
         p = lvla
         lvla = indentBlock sc $ IndentMany mn      (l sbla) lvlb <$ b sbla
@@ -219,7 +222,7 @@ prop_indentBlock mn' = forAll mkBlock $ \(l0,l1,l2,l3,l4) ->
         ib   = fromMaybe 2 mn
 
 prop_indentMany :: Property
-prop_indentMany = forAll (mkIndent "xxx" 0) (checkParser p r)
+prop_indentMany = forAll (mkIndent sbla 0) (checkParser p r)
   where r = Right (sbla, [])
         p = lvla
         lvla = indentBlock sc $ IndentMany Nothing (l sbla) lvlb <$ b sbla