Report error contexts.

Data/Attoparsec/ByteString.hs

@@ -92,6 +92,7 @@ module Data.Attoparsec.ByteString
     ) where
 
 import Data.Attoparsec.Combinator
+import Data.List (intercalate)
 import qualified Data.Attoparsec.ByteString.Internal as I
 import qualified Data.Attoparsec.Internal as I
 import qualified Data.ByteString as B
@@ -218,6 +219,7 @@ maybeResult _            = Nothing
 -- | Convert a 'Result' value to an 'Either' value. A 'T.Partial'
 -- result is treated as failure.
 eitherResult :: Result r -> Either String r
-eitherResult (T.Done _ r)     = Right r
-eitherResult (T.Fail _ _ msg) = Left msg
-eitherResult _                = Left "Result: incomplete input"
+eitherResult (T.Done _ r)        = Right r
+eitherResult (T.Fail _ [] msg)   = Left msg
+eitherResult (T.Fail _ ctxs msg) = Left (intercalate " > " ctxs ++ ": " ++ msg)
+eitherResult _                   = Left "Result: incomplete input"

Data/Attoparsec/ByteString/Internal.hs

@@ -74,6 +74,7 @@ import Data.Attoparsec.Internal
 import Data.Attoparsec.Internal.Fhthagn (inlinePerformIO)
 import Data.Attoparsec.Internal.Types hiding (Parser, Failure, Success)
 import Data.ByteString (ByteString)
+import Data.List (intercalate)
 import Data.Word (Word8)
 import Foreign.ForeignPtr (withForeignPtr)
 import Foreign.Ptr (castPtr, minusPtr, plusPtr)
@@ -416,9 +417,10 @@ parse m s = T.runParser m (buffer s) (Pos 0) Incomplete failK successK
 -- @
 parseOnly :: Parser a -> ByteString -> Either String a
 parseOnly m s = case T.runParser m (buffer s) (Pos 0) Complete failK successK of
-                  Fail _ _ err -> Left err
-                  Done _ a     -> Right a
-                  _            -> error "parseOnly: impossible error!"
+                  Fail _ [] err   -> Left err
+                  Fail _ ctxs err -> Left (intercalate " > " ctxs ++ ": " ++ err)
+                  Done _ a        -> Right a
+                  _               -> error "parseOnly: impossible error!"
 {-# INLINE parseOnly #-}
 
 get :: Parser ByteString

Data/Attoparsec/ByteString/Lazy.hs

@@ -36,6 +36,7 @@ module Data.Attoparsec.ByteString.Lazy
 
 import Control.DeepSeq (NFData(rnf))
 import Data.ByteString.Lazy.Internal (ByteString(..), chunk)
+import Data.List (intercalate)
 import qualified Data.ByteString as B
 import qualified Data.Attoparsec.ByteString as A
 import qualified Data.Attoparsec.Internal.Types as T
@@ -99,5 +100,6 @@ maybeResult _          = Nothing
 
 -- | Convert a 'Result' value to an 'Either' value.
 eitherResult :: Result r -> Either String r
-eitherResult (Done _ r)     = Right r
-eitherResult (Fail _ _ msg) = Left msg
+eitherResult (Done _ r)        = Right r
+eitherResult (Fail _ [] msg)   = Left msg
+eitherResult (Fail _ ctxs msg) = Left (intercalate " > " ctxs ++ ": " ++ msg)

Data/Attoparsec/Text.hs

@@ -131,6 +131,7 @@ import Data.Attoparsec.Text.Internal (Parser, Result, parse, takeWhile1)
 import Data.Bits (Bits, (.|.), shiftL)
 import Data.Char (isAlpha, isDigit, isSpace, ord)
 import Data.Int (Int8, Int16, Int32, Int64)
+import Data.List (intercalate)
 import Data.Text (Text)
 import Data.Word (Word8, Word16, Word32, Word64, Word)
 import qualified Data.Attoparsec.Internal as I
@@ -257,9 +258,10 @@ maybeResult _            = Nothing
 -- | Convert a 'Result' value to an 'Either' value. A 'Partial' result
 -- is treated as failure.
 eitherResult :: Result r -> Either String r
-eitherResult (T.Done _ r)     = Right r
-eitherResult (T.Fail _ _ msg) = Left msg
-eitherResult _                = Left "Result: incomplete input"
+eitherResult (T.Done _ r)        = Right r
+eitherResult (T.Fail _ [] msg)   = Left msg
+eitherResult (T.Fail _ ctxs msg) = Left (intercalate " > " ctxs ++ ": " ++ msg)
+eitherResult _                   = Left "Result: incomplete input"
 
 -- | A predicate that matches either a carriage return @\'\\r\'@ or
 -- newline @\'\\n\'@ character.

Data/Attoparsec/Text/Internal.hs

@@ -73,6 +73,7 @@ import Data.Attoparsec.Internal.Types hiding (Parser, Failure, Success)
 import qualified Data.Attoparsec.Text.Buffer as Buf
 import Data.Attoparsec.Text.Buffer (Buffer, buffer)
 import Data.Char (chr, ord)
+import Data.List (intercalate)
 import Data.String (IsString(..))
 import Data.Text.Internal (Text(..))
 import Prelude hiding (getChar, length, succ, take, takeWhile)
@@ -429,9 +430,10 @@ parse m s = runParser m (buffer s) 0 Incomplete failK successK
 -- @
 parseOnly :: Parser a -> Text -> Either String a
 parseOnly m s = case runParser m (buffer s) 0 Complete failK successK of
-                  Fail _ _ err -> Left err
-                  Done _ a     -> Right a
-                  _            -> error "parseOnly: impossible error!"
+                  Fail _ [] err   -> Left err
+                  Fail _ ctxs err -> Left (intercalate " > " ctxs ++ ": " ++ err)
+                  Done _ a        -> Right a
+                  _               -> error "parseOnly: impossible error!"
 {-# INLINE parseOnly #-}
 
 get :: Parser Text

Data/Attoparsec/Text/Lazy.hs

@@ -36,6 +36,7 @@ module Data.Attoparsec.Text.Lazy
     ) where
 
 import Control.DeepSeq (NFData(rnf))
+import Data.List (intercalate)
 import Data.Text.Lazy.Internal (Text(..), chunk)
 import qualified Data.Attoparsec.Internal.Types as T
 import qualified Data.Attoparsec.Text as A
@@ -94,5 +95,6 @@ maybeResult _          = Nothing
 
 -- | Convert a 'Result' value to an 'Either' value.
 eitherResult :: Result r -> Either String r
-eitherResult (Done _ r)     = Right r
-eitherResult (Fail _ _ msg) = Left msg
+eitherResult (Done _ r)        = Right r
+eitherResult (Fail _ [] msg)   = Left msg
+eitherResult (Fail _ ctxs msg) = Left (intercalate " > " ctxs ++ ": " ++ msg)