Remove ‘MonadThrow’ instance, add ‘Dec’

‘Dec’ stands for “default error component”. We need this because
‘String’ looses information and thus we cannot write good enough tests
with it.

Text/Megaparsec.hs

@@ -132,6 +132,7 @@ module Text.Megaparsec
     -- * Error messages
   , MessageItem (..)
   , ErrorComponent (..)
+  , Dec (..)
   , ParseError (..)
   , ShowToken (..)
   , ShowErrorComponent (..)

Text/Megaparsec/ByteString.hs

@@ -12,6 +12,7 @@
 
 module Text.Megaparsec.ByteString (Parser) where
 
+import Text.Megaparsec.Error (Dec)
 import Text.Megaparsec.Prim
 import qualified Data.ByteString as B
 
@@ -20,4 +21,4 @@ import qualified Data.ByteString as B
 -- @Parser@ type and easily change it by importing different “type
 -- modules”. This one is for strict byte-strings.
 
-type Parser = Parsec String B.ByteString
+type Parser = Parsec Dec B.ByteString

Text/Megaparsec/ByteString/Lazy.hs

@@ -12,6 +12,7 @@
 
 module Text.Megaparsec.ByteString.Lazy (Parser) where
 
+import Text.Megaparsec.Error (Dec)
 import Text.Megaparsec.Prim
 import qualified Data.ByteString.Lazy as B
 
@@ -20,4 +21,4 @@ import qualified Data.ByteString.Lazy as B
 -- @Parser@ type and easily change it by importing different “type
 -- modules”. This one is for lazy byte-strings.
 
-type Parser = Parsec String B.ByteString
+type Parser = Parsec Dec B.ByteString

Text/Megaparsec/Error.hs

@@ -20,6 +20,7 @@
 module Text.Megaparsec.Error
   ( MessageItem (..)
   , ErrorComponent (..)
+  , Dec (..)
   , ParseError (..)
   , ShowToken (..)
   , ShowErrorComponent (..)
@@ -69,11 +70,6 @@ class Ord e => ErrorComponent e where
 
   representFail :: String -> e
 
-  -- | Represent exception thrown in parser monad. (The monad implements
-  -- 'Control.Monad.Catch.MonadThrow').
-
-  representException :: Exception e' => e' -> e
-
   -- | Represent information about incorrect indentation.
 
   representIndentation
@@ -81,12 +77,19 @@ class Ord e => ErrorComponent e where
     -> Pos             -- ^ Expected indentation level
     -> e
 
-instance ErrorComponent String where
-  representFail        = id
-  representException   = ("exception: " ++) . show
-  representIndentation actual expected =
-    "incorrect indentation level (got " ++ show (unPos actual) ++
-    ", but (at least) " ++ show (unPos expected) ++ " is expected"
+-- | “Default error component”. This in our instance of 'ErrorComponent'
+-- provided out-of-box.
+--
+-- @since 5.0.0
+
+data Dec
+  = DecFail String         -- ^ 'fail' has been used in parser monad
+  | DecIndentation Pos Pos -- ^ Incorrect indentation error
+  deriving (Show, Eq, Ord, Data, Typeable)
+
+instance ErrorComponent Dec where
+  representFail        = DecFail
+  representIndentation = DecIndentation
 
 -- | The data type @ParseError@ represents parse errors. It provides the
 -- stack of source positions, set of expected and unexpected tokens as well
@@ -196,8 +199,11 @@ instance (Ord t, ShowToken t) => ShowErrorComponent (MessageItem t) where
   showErrorComponent (Label    label) = NE.toList label
   showErrorComponent EndOfInput       = "end of input"
 
-instance ShowErrorComponent String where
-  showErrorComponent = id
+instance ShowErrorComponent Dec where
+  showErrorComponent (DecFail msg) = msg
+  showErrorComponent (DecIndentation actual expected) =
+    "incorrect indentation level (got " ++ show (unPos actual) ++
+    ", but (at least) " ++ show (unPos expected) ++ " is expected"
 
 -- | Pretty-print 'ParseError'. Note that rendered 'String' always ends with
 -- a newline.

Text/Megaparsec/Prim.hs

@@ -54,7 +54,6 @@ module Text.Megaparsec.Prim
 where
 
 import Control.Monad
-import Control.Monad.Catch (Exception, MonadThrow (..))
 import Control.Monad.Cont.Class
 import Control.Monad.Error.Class
 import Control.Monad.Identity
@@ -394,15 +393,6 @@ pFail msg = ParsecT $ \s@(State _ pos _) _ _ _ eerr ->
   where d = E.singleton (representFail msg)
 {-# INLINE pFail #-}
 
-instance (ErrorComponent e, Stream s)
-    => MonadThrow (ParsecT e s m) where
-  throwM = pThrowM
-
-pThrowM :: (Exception e', ErrorComponent e) => e' -> ParsecT e s m a
-pThrowM e = ParsecT $ \s@(State _ pos _) _ _ _ eerr ->
-  eerr (ParseError pos E.empty E.empty d) s
-  where d = E.singleton (representException e)
-
 -- | Low-level creation of the 'ParsecT' type.
 
 mkPT :: Monad m => (State s -> m (Reply e s a)) -> ParsecT e s m a

Text/Megaparsec/String.hs

@@ -12,6 +12,7 @@
 
 module Text.Megaparsec.String (Parser) where
 
+import Text.Megaparsec.Error (Dec)
 import Text.Megaparsec.Prim
 
 -- | Different modules corresponding to various types of streams (@String@,
@@ -19,4 +20,4 @@ import Text.Megaparsec.Prim
 -- @Parser@ type and easily change it by importing different “type
 -- modules”. This one is for strings.
 
-type Parser = Parsec String String
+type Parser = Parsec Dec String

Text/Megaparsec/Text.hs

@@ -12,6 +12,7 @@
 
 module Text.Megaparsec.Text (Parser) where
 
+import Text.Megaparsec.Error (Dec)
 import Text.Megaparsec.Prim
 import qualified Data.Text as T
 
@@ -20,4 +21,4 @@ import qualified Data.Text as T
 -- @Parser@ type and easily change it by importing different “type
 -- modules”. This one is for strict text.
 
-type Parser = Parsec String T.Text
+type Parser = Parsec Dec T.Text

Text/Megaparsec/Text/Lazy.hs

@@ -12,6 +12,7 @@
 
 module Text.Megaparsec.Text.Lazy (Parser) where
 
+import Text.Megaparsec.Error (Dec)
 import Text.Megaparsec.Prim
 import qualified Data.Text.Lazy as T
 
@@ -20,4 +21,4 @@ import qualified Data.Text.Lazy as T
 -- @Parser@ type and easily change it by importing different “type
 -- modules”. This one is for lazy text.
 
-type Parser = Parsec String T.Text
+type Parser = Parsec Dec T.Text