diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -6,6 +6,10 @@
 
 The library does not define a data type nor parser for a whole XML document. Instead, parsers operate on XML tokens of various sizes, to allow for a usage in a streaming context (e.g. using [`streaming-attoparsec`](https://hackage.haskell.org/package/streaming-attoparsec)).
 
+The implementation is based on the following specifications:
+- [Extensible Markup Language (XML) 1.0 (Fifth Edition)](https://www.w3.org/TR/REC-xml/)
+- [Namespaces in XML 1.0 (Third Edition)](https://www.w3.org/TR/xml-names/)
+
 
 ## Caveats
 
diff --git a/hextream.cabal b/hextream.cabal
--- a/hextream.cabal
+++ b/hextream.cabal
@@ -1,7 +1,7 @@
 cabal-version:       2.4
 
 name:                hextream
-version:             0.1.0.1
+version:             0.2.0.0
 synopsis:            Streaming-friendly XML parsers
 description:         Cf README file
 homepage:            https://github.com/k0ral/hextream
diff --git a/src/Data/XML/Parser/High.hs b/src/Data/XML/Parser/High.hs
--- a/src/Data/XML/Parser/High.hs
+++ b/src/Data/XML/Parser/High.hs
@@ -222,9 +222,9 @@
       AnyContent f       -> f mempty
       WithContent parser -> unexpected "Expected non-empty tag"
   processName name = runNameParser parseName name
-    & maybe (unexpected "Unexpected name") return
+    & either unexpected return
   processAttributes state attributes = runAttrParser (parseAttributes state) (normalizeAttributes entityDecoder attributes)
-    & maybe (unexpected "Unexpected attributes") return
+    & either unexpected return
 
 -- | Simplified version of 'tag':
 --
@@ -259,7 +259,7 @@
   <|> (TokenTextContent <$> textContent entityDecoder)
   where tokenTag = tag entityDecoder anyName (\name -> (name,) <$> forwardAttrs) $ \(name, attributes) ->
           TokenTag name attributes <$> AnyContent pure
-        forwardAttrs = AttrParser Just
+        forwardAttrs = AttrParser Right
 
 -- | Same as @anyToken (decodePredefinedEntities <> decodeHtmlEntities)@, provided for convenience.
 anyToken' :: CharParsing m => Monad m => TokenParser m Token
diff --git a/src/Data/XML/Parser/High/AttrParser.hs b/src/Data/XML/Parser/High/AttrParser.hs
--- a/src/Data/XML/Parser/High/AttrParser.hs
+++ b/src/Data/XML/Parser/High/AttrParser.hs
@@ -2,6 +2,7 @@
 {-# LANGUAGE DerivingVia                #-}
 {-# LANGUAGE FlexibleInstances          #-}
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE NoImplicitPrelude          #-}
 {-# LANGUAGE StandaloneDeriving         #-}
 -- | All documentation examples assume the following setup:
 --
@@ -18,11 +19,14 @@
 
 import           Control.Applicative
 import           Control.Arrow
-import           Control.Monad
-import           Data.Map                 (Map)
-import qualified Data.Map                 as Map
-import           Data.Text                (Text)
+import           Control.Monad.Compat
+import           Control.Monad.Fail.Compat
+import           Data.Map                  (Map)
+import qualified Data.Map                  as Map
+import           Data.Text                 (Text)
+import qualified Data.Text                 as Text
 import           Data.XML.Parser.Low.Name
+import           Prelude.Compat
 
 -- $setup
 -- >>> :set -XOverloadedStrings
@@ -30,13 +34,13 @@
 -- >>> import Data.XML.Parser.High
 
 -- | How to parse tag attributes.
-newtype AttrParser a = AttrParser { runAttrParser :: Map QName Text -> Maybe a }
+newtype AttrParser a = AttrParser { runAttrParser :: Map QName Text -> Either String a }
 
 deriving instance Functor AttrParser
-deriving via (WrappedArrow (Kleisli Maybe) (Map QName Text)) instance Applicative AttrParser
+deriving via (WrappedArrow (Kleisli (Either String)) (Map QName Text)) instance Applicative AttrParser
 
 -- | Can be combined with @\<|\>@
-deriving via (WrappedArrow (Kleisli Maybe) (Map QName Text)) instance Alternative AttrParser
+deriving via (WrappedArrow (Kleisli (Either String)) (Map QName Text)) instance Alternative AttrParser
 
 -- | Can be combined with @>>=@. Attributes map is forwarded without change.
 instance Monad AttrParser where
@@ -45,6 +49,9 @@
     let AttrParser g' = g a
     g' attributes
 
+instance MonadFail AttrParser where
+  fail message = AttrParser $ const $ Left message
+
 -- | Parse any set of attributes.
 --
 -- >>> parseOnly (runTokenParser $ tag' anyName anyAttr noContent) "<tag></tag>"
@@ -61,7 +68,7 @@
 -- >>> parseOnly (runTokenParser $ tag' anyName noAttr noContent) "<tag key='value'></tag>"
 -- Left ...
 noAttr :: AttrParser ()
-noAttr = AttrParser $ \attributes -> if null attributes then Just () else Nothing
+noAttr = AttrParser $ \attributes -> if null attributes then Right () else Left $ "Expected no attribute, instead got: " <> show attributes
 
 -- | Parse attribute by name, and return its value.
 --
@@ -70,7 +77,7 @@
 -- >>> parseOnly (runTokenParser $ tag' anyName (attrValue "foo") noContent) "<tag foo='bar'></tag>"
 -- Right ()
 attrValue :: QName -> AttrParser Text
-attrValue name = AttrParser $ Map.lookup name
+attrValue name = AttrParser $ maybe (Left $ "Missing attribute named " <> show name) Right . Map.lookup name
 
 -- | Assert that an attribute exists, with given name and value.
 --
@@ -81,4 +88,4 @@
 -- >>> parseOnly (runTokenParser $ tag' anyName (hasAttr "foo" "bar") noContent) "<tag foo='bar'></tag>"
 -- Right ()
 hasAttr :: QName -> Text -> AttrParser ()
-hasAttr name value = attrValue name >>= \value' -> guard (value == value')
+hasAttr name value = attrValue name >>= \value' -> if value == value' then pure () else fail $ "Expected attribute value " <> Text.unpack value <> ", instead got: " <> Text.unpack value'
diff --git a/src/Data/XML/Parser/High/NameParser.hs b/src/Data/XML/Parser/High/NameParser.hs
--- a/src/Data/XML/Parser/High/NameParser.hs
+++ b/src/Data/XML/Parser/High/NameParser.hs
@@ -2,6 +2,7 @@
 {-# LANGUAGE DerivingVia                #-}
 {-# LANGUAGE FlexibleInstances          #-}
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE NoImplicitPrelude          #-}
 {-# LANGUAGE StandaloneDeriving         #-}
 {-# LANGUAGE TypeFamilies               #-}
 -- | All documentation examples assume the following setup:
@@ -17,9 +18,11 @@
 
 import           Control.Applicative
 import           Control.Arrow
-import           Control.Monad
+import           Control.Monad.Compat
+import           Control.Monad.Fail.Compat
 import           Data.String
 import           Data.XML.Parser.Low
+import           Prelude.Compat
 
 -- $setup
 -- >>> :set -XOverloadedStrings
@@ -27,25 +30,35 @@
 -- >>> import Data.XML.Parser.High
 
 -- | How to parse tag names.
-newtype NameParser a = NameParser { runNameParser :: QName -> Maybe a }
+newtype NameParser a = NameParser { runNameParser :: QName -> Either String a }
 
 deriving instance Functor NameParser
-deriving via (WrappedArrow (Kleisli Maybe) QName) instance Applicative NameParser
+deriving via (WrappedArrow (Kleisli (Either String)) QName) instance Applicative NameParser
 
 -- | Can be combined with @\<|\>@
-deriving via (WrappedArrow (Kleisli Maybe) QName) instance Alternative NameParser
+deriving via (WrappedArrow (Kleisli (Either String)) QName) instance Alternative NameParser
 
 -- | Match a single 'QName' in a concise way.
 --
 -- >>> parseOnly (runTokenParser $ tag' "foo" anyAttr anyContent) "<foo></foo>"
 -- Right ()
 instance (a ~ ()) => IsString (NameParser a) where
-  fromString s = NameParser $ \(QName _ name) ->
-    unless (fromString s == name) mempty
+  fromString s = anyName >>= \(QName _ name) ->
+    when (fromString s /= name) $ fail $ "Expected tag named " <> show s <> ", instead got: " <> show name
 
+-- | Can be combined with @>>=@. Qualified name is forwarded without change.
+instance Monad NameParser where
+  (NameParser f) >>= g = NameParser $ \name -> do
+    a <- f name
+    let NameParser g' = g a
+    g' name
+
+instance MonadFail NameParser where
+  fail message = NameParser $ const $ Left message
+
 -- | Match any qualified name.
 anyName :: NameParser QName
-anyName = NameParser Just
+anyName = NameParser Right
 
 -- | Match any qualified name, except for the given value.
 --
@@ -54,6 +67,6 @@
 -- >>> parseOnly (runTokenParser $ tag' (anyNameExcept "foo") anyAttr anyContent) "<bar></bar>"
 -- Right ()
 anyNameExcept :: QName -> NameParser QName
-anyNameExcept name = NameParser $ \name' -> do
-  guard $ name /= name'
-  return name'
+anyNameExcept name = NameParser $ \name' -> if name == name'
+  then Left $ "Expected any tag name except " <> show name
+  else Right name'