diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,10 @@
+0.6
+-----------------------------------------------------------------------------
+- Move `def` and `opt` from `Data.Predicate` to `Network.Wai.Predicate` and
+  make both specific to `Predicate r Error t`.
+- Remove `mapOkay` and `mapFail`.
+- Update `bytestring-from` dependency.
+
 0.5
 -----------------------------------------------------------------------------
 - Update dependencies constraints.
diff --git a/src/Data/Predicate.hs b/src/Data/Predicate.hs
--- a/src/Data/Predicate.hs
+++ b/src/Data/Predicate.hs
@@ -17,10 +17,6 @@
     , (.&.)
     , (.|.)
     , (|||)
-    , opt
-    , def
-    , mapOkay
-    , mapFail
     , exec
     -- * Result
     , module Data.Predicate.Result
@@ -108,31 +104,6 @@
 -- | Alias of 'orElse'.
 (|||) :: Predicate a f t -> Predicate a f t' -> Predicate a f (Either t t')
 (|||) = orElse
-
--- | Like 'fmap', but only maps the @Okay@ metadata to another result.
-mapOkay :: (t -> Result f t') -> Predicate a f t -> Predicate a f t'
-mapOkay f p a =
-    case p a of
-        Okay _ x -> f x
-        Fail   x -> Fail x
-
--- | Like 'mapOkay', but for the @Fail@ case.
-mapFail :: (f -> Result f' t) -> Predicate a f t -> Predicate a f' t
-mapFail f p a =
-    case p a of
-        Fail   x -> f x
-        Okay d x -> Okay d x
-
--- | A predicate modifier which makes the given predicate optional,
--- i.e. the @Okay@ metadata type becomes a 'Maybe' and in the failure-case
--- 'Nothing' is returned.
-opt :: Predicate a f t -> Predicate a f (Maybe t)
-opt = fmap (result (const $ return Nothing) (\d x -> Okay d (Just x)))
-
--- | A predicate modifier which returns as @Okay@ metadata the provided default
--- value if the given predicate fails.
-def :: t -> Predicate a f t -> Predicate a f t
-def t = fmap (result (const $ return t) Okay)
 
 exec :: Predicate a f t -> a -> (f -> b) -> (t -> b) -> b
 exec p a g f = case p a of
diff --git a/src/Network/Wai/Predicate.hs b/src/Network/Wai/Predicate.hs
--- a/src/Network/Wai/Predicate.hs
+++ b/src/Network/Wai/Predicate.hs
@@ -8,6 +8,9 @@
     ( module Data.Predicate
     , request
 
+    , def
+    , opt
+
     , query
     , hasQuery
 
@@ -29,12 +32,12 @@
     , module Network.Wai.Predicate.Error
     ) where
 
+import Control.Monad (when)
 import Data.ByteString (ByteString)
 import Data.ByteString.From
 import Data.CaseInsensitive (original)
 import Data.List (find)
-import Data.Monoid
-import Data.Maybe (isJust)
+import Data.Maybe (isNothing)
 import Data.Predicate
 import Data.Vault.Lazy (Key)
 import Data.Word
@@ -49,55 +52,90 @@
 
 import qualified Data.Vault.Lazy as Vault
 
-request :: (HasRequest r) => Predicate r f Request
+def :: a -> Predicate r Error a -> Predicate r Error a
+def a = fmap $
+    result (\e -> if not (TypeError `isReasonOf` e) then return a else Fail e)
+           Okay
+
+opt :: Predicate r Error a -> Predicate r Error (Maybe a)
+opt = fmap $
+    result (\e -> if not (TypeError `isReasonOf` e) then return Nothing else Fail e)
+           (\d -> Okay d . Just)
+
+request :: HasRequest r => Predicate r f Request
 request = return . getRequest
 
 query :: (HasQuery r, FromByteString a) => ByteString -> Predicate r Error a
-query k r = case lookupQuery k r of
-    [] -> Fail (err status400 ("Missing query '" <> k <> "'."))
-    qq -> either (Fail . err status400) return (readValues qq)
+query k r =
+    case lookupQuery k r of
+        [] -> Fail . addLabel "query" $ notAvailable k
+        qq -> either (Fail . addLabel "query" . typeError k)
+                     return
+                     (readValues qq)
 
-hasQuery :: (HasQuery r) => ByteString -> Predicate r Error ()
+hasQuery :: HasQuery r => ByteString -> Predicate r Error ()
 hasQuery k r =
-    if null (lookupQuery k r)
-        then Fail (err status400 ("Missing query '" <> k <> "'."))
-        else return ()
+    when (null (lookupQuery k r)) $
+        (Fail . addLabel "query" $ notAvailable k)
 
 header :: (HasHeaders r, FromByteString a) => HeaderName -> Predicate r Error a
-header k r = case lookupHeader k r of
-    [] -> Fail (err status400 ("Missing header '" <> original k <> "'."))
-    hh -> either (Fail . err status400) return (readValues hh)
+header k r =
+    case lookupHeader k r of
+        [] -> Fail . addLabel "header" $ notAvailable (original k)
+        hh -> either (Fail . addLabel "header" . typeError (original k))
+                     return
+                     (readValues hh)
 
-hasHeader :: (HasHeaders r) => HeaderName -> Predicate r Error ()
+hasHeader :: HasHeaders r => HeaderName -> Predicate r Error ()
 hasHeader k r =
-    if isJust $ find ((k ==) . fst) (headers r)
-        then return ()
-        else Fail (err status400 ("Missing header '" <> original k <> "'."))
+    when (isNothing (find ((k ==) . fst) (headers r))) $
+        (Fail . addLabel "header" $ notAvailable (original k))
 
 segment :: (HasPath r, FromByteString a) => Word -> Predicate r Error a
-segment i r = case lookupSegment i r of
-    Nothing -> Fail (err status400 "Path segment index out of bounds.")
-    Just  s -> either (Fail . err status400) return (readValues [s])
+segment i r =
+    case lookupSegment i r of
+        Nothing -> Fail $
+            e400 & setMessage "Path segment index out of bounds."
+                 . addLabel "path"
+        Just  s -> either (\m -> Fail (e400 & addLabel "path" . setReason TypeError . setMessage m))
+                          return
+                          (readValues [s])
 
-hasSegment :: (HasPath r) => Word -> Predicate r Error ()
+hasSegment :: HasPath r => Word -> Predicate r Error ()
 hasSegment i r =
-    if isJust $ lookupSegment i r
-        then return ()
-        else Fail (err status400 "Path segment index out of bounds.")
+    when (isNothing (lookupSegment i r)) $
+        Fail (e400 & addLabel "path" . setMessage "Path segment index out of bounds.")
 
 cookie :: (HasCookies r, FromByteString a) => ByteString -> Predicate r Error a
-cookie k r = case lookupCookie k r of
-    [] -> Fail (err status400 ("Missing cookie '" <> k <> "'."))
-    cc -> either (Fail . err status400) return (readValues cc)
+cookie k r =
+    case lookupCookie k r of
+        [] -> Fail . addLabel "cookie" $ notAvailable k
+        cc -> either (Fail . addLabel "cookie" . typeError k)
+                     return
+                     (readValues cc)
 
-hasCookie :: (HasCookies r) => ByteString -> Predicate r Error ()
+hasCookie :: HasCookies r => ByteString -> Predicate r Error ()
 hasCookie k r =
-    if null (lookupCookie k r)
-        then Fail (err status400 ("Missing cookie '" <> k <> "'."))
-        else return ()
+    when (null (lookupCookie k r)) $
+        (Fail . addLabel "cookie" $ notAvailable k)
 
 fromVault :: HasVault r => Key a -> Predicate r Error a
-fromVault k r = case Vault.lookup k (requestVault r) of
-    Nothing -> Fail (err status500 "Vault does not contain key.")
-    Just  a -> return a
+fromVault k r =
+    case Vault.lookup k (requestVault r) of
+        Just  a -> return a
+        Nothing -> Fail $
+            e500 & setReason NotAvailable
+                 . setMessage "Vault does not contain key."
+                 . addLabel "vault"
+
+-----------------------------------------------------------------------------
+-- Internal
+
+notAvailable :: ByteString -> Error
+notAvailable k = e400 & setReason NotAvailable . setSource k
+{-# INLINE notAvailable #-}
+
+typeError :: ByteString -> ByteString -> Error
+typeError k m = e400 & setReason TypeError . setSource k . setMessage m
+{-# INLINE typeError #-}
 
diff --git a/src/Network/Wai/Predicate/Accept.hs b/src/Network/Wai/Predicate/Accept.hs
--- a/src/Network/Wai/Predicate/Accept.hs
+++ b/src/Network/Wai/Predicate/Accept.hs
@@ -17,14 +17,14 @@
 import Data.Maybe
 import Data.Predicate
 import Data.Singletons.TypeLits (Symbol)
-import Network.HTTP.Types
 import Network.Wai.Predicate.Error
 import Network.Wai.Predicate.Request
 import Network.Wai.Predicate.MediaType
+import Network.Wai.Predicate.Utility
 
 import qualified Network.Wai.Predicate.Parser.MediaType as M
 
-accept :: (HasHeaders r)
+accept :: HasHeaders r
        => ByteString
        -> ByteString
        -> Predicate r Error (Media (t :: Symbol) (s :: Symbol))
@@ -34,7 +34,7 @@
         then return (Media t s 1.0 [])
         else case findMediaType t s mtypes of
             m:_ -> Okay (1.0 - mediaQuality m) m
-            []  -> Fail (err status406 msg)
+            []  -> Fail (e406 & setMessage msg)
       where
         msg = "Expected 'Accept: " <> t <> "/" <> s <> "'."
 
diff --git a/src/Network/Wai/Predicate/Content.hs b/src/Network/Wai/Predicate/Content.hs
--- a/src/Network/Wai/Predicate/Content.hs
+++ b/src/Network/Wai/Predicate/Content.hs
@@ -17,14 +17,14 @@
 import Data.Predicate
 import Data.Singletons.TypeLits (Symbol)
 import Data.Maybe
-import Network.HTTP.Types.Status
 import Network.Wai.Predicate.Error
 import Network.Wai.Predicate.MediaType
 import Network.Wai.Predicate.Request
+import Network.Wai.Predicate.Utility
 
 import qualified Network.Wai.Predicate.Parser.MediaType as M
 
-contentType :: (HasHeaders r)
+contentType :: HasHeaders r
             => ByteString
             -> ByteString
             -> Predicate r Error (Media (t :: Symbol) (s :: Symbol))
@@ -32,7 +32,7 @@
     let mtypes = M.readMediaTypes "content-type" r in
     case findContentType t s mtypes of
         m:_ -> Okay (1.0 - mediaQuality m) m
-        []  -> Fail (err status415 msg)
+        []  -> Fail (e415 & setMessage msg)
   where
     msg = "Expected 'Content-Type: " <> t <> "/" <> s <> "'."
 
diff --git a/src/Network/Wai/Predicate/Error.hs b/src/Network/Wai/Predicate/Error.hs
--- a/src/Network/Wai/Predicate/Error.hs
+++ b/src/Network/Wai/Predicate/Error.hs
@@ -2,18 +2,70 @@
 -- License, v. 2.0. If a copy of the MPL was not distributed with this
 -- file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
-module Network.Wai.Predicate.Error where
+module Network.Wai.Predicate.Error
+    ( Error
+    , Reason (..)
+    , err
+    , e400
+    , e405
+    , e406
+    , e415
+    , e500
+    , status
+    , message
+    , reason
+    , source
+    , labels
+    , setStatus
+    , setMessage
+    , setReason
+    , setSource
+    , addLabel
+    , isReasonOf
+    ) where
 
 import Data.ByteString (ByteString)
 import Network.HTTP.Types.Status
 
+data Reason
+    = NotAvailable
+    | TypeError
+    deriving (Eq, Show)
+
 -- | The error type used as meta-data for @Fail@ in all WAI predicates.
 data Error = Error
     { status  :: !Status          -- ^ HTTP status code
     , message :: Maybe ByteString -- ^ optional status message
+    , reason  :: Maybe Reason     -- ^ optional reason for this error
+    , source  :: Maybe ByteString -- ^ optional source of this erro
+    , labels  :: [ByteString]     -- ^ optional free-text labels
     } deriving (Eq, Show)
 
--- | Convenience function to construct 'Error' values from
--- status code and body message.
-err :: Status -> ByteString -> Error
-err s = Error s . Just
+err :: Status -> Error
+err s = Error s Nothing Nothing Nothing []
+
+setStatus :: Status -> Error -> Error
+setStatus s e = e { status = s }
+
+setMessage :: ByteString -> Error -> Error
+setMessage m e = e { message = Just m }
+
+setReason :: Reason -> Error -> Error
+setReason r e = e { reason = Just r }
+
+setSource :: ByteString -> Error -> Error
+setSource s e = e { source = Just s }
+
+addLabel :: ByteString -> Error -> Error
+addLabel l e = e { labels = l : labels e }
+
+isReasonOf :: Reason -> Error -> Bool
+isReasonOf r e = maybe False (r ==) (reason e)
+
+e400, e405, e406, e415, e500 :: Error
+e400 = err status400
+e405 = err status405
+e406 = err status406
+e415 = err status415
+e500 = err status500
+
diff --git a/src/Network/Wai/Predicate/Utility.hs b/src/Network/Wai/Predicate/Utility.hs
--- a/src/Network/Wai/Predicate/Utility.hs
+++ b/src/Network/Wai/Predicate/Utility.hs
@@ -4,7 +4,7 @@
 
 {-# LANGUAGE OverloadedStrings #-}
 
-module Network.Wai.Predicate.Utility (readValues) where
+module Network.Wai.Predicate.Utility (readValues, (&)) where
 
 import Data.ByteString (ByteString)
 import Data.ByteString.From
@@ -18,3 +18,7 @@
     res (Right x) _         = Right x
     res _         (Left  x) = Left (fromString x)
 
+infixl 1 &
+(&) :: a -> (a -> b) -> b
+a & f = f a
+{-# INLINE (&) #-}
diff --git a/test/Tests/Wai/Predicate.hs b/test/Tests/Wai/Predicate.hs
--- a/test/Tests/Wai/Predicate.hs
+++ b/test/Tests/Wai/Predicate.hs
@@ -4,9 +4,9 @@
 module Tests.Wai.Predicate (tests) where
 
 import Data.ByteString (ByteString)
-import Network.HTTP.Types.Status
 import Network.Wai.Predicate
 import Network.Wai.Predicate.Request
+import Network.Wai.Predicate.Utility
 import Test.Tasty
 import Test.Tasty.HUnit
 import Tests.Wai.Util
@@ -29,7 +29,8 @@
     Okay 0 (Media "application" "json" 1.0 []) @=? (accept "application" "json") rq0
 
     let rq1 = fromRequest . withHeader "Accept" "foo/bar" $ defRequest "/"
-    Fail (err status406 ("Expected 'Accept: application/json'.")) @=? (accept "application" "json") rq1
+    Fail (e406 & setMessage "Expected 'Accept: application/json'.")
+        @=? (accept "application" "json") rq1
 
 testAcceptThrift :: IO ()
 testAcceptThrift = do
@@ -37,7 +38,8 @@
     Okay 0 (Media "application" "x-thrift" 1.0 []) @=? (accept "application" "x-thrift") rq0
 
     let rq1 = fromRequest . json $ defRequest "/"
-    Fail (err status406 ("Expected 'Accept: application/x-thrift'.")) @=? (accept "application" "x-thrift") rq1
+    Fail (e406 & setMessage "Expected 'Accept: application/x-thrift'.")
+        @=? (accept "application" "x-thrift") rq1
 
 testAcceptAll :: IO ()
 testAcceptAll = do
@@ -51,7 +53,8 @@
     Okay 0 (Media "text" "plain" 1.0 []) @=? (contentType "text" "plain") rq0
 
     let rq1 = fromRequest . withHeader "Content-Type" "text/html" $ defRequest "/"
-    Fail (err status415 ("Expected 'Content-Type: text/plain'.")) @=? (contentType "text" "plain") rq1
+    Fail (e415 & setMessage "Expected 'Content-Type: text/plain'.")
+        @=? (contentType "text" "plain") rq1
 
 testContentTypeAll :: IO ()
 testContentTypeAll = do
@@ -64,7 +67,8 @@
     Okay 0 ("y" :: ByteString) @=? (query "x") rq0
 
     let rq1 = fromRequest $ defRequest "/"
-    Fail (err status400 ("Missing query 'x'.")) @=? (query "x" :: Predicate Req Error ByteString) rq1
+    Fail (e400 & setReason NotAvailable . setSource "x" . addLabel "query")
+        @=? (query "x" :: Predicate Req Error ByteString) rq1
 
 testQueryOpt :: IO ()
 testQueryOpt = do
diff --git a/wai-predicates.cabal b/wai-predicates.cabal
--- a/wai-predicates.cabal
+++ b/wai-predicates.cabal
@@ -1,5 +1,5 @@
 name:                wai-predicates
-version:             0.5
+version:             0.6
 synopsis:            WAI request predicates
 license:             OtherLicense
 license-file:        LICENSE
@@ -56,7 +56,7 @@
         attoparsec       >= 0.10   && < 0.12
       , base             == 4.*
       , bytestring       >= 0.9    && < 0.11
-      , bytestring-from  == 0.2.*
+      , bytestring-from  >= 0.2    && < 0.4
       , cookie           == 0.4.*
       , case-insensitive >= 1.1    && < 1.3
       , http-types       == 0.8.*