diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,12 @@
+0.4
+-----------------------------------------------------------------------------
+- Use `singletons` library instead of `GHC.TypeLits` from base.
+- Remove `mapResult` (use `fmap` instead).
+
+0.3.2
+-----------------------------------------------------------------------------
+- Update dependencies constraints.
+
 0.3.1
 -----------------------------------------------------------------------------
 - Add `fromVault` predicate and `HasVault` type-class.
diff --git a/src/Data/Predicate.hs b/src/Data/Predicate.hs
--- a/src/Data/Predicate.hs
+++ b/src/Data/Predicate.hs
@@ -5,8 +5,8 @@
 {-# LANGUAGE TypeOperators #-}
 
 module Data.Predicate
-    ( Predicate
-    , module Data.Predicate.Result
+    ( -- * Predicate
+      Predicate
     , constant
     , failure
     , true
@@ -19,10 +19,12 @@
     , (|||)
     , opt
     , def
-    , mapResult
     , mapOkay
     , mapFail
     , exec
+    -- * Result
+    , module Data.Predicate.Result
+    -- * Product
     , module Data.Predicate.Product
     ) where
 
@@ -58,7 +60,7 @@
 
 -- | A predicate corresponding to the logical AND connective
 -- of two predicate.
-and, (.&.) :: Predicate a f t -> Predicate a f t' -> Predicate a f (t ::: t')
+and :: Predicate a f t -> Predicate a f t' -> Predicate a f (t ::: t')
 and f g x = f x `cmp` g x
   where
     cmp (Okay d y) (Okay w z) = Okay (d + w) (y ::: z)
@@ -72,7 +74,7 @@
 -- If both arguments evaluate to @Okay@ the one with the
 -- smaller \"delta\" will be preferred, or--if equal--the
 -- left-hand argument.
-or, (.|.) :: Predicate a f t -> Predicate a f t -> Predicate a f t
+or :: Predicate a f t -> Predicate a f t -> Predicate a f t
 or f g x = f x `cmp` g x
   where
     cmp a@(Okay d _) b@(Okay w _)  = if w < d then b else a
@@ -87,7 +89,7 @@
 -- If both arguments evaluate to @Okay@ the one with the
 -- smaller \"delta\" will be preferred, or--if equal--the
 -- left-hand argument.
-orElse, (|||) :: Predicate a f t -> Predicate a f t' -> Predicate a f (Either t t')
+orElse :: Predicate a f t -> Predicate a f t' -> Predicate a f (Either t t')
 orElse f g x = f x `cmp` g x
   where
     cmp (Okay d y) (Okay w z) = if w < d then Okay w (Right z) else Okay d (Left y)
@@ -95,22 +97,26 @@
     cmp (Fail   _) (Okay d y) = Okay d (Right y)
     cmp (Fail   _) (Fail   y) = Fail y
 
+-- | Alias of 'and'.
+(.&.) :: Predicate a f t -> Predicate a f t' -> Predicate a f (t ::: t')
 (.&.) = and
+
+-- | Alias of 'or'.
+(.|.) :: Predicate a f t -> Predicate a f t -> Predicate a f t
 (.|.) = or
-(|||) = orElse
 
--- | Map the result of the given predicate to some other result.
-mapResult :: (Result f t -> Result f' t') -> Predicate a f t -> Predicate a f' t'
-mapResult f p = f . p
+-- | Alias of 'orElse'.
+(|||) :: Predicate a f t -> Predicate a f t' -> Predicate a f (Either t t')
+(|||) = orElse
 
--- | Like 'mapResult', but only maps the @Okay@ metadata to another result.
+-- | 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 'mapResult', but only maps the @Fail@ metadata to another result.
+-- | 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
@@ -121,12 +127,12 @@
 -- 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 = mapResult (result (const $ return Nothing) (\d x -> Okay d (Just x)))
+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 = mapResult (result (const $ return t) Okay)
+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/Accept.hs b/src/Network/Wai/Predicate/Accept.hs
--- a/src/Network/Wai/Predicate/Accept.hs
+++ b/src/Network/Wai/Predicate/Accept.hs
@@ -16,7 +16,7 @@
 import Data.Monoid hiding (All)
 import Data.Maybe
 import Data.Predicate
-import GHC.TypeLits
+import Data.Singletons.TypeLits (Symbol)
 import Network.HTTP.Types
 import Network.Wai.Predicate.Error
 import Network.Wai.Predicate.Request
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
@@ -15,7 +15,7 @@
 import Data.ByteString (ByteString)
 import Data.Monoid hiding (All)
 import Data.Predicate
-import GHC.TypeLits
+import Data.Singletons.TypeLits (Symbol)
 import Data.Maybe
 import Network.HTTP.Types.Status
 import Network.Wai.Predicate.Error
diff --git a/src/Network/Wai/Predicate/MediaType.hs b/src/Network/Wai/Predicate/MediaType.hs
--- a/src/Network/Wai/Predicate/MediaType.hs
+++ b/src/Network/Wai/Predicate/MediaType.hs
@@ -9,7 +9,8 @@
 
 import Data.ByteString (ByteString)
 import Data.ByteString.Char8 (pack)
-import GHC.TypeLits
+import Data.Singletons
+import Data.Singletons.TypeLits
 
 data Media (t :: Symbol) (s :: Symbol) = Media
     { rawType      :: !ByteString
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:             0.3.2
+version:             0.4
 synopsis:            WAI request predicates
 license:             OtherLicense
 license-file:        LICENSE
@@ -58,6 +58,7 @@
       , cookie           == 0.4.*
       , case-insensitive >= 1.1    && < 1.3
       , http-types       == 0.8.*
+      , singletons       == 0.10.*
       , transformers     == 0.3.*
       , vault            == 0.3.*
       , vector           == 0.10.*