diff --git a/simpleconfig.cabal b/simpleconfig.cabal
--- a/simpleconfig.cabal
+++ b/simpleconfig.cabal
@@ -1,11 +1,11 @@
--- This file has been generated from package.yaml by hpack version 0.20.0.
+-- This file has been generated from package.yaml by hpack version 0.28.2.
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: 1f3e3450009bfd77e7430ed520edd3aa0c946c5dd117d5f40be0ee518511d9b5
+-- hash: 3985e307c00f6905bfb6d5a656f0c09031bd4c0a8d158c15cb73db132796ebaa
 
 name:           simpleconfig
-version:        0.0.9
+version:        0.0.10
 synopsis:       Short description of your package
 description:    Please see the README on Github at <https://github.com/koterpillar/simpleconfig#readme>
 category:       Web
@@ -18,7 +18,6 @@
 license-file:   LICENSE
 build-type:     Simple
 cabal-version:  >= 1.10
-
 extra-source-files:
     README.md
 
@@ -27,31 +26,34 @@
   location: https://github.com/koterpillar/simpleconfig
 
 library
+  exposed-modules:
+      Config.Simple
+  other-modules:
+      Paths_simpleconfig
   hs-source-dirs:
       src
   build-depends:
       base >=4.7 && <5
     , containers
+    , either
     , lens
-  exposed-modules:
-      Config.Simple
-  other-modules:
-      Paths_simpleconfig
   default-language: Haskell2010
 
 test-suite simpleconfig-test
   type: exitcode-stdio-1.0
   main-is: Spec.hs
+  other-modules:
+      Paths_simpleconfig
   hs-source-dirs:
       test
   ghc-options: -threaded -rtsopts -with-rtsopts=-N
   build-depends:
       base
     , containers
+    , either
     , generic-deriving
+    , hspec
     , lens
     , simpleconfig
     , text
-  other-modules:
-      Paths_simpleconfig
   default-language: Haskell2010
diff --git a/src/Config/Simple.hs b/src/Config/Simple.hs
--- a/src/Config/Simple.hs
+++ b/src/Config/Simple.hs
@@ -12,6 +12,7 @@
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE TypeSynonymInstances #-}
+{-# LANGUAGE UndecidableInstances #-}
 
 module Config.Simple
   ( ConfigBool
@@ -20,14 +21,19 @@
   , Partial
   , Complete
   , LensFor(..)
+  , AccumFor(..)
   , configLens
+  , configLensPartial
   , fromPartialConfig
   ) where
 
 import Control.Lens
 
+import Data.Either.Validation
 import Data.Monoid (Any(..), Last(..))
 import Data.Set (Set)
+import qualified Data.Set as Set
+import Data.String (IsString(..))
 
 import GHC.Generics
 import qualified GHC.Generics as G
@@ -39,25 +45,41 @@
 
 data CComplete
 
+data CAccum
+
 data CLensFor k c
 
+type family ConfigLensTarget k where
+  ConfigLensTarget CComplete = CComplete
+  ConfigLensTarget CPartial = CAccum
+
+type family ConfigLensFor k s a where
+  ConfigLensFor CComplete s a = LensFor s a
+  ConfigLensFor CPartial s a = AccumFor s a
+
 newtype LensFor s a =
   LensFor (Lens' s a)
 
+newtype AccumFor s a =
+  AccumFor (a -> s -> s)
+
 type family ConfigLast a k where
   ConfigLast a CPartial = Last a
   ConfigLast a CComplete = a
-  ConfigLast a (CLensFor k root) = LensFor root (ConfigLast a k)
+  ConfigLast a CAccum = a
+  ConfigLast a (CLensFor k root) = ConfigLensFor k root (ConfigLast a (ConfigLensTarget k))
 
 type family ConfigBool k where
   ConfigBool CPartial = Any
   ConfigBool CComplete = Bool
-  ConfigBool (CLensFor k root) = LensFor root (ConfigBool k)
+  ConfigBool CAccum = Bool
+  ConfigBool (CLensFor k root) = ConfigLensFor k root (ConfigBool (ConfigLensTarget k))
 
 type family ConfigSet a k where
   ConfigSet a CPartial = Set a
   ConfigSet a CComplete = Set a
-  ConfigSet a (CLensFor k root) = LensFor root (ConfigSet a k)
+  ConfigSet a CAccum = a
+  ConfigSet a (CLensFor k root) = ConfigLensFor k root (ConfigSet a (ConfigLensTarget k))
 
 type Partial config = config CPartial
 
@@ -69,29 +91,45 @@
      ( Generic (Partial config)
      , Generic (Complete config)
      , GFromPartialConfig (Rep (Partial config)) (Rep (Complete config))
+     , IsString text
      )
   => Partial config
-  -> Maybe (Complete config)
+  -> Validation [text] (Complete config)
 fromPartialConfig = fmap G.to . gFromPartialConfig . G.from
 
 class GFromPartialConfig (repPartial :: * -> *) (repComplete :: * -> *) where
-  gFromPartialConfig :: repPartial x -> Maybe (repComplete x)
+  gFromPartialConfig :: IsString text => repPartial x -> Validation [text] (repComplete x)
 
 instance GFromPartialConfig fp fc =>
-         GFromPartialConfig (M1 i m fp) (M1 i m fc) where
+         GFromPartialConfig (D1 m fp) (D1 m fc) where
   gFromPartialConfig (M1 x) = M1 <$> gFromPartialConfig x
 
+instance GFromPartialConfig fp fc =>
+         GFromPartialConfig (C1 m fp) (C1 m fc) where
+  gFromPartialConfig (M1 x) = M1 <$> gFromPartialConfig x
+
 instance (GFromPartialConfig ap ac, GFromPartialConfig bp bc) =>
          GFromPartialConfig (ap :*: bp) (ac :*: bc) where
   gFromPartialConfig (a :*: b) =
     liftA2 (:*:) (gFromPartialConfig a) (gFromPartialConfig b)
 
+instance (GFromPartialConfigMaybe fp fc, Selector m) =>
+         GFromPartialConfig (S1 m fp) (S1 m fc) where
+  gFromPartialConfig v@(M1 x) =
+    fmap M1 $
+    case gFromPartialConfigMaybe x of
+      Just r -> Success r
+      Nothing -> Failure [fromString $ selName v]
+
+class GFromPartialConfigMaybe (repPartial :: * -> *) (repComplete :: * -> *) where
+  gFromPartialConfigMaybe :: repPartial x -> Maybe (repComplete x)
+
 class GFromPartialConfigMember partial complete where
   gFromPartialConfigMember :: partial -> Maybe complete
 
 instance GFromPartialConfigMember partial complete =>
-         GFromPartialConfig (Rec0 partial) (Rec0 complete) where
-  gFromPartialConfig (K1 a) = K1 <$> gFromPartialConfigMember a
+         GFromPartialConfigMaybe (Rec0 partial) (Rec0 complete) where
+  gFromPartialConfigMaybe (K1 a) = K1 <$> gFromPartialConfigMember a
 
 instance GFromPartialConfigMember Any Bool where
   gFromPartialConfigMember = Just . getAny
@@ -102,20 +140,38 @@
 instance GFromPartialConfigMember (Last a) a where
   gFromPartialConfigMember = getLast
 
-configLens ::
+configLens' ::
      forall config k.
      ( Generic (config k)
      , Generic (LensConfig k config)
      , GLensFor (config k) (Rep (config k)) (Rep (LensConfig k config))
      )
   => LensConfig k config
-configLens = G.to $ gToLensFor rootLens
+configLens' = G.to $ gToLensFor rootLens
   where
     rootLens ::
          forall x. Generic (config k)
       => Lens' (config k) (Rep (config k) x)
     rootLens = G.generic
 
+configLens ::
+     forall config.
+     ( Generic (config CComplete)
+     , Generic (LensConfig CComplete config)
+     , GLensFor (config CComplete) (Rep (config CComplete)) (Rep (LensConfig CComplete config))
+     )
+  => LensConfig CComplete config
+configLens = configLens'
+
+configLensPartial ::
+     forall config.
+     ( Generic (config CPartial)
+     , Generic (LensConfig CPartial config)
+     , GLensFor (config CPartial) (Rep (config CPartial)) (Rep (LensConfig CPartial config))
+     )
+  => LensConfig CPartial config
+configLensPartial = configLens'
+
 class GLensFor root rep repLens where
   gToLensFor :: Lens' root (rep x) -> repLens x
 
@@ -129,3 +185,12 @@
 
 instance GLensFor root (Rec0 x) (Rec0 (LensFor root x)) where
   gToLensFor rootLens = K1 $ LensFor $ rootLens . G._K1
+
+instance GLensFor root (Rec0 (Last x)) (Rec0 (AccumFor root x)) where
+  gToLensFor rootLens = K1 $ AccumFor $ \v s -> s & rootLens . G._K1 <>~ Last (Just v)
+
+instance GLensFor root (Rec0 Any) (Rec0 (AccumFor root Bool)) where
+  gToLensFor rootLens = K1 $ AccumFor $ \v s -> s & rootLens . G._K1 <>~ Any v
+
+instance Ord a => GLensFor root (Rec0 (Set a)) (Rec0 (AccumFor root a)) where
+  gToLensFor rootLens = K1 $ AccumFor $ \v s -> s & rootLens . G._K1 <>~ Set.singleton v
diff --git a/test/Spec.hs b/test/Spec.hs
--- a/test/Spec.hs
+++ b/test/Spec.hs
@@ -11,7 +11,9 @@
 import Control.Lens
 import Control.Monad
 
+import Data.Either.Validation
 import Data.Foldable
+import Data.Maybe
 import Data.Monoid (Last(..))
 import Data.Set (Set)
 import qualified Data.Set as Set
@@ -23,6 +25,8 @@
 
 import GHC.Generics
 
+import Test.Hspec
+
 import Config.Simple
 
 data ConfigF k = Config
@@ -49,14 +53,32 @@
 
 Config (LensFor address) (LensFor dryRun) (LensFor widgets) = configLens
 
+Config (AccumFor address') (AccumFor dryRun') (AccumFor widgets') =
+  configLensPartial
+
+isSuccess :: Validation e a -> Bool
+isSuccess (Success _) = True
+isSuccess (Failure _) = False
+
 main :: IO ()
-main = do
-  let config' =
-        mempty & (address <>~ pure "Silverpond") & (dryRun <>~ Any True) &
-        (widgets <>~ Set.singleton "blah") &
-        (address <>~ pure "SEEK")
-  print config'
-  let (Just config) = fromPartialConfig config'
-  Text.putStrLn $ "Address = " <> config ^. address
-  when (config ^. dryRun) $ Text.putStrLn "Dry run"
-  for_ (config ^. widgets) $ \widget -> Text.putStrLn $ "Widget = " <> widget
+main =
+  hspec $
+  describe "fromPartialConfig" $ do
+    context "with a complete config" $ do
+      let config' =
+            mempty & address' "Silverpond" & dryRun' True & widgets' "blah" &
+            address' "SEEK"
+      let result = fromPartialConfig config'
+      it "should succeed" $ result `shouldSatisfy` isSuccess
+      let (Success config) = fromPartialConfig config'
+      it "should override Last members with later assignments" $
+        config ^. address `shouldBe` "SEEK"
+      it "should turn on Any members" $ config ^. dryRun `shouldBe` True
+      it "should record set members" $
+        config ^. widgets `shouldBe` Set.fromList ["blah"]
+    context "with incomplete config" $ do
+      let config' = mempty & dryRun' True
+      let result = fromPartialConfig config'
+      it "should return a failure" $ result `shouldNotSatisfy` isSuccess
+      let (Failure fields) = fromPartialConfig config'
+      it "should return missing fields" $ fields `shouldBe` ["_address"]