diff --git a/composite-base.cabal b/composite-base.cabal
--- a/composite-base.cabal
+++ b/composite-base.cabal
@@ -1,13 +1,7 @@
 cabal-version: 1.12
 
--- This file has been generated from package.yaml by hpack version 0.34.7.
---
--- see: https://github.com/sol/hpack
---
--- hash: e7d77c01ec2d16ffde579ab8162ffa4762d032bef3c7bdfdbcecb769700420d8
-
 name:           composite-base
-version:        0.8.2.1
+version:        0.8.2.2
 synopsis:       Shared utilities for composite-* packages.
 description:    Shared helpers for the various composite packages.
 category:       Records
@@ -59,7 +53,7 @@
       base >=4.12 && <5
     , deepseq ==1.4.*
     , exceptions >=0.8.3 && <0.11
-    , lens >=4.15.4 && <5.2
+    , lens >=4.15.4 && <5.3
     , monad-control >=1.0.2.2 && <1.1
     , mtl >=2.2.1 && <2.3
     , profunctors >=5.2.1 && <5.7
@@ -112,7 +106,7 @@
     , deepseq ==1.4.*
     , exceptions >=0.8.3 && <0.11
     , hspec
-    , lens >=4.15.4 && <5.2
+    , lens >=4.15.4 && <5.3
     , monad-control >=1.0.2.2 && <1.1
     , mtl >=2.2.1 && <2.3
     , profunctors >=5.2.1 && <5.7
diff --git a/src/Composite/CoRecord.hs b/src/Composite/CoRecord.hs
--- a/src/Composite/CoRecord.hs
+++ b/src/Composite/CoRecord.hs
@@ -5,22 +5,25 @@
 
 import Prelude
 import Composite.Record (AllHave, HasInstances, (:->)(getVal, Val), reifyDicts, reifyVal, val, zipRecsWith)
-import Control.Lens (Prism', prism')
+import Control.Lens (Prism', Wrapped, Unwrapped, prism', review, view, _Wrapped')
+import Control.Monad.Except (ExceptT, throwError, withExceptT)
 import Data.Functor.Contravariant (Contravariant(contramap))
 import Data.Functor.Identity (Identity(Identity), runIdentity)
-import Data.Kind (Constraint)
+import Data.Kind (Constraint, Type)
 import Data.Maybe (fromMaybe)
 import Data.Profunctor (dimap)
 import Data.Proxy (Proxy(Proxy))
+import Data.Text (Text, pack)
 import Data.Vinyl.Core (Dict(Dict), Rec((:&), RNil), RMap, RecApplicative, RecordToList, ReifyConstraint, recordToList, reifyConstraint, rmap, rpure)
 import Data.Vinyl.Functor (Compose(Compose, getCompose), Const(Const), (:.))
 import Data.Vinyl.Lens (RElem, type (∈), type (⊆), rget, rput, rreplace)
 import Data.Vinyl.TypeLevel (RecAll, RIndex)
+import GHC.TypeLits (KnownSymbol, symbolVal)
 
 -- FIXME? replace with int-index/union or at least lift ideas from there. This encoding is awkward to work with and not compositional.
 
 -- |@CoRef f rs@ represents a single value of type @f r@ for some @r@ in @rs@.
-data CoRec :: (u -> *) -> [u] -> * where
+data CoRec :: (u -> Type) -> [u] -> Type where
   -- |Witness that @r@ is an element of @rs@ using '∈' ('RElem' with 'RIndex') from Vinyl.
   CoVal :: r ∈ rs => !(f r) -> CoRec f rs
 
@@ -160,10 +163,10 @@
 
 -- |Given a list of constraints @cs@ required to apply some function, apply the function to whatever value @r@ (not @f r@) which the 'CoRec' contains.
 onCoRec
-  :: forall (cs :: [* -> Constraint]) (f :: * -> *) (rs :: [*]) (b :: *) (proxy :: [* -> Constraint] -> *).
+  :: forall (cs :: [Type -> Constraint]) (f :: Type -> Type) (rs :: [Type]) (b :: Type) (proxy :: [Type -> Constraint] -> Type).
      (AllHave cs rs, Functor f, RecApplicative rs)
   => proxy cs
-  -> (forall (a :: *). HasInstances a cs => a -> b)
+  -> (forall (a :: Type). HasInstances a cs => a -> b)
   -> CoRec f rs
   -> f b
 onCoRec p f (CoVal x) = go <$> x
@@ -173,10 +176,10 @@
 
 -- |Given a list of constraints @cs@ required to apply some function, apply the function to whatever value @r@ which the 'Field' contains.
 onField
-  :: forall (cs :: [* -> Constraint]) (rs :: [*]) (b :: *) (proxy :: [* -> Constraint] -> *).
+  :: forall (cs :: [Type -> Constraint]) (rs :: [Type]) (b :: Type) (proxy :: [Type -> Constraint] -> Type).
      (AllHave cs rs, RecApplicative rs)
   => proxy cs
-  -> (forall (a :: *). HasInstances a cs => a -> b)
+  -> (forall (a :: Type). HasInstances a cs => a -> b)
   -> Field rs
   -> b
 onField p f x = runIdentity (onCoRec p f x)
@@ -245,3 +248,64 @@
 widenField r =
   fromMaybe (error "widenField should be provably total, isn't") $
     firstField (rreplace (fieldToRec r) (rpure Nothing))
+
+-- |Constraint showing that @rs ⊆ ss@ and giving enough power to widen a @'CoRec' f rs@ to @'CoRec' f ss@.
+type WiderCoRec rs ss = (FoldRec ss ss, RecApplicative rs, RecApplicative ss, rs ⊆ ss)
+
+-- |Constraint on a pair of types @r@ and @s@ which are 'Wrapped' around @'CoRec' f rs@ and @'CoRec' f ss@ respectively and observes that those types can be unwrapped and the former can be widened to the latter.
+type WrappedWiderCoRec f r rs s ss = (Wrapped r, Unwrapped r ~ CoRec f rs, Wrapped s, Unwrapped s ~ CoRec f ss, WiderCoRec rs ss)
+
+-- |'WrappedWiderCoRec' specialized to 'CoRec' 'Identity', i.e. 'Field'.
+type WrappedWiderField r rs s ss = WrappedWiderCoRec Identity r rs s ss
+
+-- |Widen a @'CoRec' f rs@ wrapped in @r@ to a @'CoRec' f ss@ wrapped in @s@.
+widenWrappedCoRec :: WrappedWiderCoRec f r rs s ss => r -> s
+widenWrappedCoRec = review _Wrapped' . widenCoRec . view _Wrapped'
+
+-- |Widen a @'Field' rs@ wrapped in @r@ to a @'Field' ss@ wrapped in @s@.
+widenWrappedField :: WrappedWiderField r rs s ss => r -> s
+widenWrappedField = review _Wrapped' . widenCoRec . view _Wrapped'
+
+-- |Widen a wrapped 'CoRec' using 'widenWrappedCoRec' and then throw it as an 'ExceptT' error, as a convenience when using 'CoRec' / 'Field' for building up error types by set union.
+throwWidened :: (Monad m, WrappedWiderCoRec f e er e' er') => e -> ExceptT e' m a
+throwWidened = throwError . widenWrappedCoRec
+
+-- |Catch a wrapped 'CoRec' thrown by an 'ExceptT' and widen it using 'widenWrappedCoRec' then rethrow it, as a convenience when using 'CoRec' / 'Field' for building up error types by set union.
+rethrowWidened :: (Functor m, WrappedWiderCoRec f e er e' er') => ExceptT e m a -> ExceptT e' m a
+rethrowWidened = withExceptT widenWrappedCoRec
+
+-- |Specialized type of 'Case' for 'Val', with the type parameters in a convenient order for type application, e.g.:
+--
+-- @
+--   valCase @"foo" (\ a -> ...)
+--     :& valCase @"bar" (\ b -> ...)
+--     :& RNil
+-- @
+valCase :: forall s a b. (a -> b) -> Case b (s :-> a)
+valCase f = Case $ \(Val a) -> f a
+
+-- |Specialized type of 'Case'' for 'Val', with the type parameters in a convenient order for type application, e.g.:
+--
+-- @
+--   valCase' @"foo" (\ fa -> ...)
+--     :& valCase' @"bar" (\ fb -> ...)
+--     :& RNil
+-- @
+valCase' :: forall s f a b. Functor f => (f a -> b) -> Case' f b (s :-> a)
+valCase' f = Case' $ \fva -> f (fmap getVal fva)
+
+-- |Make a 'Case' which yields the symbol text for a field @s :-> ()@. E.g.:
+--
+-- @
+--   keywordCase @"foo" (Identity (Val ())) == "foo"
+-- @
+keywordCase :: KnownSymbol s => Case Text (s :-> ())
+keywordCase = keywordCase' id
+
+-- |Make a 'Case'' which yields the symbol text for a field @s :-> ()@ as projected through the given function. E.g.:
+--
+-- @
+--   keywordCase @"foo" (<> "bar") (Identity (Val ())) == "foobar"
+-- @
+keywordCase' :: KnownSymbol s => (Text -> a) -> Case a (s :-> ())
+keywordCase' f = Case $ \(Val () :: s :-> ()) -> f (pack (symbolVal (Proxy @s)))
diff --git a/src/Composite/Record.hs b/src/Composite/Record.hs
--- a/src/Composite/Record.hs
+++ b/src/Composite/Record.hs
@@ -10,21 +10,23 @@
   , ReifyNames(reifyNames)
   , RecWithContext(rmapWithContext)
   , RDelete, RDeletable, rdelete
+  , rwiden
+  , _SingleVal
   ) where
 
 import Control.DeepSeq(NFData(rnf))
 import Control.Lens (Iso, iso)
 import Control.Lens.TH (makeWrapped)
-import Data.Functor.Identity (Identity(Identity))
+import Data.Functor.Identity (Identity(Identity), runIdentity)
 import Data.Functor.Contravariant (Contravariant(contramap))
 import Data.Kind (Constraint, Type)
 import Data.List.NonEmpty (NonEmpty((:|)))
 import Data.Proxy (Proxy(Proxy))
 import Data.String (IsString)
 import Data.Text (Text, pack)
-import Data.Vinyl (Rec((:&), RNil), RecApplicative, rcast, recordToList, rpure)
+import Data.Vinyl (Rec((:&), RNil), RecApplicative, RMap, rcast, rdowncast, recordToList, rmap, rpure)
 import qualified Data.Vinyl as Vinyl
-import Data.Vinyl.Functor (Compose(Compose), Const(Const), (:.))
+import Data.Vinyl.Functor (Compose(Compose, getCompose), Const(Const), (:.))
 import Data.Vinyl.Lens (type (∈), type (⊆))
 import qualified Data.Vinyl.TypeLevel as Vinyl
 import Data.Vinyl.XRec(IsoHKD(HKD, toHKD, unHKD))
@@ -80,8 +82,6 @@
   traverse k (Val a) = Val <$> k a
   {-# INLINE traverse #-}
 instance Monad ((:->) s) where
-  return = pure
-  {-# INLINE return #-}
   Val a >>= k = k a
   {-# INLINE (>>=) #-}
 
@@ -349,3 +349,14 @@
 -- |Remove an element @r@ from a @'Rec' f rs@. Note this is just a type constrained 'rcast'.
 rdelete :: RDeletable r rs => proxy r -> Rec f rs -> Rec f (RDelete r rs)
 rdelete _ = rcast
+
+-- |Widen a record from @'Record' rs@ to @'Rec' 'Maybe' ss@. Note this is just a type constrainted 'rdowncast'.
+rwiden :: (RecApplicative ss, RMap rs, RMap ss, rs ⊆ ss) => Record rs -> Rec Maybe ss
+rwiden = rmap (fmap runIdentity . getCompose) . rdowncast
+
+-- |'Iso' which observes that a record with a single field @s@ with value @a@ is isomorphic to the value alone.
+_SingleVal :: forall s a b. Iso (Record '[s :-> a]) (Record '[s :-> b]) a b
+_SingleVal =
+  let toA :: Record '[s :-> a] -> a = getVal . runIdentity . Vinyl.rget @(s :-> a)
+      fromA x = Identity (Val x) :& RNil
+   in iso toA fromA
diff --git a/src/Control/Monad/Composite/Context.hs b/src/Control/Monad/Composite/Context.hs
--- a/src/Control/Monad/Composite/Context.hs
+++ b/src/Control/Monad/Composite/Context.hs
@@ -46,6 +46,7 @@
 import qualified Control.Monad.Writer.Lazy as Lazy
 import qualified Control.Monad.Writer.Strict as Strict
 import Control.Monad.Writer.Class (MonadWriter(writer, tell, listen, pass))
+import Data.Kind (Type)
 
 import Control.Monad.IO.Unlift
   ( MonadUnliftIO
@@ -59,7 +60,7 @@
 
 -- |Class of monad (stacks) which have context reading functionality baked in. Similar to 'Control.Monad.Reader.MonadReader' but can coexist with a
 -- another monad that provides 'Control.Monad.Reader.MonadReader' and requires the context to be a record.
-class Monad m => MonadContext (c :: [*]) m | m -> c where
+class Monad m => MonadContext (c :: [Type]) m | m -> c where
   -- |Fetch the context record from the environment.
   askContext :: m (Record c)
 
@@ -123,7 +124,7 @@
 askField l = asksContext $ view l
 
 -- |Monad transformer which adds an implicit environment which is a record. Isomorphic to @ReaderT (Record c) m@.
-newtype ContextT (c :: [*]) (m :: (* -> *)) a = ContextT { runContextT :: Record c -> m a }
+newtype ContextT (c :: [Type]) (m :: (Type -> Type)) a = ContextT { runContextT :: Record c -> m a }
 
 -- |Run some action in a given context, equivalent to 'runContextT' but with the arguments flipped.
 runInContext :: Record c -> ContextT c m a -> m a