diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,12 @@
 # Revision history for pa-json
 
+## 0.4.0.0 -- 2024-09-04
+
+- Add `eachInArrayNonEmpty`
+- Add `choice`, `choiceDef`, `choiceImpl`, `mkSingleChoice`
+- Add `ensureNotEmpty`
+- Add `mkUtcTime`, `mkMaybe`
+
 ## 0.3.0.0 -- 2023-10-15
 
 - rename `jsonArray` to `mkJsonArray`
diff --git a/pa-json.cabal b/pa-json.cabal
--- a/pa-json.cabal
+++ b/pa-json.cabal
@@ -1,7 +1,7 @@
 cabal-version:      3.0
 -- !!! ATTN: file autogenerated from pa-template.cabal.mustache on toplevel !!!
 name:               pa-json
-version:            0.3.0.0
+version:            0.4.0.0
 synopsis:           Our JSON parsers/encoders
 description:        The interface of `aeson` is unfortunately extremely … suboptimal. Here’s some wrappers trying to improve the situation, which we use internally.
 license:            BSD-3-Clause
@@ -61,6 +61,10 @@
     -- to enable the `type` keyword in import lists (ormolu uses this automatically)
     ExplicitNamespaces
 
+    -- allows defining pattern synonyms, but also the `import Foo (pattern FooPattern)` import syntax
+    PatternSynonyms
+
+
   default-language: GHC2021
 
 library
@@ -83,6 +87,7 @@
       bytestring,
       base64-bytestring,
       containers,
+      mono-traversable,
       scientific,
       time,
       text,
diff --git a/src/Json.hs b/src/Json.hs
--- a/src/Json.hs
+++ b/src/Json.hs
@@ -1,19 +1,22 @@
-{-# LANGUAGE KindSignatures #-}
-{-# LANGUAGE UndecidableInstances #-}
+{-# LANGUAGE QuasiQuotes #-}
 
 module Json where
 
 import Data.Aeson (FromJSON (parseJSON), ToJSON (toEncoding, toJSON), Value (..), withObject)
 import Data.Aeson qualified as Json
+import Data.Aeson.BetterErrors (throwCustomError)
 import Data.Aeson.BetterErrors qualified as Json
 import Data.Aeson.Types qualified
 import Data.Error.Tree
 import Data.Map.Strict qualified as Map
 import Data.Maybe (catMaybes)
+import Data.MonoTraversable (MonoFoldable)
+import Data.MonoTraversable qualified as MonoFoldable
 import Data.Set (Set)
 import Data.Set qualified as Set
 import Data.Text qualified as Text
 import Data.Time (UTCTime)
+import Data.Time.Format.ISO8601 qualified as ISO8601
 import Data.Vector qualified as Vector
 import FieldParser (FieldParser)
 import FieldParser qualified as Field
@@ -188,6 +191,64 @@
   Json.ParseT err m (Label label (Maybe a))
 keyLabelMay' Proxy key parser = label @label <$> Json.keyMay key parser
 
+-- | Parse each value in the array and check that the array is not empty.
+eachInArrayNonEmpty ::
+  (Monad m) =>
+  Json.ParseT Error m a ->
+  Json.ParseT Error m (NonEmpty a)
+eachInArrayNonEmpty inner =
+  Json.eachInArray inner <&> nonEmpty >>= \case
+    Nothing -> Json.throwCustomError "Array must not be empty"
+    Just a -> pure a
+
+-- | Try to parse a choice, match on the `tag` and use the corresponding parser for `value`
+-- If no tag matches, throw a parsing error
+--
+-- A choice is the conventional encoding of `tag/value`.
+choice ::
+  (Monad m) =>
+  -- | List of `tag`s with their parsers for `value`
+  [(Text, Json.ParseT Error m res)] ->
+  Json.ParseT Error m res
+choice matches = do
+  choiceImpl $ \tag ->
+    matches
+      & findMaybe (\(t, v) -> if t == tag then Just v else Nothing)
+      & \case
+        Just res -> res
+        Nothing -> Json.throwCustomError [fmt|Choice tag was "{tag}" but we only know these tags: {matches <&> fst & show}|]
+
+-- | Try to parse a choice, match on the `tag` and use the corresponding parser for `value`
+-- If no tag matches, return the default value.
+--
+-- A choice is the conventional encoding of `tag/value`.
+choiceDef ::
+  (Monad m) =>
+  -- | Default value if nothing matches.
+  res ->
+  -- | List of `tag`s with their parsers for `value`
+  [(Text, Json.ParseT Error m res)] ->
+  Json.ParseT Error m res
+choiceDef def matches = do
+  choiceImpl $ \tag ->
+    matches
+      & findMaybe (\(t, v) -> if t == tag then Just v else Nothing)
+      & fromMaybe (pure def)
+
+choiceImpl :: (Monad m) => (Text -> Json.ParseT Error m b) -> Json.ParseT Error m b
+choiceImpl inner = do
+  let keyOrErr k i err =
+        Json.keyMay k i >>= \case
+          Nothing -> Json.throwCustomError err
+          Just a -> pure a
+  -- TODO: throw error on extra fields?
+  tag <- keyOrErr "tag" Json.asText "Choice needs a tag"
+  keyOrErr "value" (inner tag) "Choice needs a value"
+
+-- | Ensure the given structure (e.g. a text) is not empty, or throw the given error.
+ensureNotEmpty :: (MonoFoldable mono, Monad m) => err -> mono -> Json.ParseT err m mono
+ensureNotEmpty err val = if MonoFoldable.onull val then throwCustomError err else pure val
+
 -- NOTE: keyRenamed Test in "Json.JsonTest", due to import cycles.
 
 -- | Like 'Json.key', but allows a list of keys that are tried in order.
@@ -241,3 +302,18 @@
 -- | Create a json array from a list of json values.
 mkJsonArray :: [Value] -> Value
 mkJsonArray xs = xs & Vector.fromList & Array
+
+-- | Encode a single possibility into a @tag/value@ object.
+mkSingleChoice :: Text -> Value -> Value
+mkSingleChoice key val =
+  Json.object
+    [ ("tag", String key),
+      ("value", val)
+    ]
+
+-- | Encode UTCTime as 'String', as an ISO8601 timestamp with timezone (@yyyy-mm-ddThh:mm:ss[.sss]Z@)
+mkUtcTime :: UTCTime -> Value
+mkUtcTime = String . stringToText . ISO8601.iso8601Show @UTCTime
+
+mkMaybe :: (a -> Value) -> Maybe a -> Value
+mkMaybe f ma = ma <&> f & toJSON @(Maybe Value)