Add an instance for tagging values

source/library/Witch/Instances.hs

@@ -23,6 +23,7 @@ import qualified Data.Map as Map
 import qualified Data.Ratio as Ratio
 import qualified Data.Sequence as Seq
 import qualified Data.Set as Set
+import qualified Data.Tagged as Tagged
 import qualified Data.Text as Text
 import qualified Data.Text.Encoding as Text
 import qualified Data.Text.Lazy as LazyText
@@ -1272,6 +1273,11 @@ instance From.From Time.NominalDiffTime Time.CalendarDiffTime where
 instance From.From Time.ZonedTime Time.UTCTime where
   from = Time.zonedTimeToUTC
 
+-- Tagged
+
+-- | Uses @coerce@. Essentially the same as 'Tagged.Tagged'.
+instance From.From a (Tagged.Tagged t a)
+
 --
 
 realFloatToRational ::

source/test-suite/Main.hs

@@ -20,6 +20,7 @@ import qualified Data.Map as Map
 import qualified Data.Ratio as Ratio
 import qualified Data.Sequence as Seq
 import qualified Data.Set as Set
+import qualified Data.Tagged as Tagged
 import qualified Data.Text as Text
 import qualified Data.Text.Lazy as LazyText
 import qualified Data.Time as Time
@@ -2070,6 +2071,11 @@ spec = describe "Witch" $ do
       it "works" $ do
         f (Time.ZonedTime (Time.LocalTime (Time.ModifiedJulianDay 0) (Time.TimeOfDay 0 0 0)) Time.utc) `shouldBe` Time.UTCTime (Time.ModifiedJulianDay 0) 0
 
+    describe "From a (Tagged t a)" $ do
+      let f = Witch.from @Bool @(Tagged.Tagged () Bool)
+      it "works" $ do
+        f False `shouldBe` Tagged.Tagged False
+
 newtype Age
   = Age Int.Int8
   deriving (Eq, Show)

witch.cabal

@@ -26,6 +26,7 @@ common library
     , base >= 4.10 && < 4.18
     , bytestring >= 0.10.8 && < 0.12
     , containers >= 0.5.10 && < 0.7
+    , tagged >= 0.8.6 && < 0.9
     , text >= 1.2.3 && < 1.3 || >= 2.0 && < 2.1
     , time >= 1.9.1 && < 1.13
   default-language: Haskell2010