Merge pull request #2508 from github/more-flags

Use a Data.Flag rather than a Bool for force-indexing checks.

src/Data/Flag.hs

@@ -1,4 +1,4 @@
-{-# LANGUAGE RankNTypes, KindSignatures #-}
+{-# LANGUAGE DerivingStrategies, GeneralizedNewtypeDeriving, RankNTypes, KindSignatures #-}
 
 -- | -- This technique is due to Oleg Grenrus: <http://oleg.fi/gists/posts/2019-03-21-flag.html>
 -- The implementation is clean-room due to unclear licensing of the original post.
@@ -10,7 +10,7 @@ module Data.Flag
   , choose
   ) where
 
-import Data.Coerce
+import Prologue
 
 -- | To declare a new flag, declare a singly-inhabited type:
 -- @data MyFlag = MyFlag@
@@ -19,6 +19,8 @@ import Data.Coerce
 -- working with multiple flag values in flight, as the 'toBool' deconstructor provides a witness
 -- that you really want the given semantic flag value from the flag datum.
 newtype Flag (t :: *) = Flag Bool
+  deriving stock (Eq, Show)
+  deriving newtype NFData
 
 -- | The constructor for a 'Flag'. You specify @t@ with a visible type application.
 flag :: t -> Bool -> Flag t