Address come comments

Author: bezirg

plutus-tx/src/PlutusTx/Eq/Class.hs

@@ -6,19 +6,16 @@ module PlutusTx.Eq.Class
 import PlutusTx.Bool
 import PlutusTx.Builtins qualified as Builtins
 
-{- HLINT ignore -}
-
 infix 4 ==
 
--- Copied from the GHC definition
+{- | The 'Eq' class defines equality ('==').
 
--- | The 'Eq' class defines equality ('==').
+(/=) deliberately omitted, to make this a one-method class which has a
+simpler representation
+-}
 class Eq a where
   (==) :: a -> a -> Bool
 
--- (/=) deliberately omitted, to make this a one-method class which has a
--- simpler representation
-
 infix 4 /=
 (/=) :: (Eq a) => a -> a -> Bool
 x /= y = not (x == y)

plutus-tx/src/PlutusTx/Eq/TH.hs

@@ -10,6 +10,10 @@ import Language.Haskell.TH as TH
 import Language.Haskell.TH.Datatype as TH
 import Data.Deriving.Internal (varTToName)
 
+{- | derive a PlutusTx.Eq instance for a datatype/newtype, similar to Haskell's `deriving stock Eq`.
+
+One shortcoming compared to Haskell's deriving is that you cannot `PlutusTx.deriveEq` for polymorphic phantom types.
+-}
 deriveEq :: TH.Name -> TH.Q [TH.Dec]
 deriveEq name = do
   TH.DatatypeInfo

plutus-tx/test/Eq/Spec.hs

@@ -23,6 +23,10 @@ data SomeLargeADT a b c d e =
   deriving stock HS.Eq
 deriveEq ''SomeLargeADT
 
+newtype PhantomADT e = PhantomADT ()
+  deriving stock HS.Eq
+deriveEq ''PhantomADT
+
 eqTests :: TestTree
 eqTests =
   let v1 :: SomeLargeADT () BuiltinString () () () = SomeLargeADT1 1 () Tx.True "foobar" () ()
@@ -36,6 +40,8 @@ eqTests =
     [testCase "reflexive1" $ (v1 Tx.== v1) @?= (v1 HS.== v1)
     , testCase "reflexive2" $ (v2 Tx.== v2) @?= (v2 HS.== v2)
     , testCase "reflexive3" $ (v3 Tx.== v3) @?= (v3 HS.== v3)
+    -- currently does not work with polymorphic phantom types, remove the type annotation when support is added
+    , testCase "phantom" $ (PhantomADT @() () Tx.== PhantomADT ()) @?= (PhantomADT () HS.== PhantomADT ())
     , testCase "shortcircuit" $ (v3 Tx.== v3Error1) @?= (v3 Tx.== v3Error1) -- should not throw an error
     , testCase "throws" $ try @SomeException (evaluate $ v3 Tx.== v3Error2) >>= assertBool "did not throw error" . isLeft -- should throw erro
     ]