Add UnsafeRefEq and UnsafeRefEqFallback newtypes with Eq instances

bower.json

@@ -19,5 +19,8 @@
   "devDependencies": {
     "purescript-console": "^4.0.0",
     "purescript-exceptions": "^4.0.0"
+  },
+  "dependencies": {
+    "purescript-prelude": "^4.1.0"
   }
 }

src/Unsafe/Reference.purs

@@ -1,11 +1,32 @@
 module Unsafe.Reference
   ( unsafeRefEq
   , reallyUnsafeRefEq
+  , UnsafeRefEq (..)
+  , UnsafeRefEqFallback (..)
   ) where
 
+import Prelude
+
 -- | Compares two values of the same type using strict (`===`) equality.
 unsafeRefEq :: forall a. a -> a -> Boolean
 unsafeRefEq = reallyUnsafeRefEq
 
 -- | Compares two values of different types using strict (`===`) equality.
 foreign import reallyUnsafeRefEq :: forall a b. a -> b -> Boolean
+
+-- | The `Eq` instance is defined by `unsafeRefEq`.
+newtype UnsafeRefEq a = UnsafeRefEq a
+
+instance eqUnsafeRefEq :: Eq (UnsafeRefEq a) where
+  eq (UnsafeRefEq l) (UnsafeRefEq r) = unsafeRefEq l r
+
+-- | The `Eq` instance first checks `unsafeRefEq`, if `false` falls back to
+-- | the underlying `Eq` instance.
+newtype UnsafeRefEqFallback a = UnsafeRefEqFallback a
+
+instance eqUnsafeRefEqFallback ::
+  Eq a =>
+  Eq (UnsafeRefEqFallback a) where
+    eq (UnsafeRefEqFallback l) (UnsafeRefEqFallback r) =
+      unsafeRefEq l r || l == r
+

test/Main.purs

@@ -5,7 +5,7 @@ import Prelude
 import Effect (Effect)
 import Effect.Exception (throwException, error)
 
-import Unsafe.Reference (unsafeRefEq, reallyUnsafeRefEq)
+import Unsafe.Reference (unsafeRefEq, reallyUnsafeRefEq, UnsafeRefEq(..), UnsafeRefEqFallback(..))
 
 data Foo = Foo String
 data Bar = Bar String
@@ -17,6 +17,10 @@ assert :: Boolean -> String -> Effect Unit
 assert true _ = pure unit
 assert _ desc = throwException (error desc)
 
+data X = Y Unit | Z
+
+derive instance eqX :: Eq X
+
 main :: Effect Unit
 main = do
   let
@@ -32,3 +36,7 @@ main = do
   assert (unsafeRefEq foo3 foo4) "unsafeRefEq newtype"
   assert (reallyUnsafeRefEq foo3 bar2) "reallyUnsafeRefEq newtype"
   assert (not reallyUnsafeRefEq foo1 bar1) "not reqallyUnsafeRefEq data"
+  assert (UnsafeRefEq 0 == UnsafeRefEq 0) "UnsafeRefEq eq with identical values"
+  assert (UnsafeRefEq (Y unit) /= UnsafeRefEq (Y unit)) "UnsafeRefEq not eq with non-identical values"
+  assert (UnsafeRefEqFallback (Y unit) == UnsafeRefEqFallback (Y unit)) "UnsafeRefEqFallback eq with non-identical but equal values"
+  assert (UnsafeRefEqFallback (Y unit) /= UnsafeRefEqFallback Z) "UnsafeRefEqFallback not eq with non equal values"