Allow the construction of self-referential Refs

src/Effect/Ref.js

@@ -6,6 +6,14 @@ exports.new = function (val) {
   };
 };
 
+exports.newWithSelf = function (f) {
+  return function () {
+    var ref = { value: null };
+    ref.value = f(ref);
+    return ref;
+  };
+};
+
 exports.read = function (ref) {
   return function () {
     return ref.value;

src/Effect/Ref.purs

@@ -22,6 +22,7 @@
 module Effect.Ref
   ( Ref
   , new
+  , newWithSelf
   , read
   , modify'
   , modify
@@ -42,6 +43,10 @@ type role Ref representational
 -- | Create a new mutable reference containing the specified value.
 foreign import new :: forall s. s -> Effect (Ref s)
 
+-- | Create a new mutable reference containing a value that can refer to the
+-- | `Ref` being created.
+foreign import newWithSelf :: forall s. (Ref s -> s) -> Effect (Ref s)
+
 -- | Read the current value of a mutable reference.
 foreign import read :: forall s. Ref s -> Effect s
 

test/Main.purs

@@ -28,3 +28,27 @@ main = do
   -- now it is 2 when we read out the value
   curr3 <- Ref.read ref
   assertEqual { actual: curr3, expected: 2 }
+
+  selfRef
+
+newtype RefBox = RefBox { ref :: Ref.Ref RefBox, value :: Int }
+
+selfRef :: Effect Unit
+selfRef = do
+  -- Create a self-referential `Ref`
+  ref <- Ref.newWithSelf \ref -> RefBox { ref, value: 0 }
+
+  -- Grab the `Ref` from within the `Ref`
+  ref' <- Ref.read ref <#> \(RefBox r) -> r.ref
+
+  -- Modify the `ref` and check that value in `ref'` changes
+  Ref.modify_ (\(RefBox r) -> RefBox (r { value = 1 })) ref
+  assertEqual
+    <<< { expected: 1, actual: _ }
+    =<< (Ref.read ref' <#> \(RefBox { value }) -> value)
+
+  -- Modify the `ref'` and check that value in `ref` changes
+  Ref.modify_ (\(RefBox r) -> RefBox (r { value = 2 })) ref'
+  assertEqual
+    <<< { expected: 2, actual: _ }
+    =<< (Ref.read ref <#> \(RefBox { value }) -> value)