Implement constObservable

src/lib/Qd/Observable.hs

@@ -19,6 +19,7 @@ module Qd.Observable (
   joinObservableEither,
   mergeObservable,
   mergeObservable',
+  constObservable,
   FnObservable(..),
 ) where
 
@@ -249,6 +250,17 @@ instance Observable v (FnObservable v) where
   }
 
 
+newtype ConstObservable a = ConstObservable (Maybe a)
+instance Observable a (ConstObservable a) where
+  getValue (ConstObservable x) = return x
+  subscribe (ConstObservable x) callback = do
+    callback (Current, x)
+    return $ SubscriptionHandle { unsubscribe = return () }
+-- | Create an observable that contains a constant value.
+constObservable :: Maybe a -> SomeObservable a
+constObservable = SomeObservable . ConstObservable
+
+
 -- TODO implement
 _cacheObservable :: Observable v o => o -> SomeObservable v
 _cacheObservable = SomeObservable