[WIP] Lazy instance

bower.json

@@ -17,7 +17,12 @@
     "package.json"
   ],
   "dependencies": {
+    "purescript-control": "matthewleon/purescript-control#lazy-function",
     "purescript-monoid": "^3.0.0",
     "purescript-prelude": "^3.0.0"
+  },
+  "devDependencies": {
+    "purescript-eff": "^3.1.0",
+    "purescript-console": "^3.0.0"
   }
 }

src/Data/Maybe.purs

@@ -5,6 +5,7 @@ import Prelude
 import Control.Alt (class Alt)
 import Control.Alternative (class Alternative)
 import Control.Extend (class Extend)
+import Control.Lazy (class Lazy, defer)
 import Control.MonadZero (class MonadZero)
 import Control.Plus (class Plus)
 
@@ -201,6 +202,9 @@ instance boundedMaybe :: Bounded a => Bounded (Maybe a) where
   top = Just top
   bottom = Nothing
 
+instance lazyMaybe :: Lazy a => Lazy (Maybe a) where
+  defer f = ((map (defer <<< const)) <$> f) unit
+
 -- | The `Show` instance allows `Maybe` values to be rendered as a string with
 -- | `show` whenever there is an `Show` instance for the type the `Maybe`
 -- | contains.

test/Main.purs

@@ -0,0 +1,21 @@
+module Test.Main where
+
+import Prelude
+import Control.Lazy as Z
+import Control.Monad.Eff (Eff)
+import Control.Monad.Eff.Console (CONSOLE, logShow)
+import Data.Maybe (Maybe(..))
+
+diverge :: forall a. a -> a
+diverge = Z.fix id
+
+{-
+explosiveMaybe :: Maybe Unit
+explosiveMaybe = Just diverge <*> Just unit
+-}
+
+deferredMaybeExplosion :: Maybe Unit
+deferredMaybeExplosion = Z.defer \_ -> Just diverge <*> Just unit
+
+main :: Eff (console :: CONSOLE) Unit
+main = logShow deferredMaybeExplosion