add Lazy instance for Aff (#142)

* add Lazy instance for Aff

* change Lazy tests so result is returned from fix

* move test_lazy after kill and avar tests

src/Control/Monad/Aff.purs

@@ -36,6 +36,7 @@ import Prelude
 import Control.Alt (class Alt)
 import Control.Alternative (class Alternative)
 import Control.Apply (lift2)
+import Control.Lazy (class Lazy)
 import Control.Monad.Eff (Eff, kind Effect)
 import Control.Monad.Eff.Class (class MonadEff, liftEff)
 import Control.Monad.Eff.Exception (Error, EXCEPTION, error)
@@ -110,6 +111,9 @@ instance monadErrorAff ∷ MonadError Error (Aff eff) where
 instance monadEffAff ∷ MonadEff eff (Aff eff) where
   liftEff = _liftEff
 
+instance lazyAff ∷ Lazy (Aff eff a) where
+  defer f = pure unit >>= f
+
 -- | Applicative for running parallel effects. Any `Aff` can be coerced to a
 -- | `ParAff` and back using the `Parallel` class.
 foreign import data ParAff ∷ # Effect → Type → Type

test/Test/Main.purs

@@ -3,6 +3,7 @@ module Test.Main where
 import Prelude
 
 import Control.Alt ((<|>))
+import Control.Lazy (fix)
 import Control.Monad.Aff (Aff, Canceler(..), runAff_, launchAff, makeAff, try, bracket, generalBracket, delay, forkAff, suspendAff, joinFiber, killFiber, never, supervise, Error, error, message)
 import Control.Monad.Aff.AVar (AVAR, makeEmptyVar, takeVar, putVar)
 import Control.Monad.Aff.Compat as AC
@@ -632,6 +633,26 @@ test_scheduler_size = assert "scheduler" do
   _ ← traverse joinFiber =<< traverse forkAff (Array.replicate 100000 (modifyRef ref (add 1)))
   eq 100000 <$> readRef ref
 
+test_lazy ∷ ∀ eff. TestAff eff Unit
+test_lazy = assert "Lazy Aff" do
+  varA ← makeEmptyVar
+  varB ← makeEmptyVar
+  fiberA <- forkAff $ fix \loop -> do
+    a <- takeVar varA
+    putVar (a + 1) varB
+    loop
+  fiberB <- forkAff $ fix \loop -> do
+    b <- takeVar varB
+    if (b > 100)
+      then do
+        killFiber (error "finished") fiberA
+        pure "done"
+      else do
+        putVar (b + 1) varA
+        loop
+  putVar 0 varA
+  eq "done" <$> joinFiber fiberB
+
 main ∷ TestEff () Unit
 main = do
   test_pure
@@ -670,6 +691,7 @@ main = do
     test_kill_parallel_alt
     test_kill_parallel_alt_finalizer
     test_avar_order
+    test_lazy
     test_efffn
     test_fiber_map
     test_fiber_apply