Merge pull request #20 from epost/patch-1

Add `finally`.

Author: jdegoes

examples/src/Examples.purs

@@ -60,6 +60,21 @@ module Examples where
     e <- attempt $ takeVar v
     either (const $ trace "Success: Killed queue dead") (const $ trace "Failure: Oh noes, queue survived!") e
 
+  test_finally :: TestAVar _
+  test_finally = do
+    v <- makeVar
+    finally
+      (putVar v 0)
+      (putVar v 2)
+    apathize $ finally
+      (throwError (error "poof!") *> putVar v 666) -- this putVar should not get executed
+      (putVar v 40)
+    n1 <- takeVar v
+    n2 <- takeVar v
+    n3 <- takeVar v
+    trace $ if n1 + n2 + n3 == 42 then "Success: effects amount to 42."
+                                  else "Failure: Expected 42."
+
   test_parRace :: TestAVar _
   test_parRace = do
     s <- runPar (Par (later' 100 $ pure "Success: Early bird got the worm") <|> 
@@ -134,6 +149,9 @@ module Examples where
     trace "Testing AVar killVar"
     test_killVar
 
+    trace "Testing finally"
+    test_finally
+
     trace "Testing Par (<|>)"
     test_parRace
 

src/Control/Monad/Aff.purs

@@ -6,6 +6,7 @@ module Control.Monad.Aff
   , attempt
   , cancel
   , cancelWith
+  , finally
   , forkAff
   , later
   , later'
@@ -90,6 +91,13 @@ module Control.Monad.Aff
   later' :: forall e a. Number -> Aff e a -> Aff e a
   later' n aff = runFn3 _setTimeout nonCanceler n aff
 
+  -- | Compute `aff1`, followed by `aff2` regardless of whether `aff1` terminated successfully.
+  finally :: forall e a b. Aff e a -> Aff e b -> Aff e a
+  finally aff1 aff2 = do
+    x <- attempt aff1
+    aff2
+    either throwError pure x
+
   -- | Forks the specified asynchronous computation so subsequent computations
   -- | will not block on the result of the computation.
   -- |