Merge pull request #98 from syaiful6/avar-op

add tryTakeVar and tryPeekVar

src/Control/Monad/Aff/AVar.purs

@@ -9,18 +9,21 @@ module Control.Monad.Aff.AVar
   , putVar
   , modifyVar
   , killVar
+  , tryTakeVar
+  , tryPeekVar
   , module Exports
   ) where
 
 import Prelude
 
 import Control.Monad.Aff (Aff, nonCanceler)
 import Control.Monad.Aff.Internal (AVar) as Exports
-import Control.Monad.Aff.Internal (AVBox, AVar, _killVar, _putVar, _takeVar, _peekVar, _makeVar)
+import Control.Monad.Aff.Internal (AVBox, AVar, _killVar, _putVar, _takeVar, _peekVar, _makeVar, _tryTakeVar, _tryPeekVar)
 import Control.Monad.Eff (kind Effect)
 import Control.Monad.Eff.Exception (Error())
 
-import Data.Function.Uncurried (runFn3, runFn2)
+import Data.Function.Uncurried (runFn4, runFn3, runFn2)
+import Data.Maybe (Maybe(..))
 
 import Unsafe.Coerce (unsafeCoerce)
 
@@ -43,10 +46,20 @@ makeVar' a = do
 takeVar :: forall e a. AVar a -> AffAVar e a
 takeVar q = fromAVBox $ runFn2 _takeVar nonCanceler q
 
+-- | A variant of `takeVar` which return immediately if the asynchronous avar
+-- | was empty. Nothing if the avar empty and `Just a` if the avar have contents `a`.
+tryTakeVar :: forall e a. AVar a -> AffAVar e (Maybe a)
+tryTakeVar q = fromAVBox $ runFn4 _tryTakeVar Nothing Just nonCanceler q
+
 -- | Reads a value from the asynchronous var but does not consume it.
 peekVar :: forall e a. AVar a -> AffAVar e a
 peekVar q = fromAVBox $ runFn2 _peekVar nonCanceler q
 
+-- | A variant of `peekVar` which return immediately when the asynchronous avar
+-- | was empty. Nothing if the avar empty and `Just a` if the avar have contents `a`.
+tryPeekVar :: forall e a. AVar a -> AffAVar e (Maybe a)
+tryPeekVar q = fromAVBox $ runFn4 _tryPeekVar Nothing Just nonCanceler q
+
 -- | Puts a new value into the asynchronous avar. If the avar has
 -- | been killed, this will result in an error.
 putVar :: forall e a. AVar a -> a -> AffAVar e Unit

src/Control/Monad/Aff/Internal.js

@@ -25,6 +25,21 @@ exports._takeVar = function (nonCanceler, avar) {
   };
 };
 
+exports._tryTakeVar = function (nothing, just, nonCanceler, avar) {
+  return function (success, error) {
+    if (avar.error !== undefined) {
+      error(avar.error);
+    } else if (avar.producers.length > 0) {
+      avar.producers.shift()(function (x) {
+        return success(just(x));
+      }, error);
+    } else {
+      success(nothing);
+    }
+    return nonCanceler;
+  };
+};
+
 exports._peekVar = function (nonCanceler, avar) {
   return function (success, error) {
     if (avar.error !== undefined) {
@@ -38,6 +53,21 @@ exports._peekVar = function (nonCanceler, avar) {
   };
 };
 
+exports._tryPeekVar = function (nothing, just, nonCanceler, avar) {
+  return function (success, error) {
+    if (avar.error !== undefined) {
+      error(avar.error);
+    } else if (avar.producers.length > 0) {
+      avar.producers[0](function (x) {
+        return success(just(x));
+      }, error);
+    } else {
+      success(nothing);
+    }
+    return nonCanceler;
+  };
+};
+
 exports._putVar = function (nonCanceler, avar, a) {
   return function (success, error) {
     if (avar.error !== undefined) {

src/Control/Monad/Aff/Internal.purs

@@ -3,7 +3,9 @@ module Control.Monad.Aff.Internal
   , AVar
   , _makeVar
   , _takeVar
+  , _tryTakeVar
   , _peekVar
+  , _tryPeekVar
   , _putVar
   , _killVar
   ) where
@@ -12,7 +14,8 @@ import Prelude
 
 import Control.Monad.Eff.Exception (Error)
 
-import Data.Function.Uncurried (Fn2, Fn3)
+import Data.Maybe (Maybe)
+import Data.Function.Uncurried (Fn2, Fn3, Fn4)
 
 foreign import data AVar :: Type -> Type
 
@@ -22,8 +25,12 @@ foreign import _makeVar :: forall c a. c -> AVBox (AVar a)
 
 foreign import _takeVar :: forall c a. Fn2 c (AVar a) (AVBox a)
 
+foreign import _tryTakeVar :: forall c a. Fn4 (forall x. Maybe x) (forall x. x -> Maybe x) c (AVar a) (AVBox (Maybe a))
+
 foreign import _peekVar :: forall c a. Fn2 c (AVar a) (AVBox a)
 
+foreign import _tryPeekVar :: forall c a. Fn4 (forall x. Maybe x) (forall x. x -> Maybe x) c (AVar a) (AVBox (Maybe a))
+
 foreign import _putVar :: forall c a. Fn3 c (AVar a) a (AVBox Unit)
 
 foreign import _killVar :: forall c a. Fn3 c (AVar a) Error (AVBox Unit)

test/Test/Main.purs

@@ -4,7 +4,7 @@ import Prelude
 
 import Control.Alt ((<|>))
 import Control.Monad.Aff (Aff, runAff, makeAff, launchAff, delay, forkAff, forkAll, Canceler(..), cancel, attempt, finally, apathize)
-import Control.Monad.Aff.AVar (AVAR, makeVar, makeVar', putVar, modifyVar, takeVar, peekVar, killVar)
+import Control.Monad.Aff.AVar (AVAR, makeVar, makeVar', putVar, modifyVar, takeVar, peekVar, killVar, tryTakeVar, tryPeekVar)
 import Control.Monad.Aff.Console (CONSOLE, log)
 import Control.Monad.Eff (Eff)
 import Control.Monad.Eff.Console (log) as Eff
@@ -129,6 +129,42 @@ test_killVar = do
   e <- attempt $ takeVar v
   either (const $ log "Success: Killed queue dead") (const $ log "Failure: Oh noes, queue survived!") e
 
+test_tryTakeVar :: TestAVar Unit
+test_tryTakeVar = do
+  timeout (Milliseconds 1000.0) do
+    v <- makeVar
+    x <- tryTakeVar v
+    case x of
+      Nothing -> log $ "Success: trying take an empty var"
+      Just _  -> throwError $ error $ "Failure: Oh noes, take an empty var should return Nothing"
+
+  timeout (Milliseconds 1000.0) do
+    v <- makeVar
+    b <- tryTakeVar v
+    putVar v 1.0
+    a <- tryTakeVar v
+    when (a /= Just 1.0 || a == b) do
+      throwError $ error ("Failure: Oh noes, tryTakeVar should take var if it available, value: " <> show a)
+    log $ "Success: value taken by tryTakeVar " <> show a
+
+test_tryPeekVar :: TestAVar Unit
+test_tryPeekVar = do
+  timeout (Milliseconds 1000.0) do
+    v <- makeVar
+    x <- tryPeekVar v
+    case x of
+      Nothing -> log $ "Success: try peek var return immediately"
+      Just _  -> throwError $ error $ "Failure: tryPeekVar return Just when peek an empty var"
+
+  timeout (Milliseconds 1000.0) do
+    v <- makeVar
+    putVar v 100.0
+    a <- tryPeekVar v
+    b <- takeVar v
+    when (a /= Just b) do
+      throwError (error "Something horrible went wrong - peeked var is not equal to taken var")
+    log ("Success: Try Peeked value not consumed")
+
 test_finally :: TestAVar Unit
 test_finally = do
   v <- makeVar
@@ -311,6 +347,12 @@ main = do
     log "Testing AVar killVar"
     test_killVar
 
+    log "Testing AVar - tryTakeVar"
+    test_tryTakeVar
+
+    log "Testing AVar - tryPeekVar"
+    test_tryPeekVar
+
     log "Testing finally"
     test_finally