Release polysemy 0.2.1.0

README.md

@@ -59,7 +59,7 @@ Or by adding `-fplugin=Polysemy.Plugin` to your package.yaml/.cabal file `ghc-op
 
 
 <sup><a name="fn1">1</a></sup>: Unfortunately this is not true in GHC 8.6.3, but
-will be true as soon as [my patch](https://gitlab.haskell.org/ghc/ghc/merge_requests/668/) lands.
+will be true in GHC 8.10.1.
 
 
 ## Examples

package.yaml

@@ -1,5 +1,5 @@
 name:                polysemy
-version:             0.2.0.0
+version:             0.2.1.0
 github:              "isovector/polysemy"
 license:             BSD3
 author:              "Sandy Maguire"

polysemy-plugin/ChangeLog.md

@@ -1,5 +1,11 @@
 # Changelog for polysemy-plugin
 
+## 0.2.1.0 (2019-05-27)
+
+- Fixed a bug in the `Alternative` instance for `Sem`, where it would choose the
+    *last* success instead of the first
+- Added `MonadPlus` and `MonadFail` instances for `Sem`
+
 ## 0.2.0.0 (2019-05-23)
 
 - Fixed a serious bug where the changes from 0.1.0.1 broke most real-world

polysemy.cabal

@@ -4,10 +4,10 @@ cabal-version: 1.12
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: 9a2044113434528f0539fb410b356387676cc468eefb618a666c1246379b43da
+-- hash: d24941def9c13073c8c65e68d2cb76cddd1df3fad645148e5dfffa914a749250
 
 name:           polysemy
-version:        0.2.0.0
+version:        0.2.1.0
 synopsis:       Higher-order, low-boilerplate, zero-cost free monads.
 description:    Please see the README on GitHub at <https://github.com/isovector/polysemy#readme>
 category:       Language
@@ -88,6 +88,7 @@ test-suite polysemy-test
   type: exitcode-stdio-1.0
   main-is: Main.hs
   other-modules:
+      AlternativeSpec
       FusionSpec
       HigherOrderSpec
       OutputSpec

src/Polysemy/Internal.hs

@@ -27,6 +27,8 @@ module Polysemy.Internal
   ) where
 
 import Control.Applicative
+import Control.Monad
+import Control.Monad.Fail
 import Control.Monad.Fix
 import Control.Monad.IO.Class
 import Data.Functor.Identity
@@ -216,6 +218,13 @@ instance (Member NonDet r) => Alternative (Sem r) where
       True  -> b
   {-# INLINE (<|>) #-}
 
+instance (Member NonDet r) => MonadPlus (Sem r) where
+  mzero = empty
+  mplus = (<|>)
+
+instance (Member NonDet r) => MonadFail (Sem r) where
+  fail = const empty
+
 
 ------------------------------------------------------------------------------
 -- | This instance will only lift 'IO' actions. If you want to lift into some

src/Polysemy/NonDet.hs

@@ -62,6 +62,6 @@ runNonDet (Sem m) = Sem $ \k -> runNonDetC $ m $ \u ->
       foldr cons nil z
     Right (Yo Empty _ _ _) -> empty
     Right (Yo (Choose ek) s _ y) -> do
-      z <- pure (ek True) <|> pure (ek False)
+      z <- pure (ek False) <|> pure (ek True)
       pure $ y $ z <$ s
 

test/AlternativeSpec.hs

@@ -0,0 +1,34 @@
+module AlternativeSpec where
+
+import Polysemy
+import Polysemy.NonDet
+import Test.Hspec
+import Control.Applicative
+
+semFail :: Member NonDet r => Maybe Bool -> Sem r Bool
+semFail mb = do
+  Just b <- pure mb
+  pure b
+
+
+runAlt :: Alternative f => Sem '[NonDet] a -> f a
+runAlt = run . runNonDet
+
+
+spec :: Spec
+spec = do
+  describe "Alternative instance" $ do
+    it "should choose the first branch" $ do
+      runAlt (pure '1' <|> pure '2') `shouldBe` (Just '1')
+    it "should failover" $ do
+      runAlt (empty <|> pure '2') `shouldBe` (Just '2')
+      runAlt (pure '1' <|> empty) `shouldBe` (Just '1')
+
+  describe "MonadFail instance" $ do
+    it "should call empty via fail" $ do
+      runAlt (semFail Nothing) `shouldBe` Nothing
+      runAlt (semFail Nothing) `shouldBe` []
+    it "should work fine for non-failing patterns" $ do
+      runAlt (semFail $ Just True) `shouldBe` Just True
+      runAlt (semFail $ Just False) `shouldBe` [False]
+