Fix Semigroup instance for Exp Maybe

Fixes #517

accelerate.cabal

@@ -543,6 +543,7 @@ library
         Data.Array.Accelerate.Test.NoFib.Issues.Issue436
         Data.Array.Accelerate.Test.NoFib.Issues.Issue437
         Data.Array.Accelerate.Test.NoFib.Issues.Issue439
+        Data.Array.Accelerate.Test.NoFib.Issues.Issue517
 
   else
     cpp-options:

src/Data/Array/Accelerate/Data/Maybe.hs

@@ -135,7 +135,7 @@ instance (Monoid (Exp a), Elt a) => Monoid (Exp (Maybe a)) where
 
 instance (Semigroup (Exp a), Elt a) => Semigroup (Exp (Maybe a)) where
   ma <> mb = cond (isNothing ma) mb
-           $ cond (isNothing mb) mb
+           $ cond (isNothing mb) ma
            $ lift (Just (fromJust ma <> fromJust mb))
 
 instance (Lift Exp a, Elt (Plain a)) => Lift Exp (Maybe a) where

src/Data/Array/Accelerate/Test/NoFib/Issues.hs

@@ -37,6 +37,7 @@ module Data.Array.Accelerate.Test.NoFib.Issues (
   module Data.Array.Accelerate.Test.NoFib.Issues.Issue436,
   module Data.Array.Accelerate.Test.NoFib.Issues.Issue437,
   module Data.Array.Accelerate.Test.NoFib.Issues.Issue439,
+  module Data.Array.Accelerate.Test.NoFib.Issues.Issue517,
 
 ) where
 
@@ -67,6 +68,7 @@ import Data.Array.Accelerate.Test.NoFib.Issues.Issue427
 import Data.Array.Accelerate.Test.NoFib.Issues.Issue436
 import Data.Array.Accelerate.Test.NoFib.Issues.Issue437
 import Data.Array.Accelerate.Test.NoFib.Issues.Issue439
+import Data.Array.Accelerate.Test.NoFib.Issues.Issue517
 
 
 test_issues :: RunN -> TestTree
@@ -96,5 +98,6 @@ test_issues runN =
     , test_issue436 runN
     , test_issue437 runN
     , test_issue439 runN
+    , test_issue517 runN
     ]
 

src/Data/Array/Accelerate/Test/NoFib/Issues/Issue517.hs

@@ -0,0 +1,45 @@
+{-# LANGUAGE RankNTypes #-}
+-- |
+-- Module      : Data.Array.Accelerate.Test.NoFib.Issues.Issue439
+-- Copyright   : [2009..2020] The Accelerate Team
+-- License     : BSD3
+--
+-- Maintainer  : Trevor L. McDonell <trevor.mcdonell@gmail.com>
+-- Stability   : experimental
+-- Portability : non-portable (GHC extensions)
+--
+-- https://github.com/AccelerateHS/accelerate/issues/517
+--
+
+module Data.Array.Accelerate.Test.NoFib.Issues.Issue517 (
+
+  test_issue517
+
+) where
+
+import Data.Array.Accelerate                              as A
+import Data.Array.Accelerate.Data.Semigroup               as A
+import Data.Array.Accelerate.Test.NoFib.Base
+
+import Test.Tasty
+import Test.Tasty.HUnit
+
+
+test_issue517 :: RunN -> TestTree
+test_issue517 runN
+  = testCase "517"
+  $ e1 @=? runN t1
+
+type Tup5 a = (a, a, a, a, a)
+
+e1 :: Scalar (Tup5 (Maybe (Max Float)))
+e1 = fromList Z [(Nothing, Just 2, Just 3, Just 5, Just 7)]
+
+t1 :: Acc (Scalar (Tup5 (Maybe (Max Float))))
+t1 = unit $
+  T5 (Nothing_ <> Nothing_)
+     (Nothing_ <> Just_ 2)
+     (Just_ 3 <> Nothing_)
+     (Just_ 4 <> Just_ 5)
+     (Just_ 7 <> Just_ 6)
+