Add a test case for ParserK.toParser

test/Streamly/Test/Data/ParserK.hs

@@ -795,6 +795,29 @@ toParser = do
             r3 <- runP1 [0,1::Int]
             fromRight undefined r3 `shouldBe` 0
 
+    -- NOTE: Without fusionBreaker this test would pass even if toParser has
+    -- incorrect implementation because of fusion rules.
+    let p2 =
+            ParserK.toParser
+                $ fusionBreaker
+                $ ParserK.adapt (Parser.takeWhile (<= 3) FL.toList)
+        runP2 xs = Stream.parseBreak p2 (Stream.fromList xs)
+    describe "toParser . adapt" $ do
+        it "(<= 3) for [1, 2, 3, 4, 5]" $ do
+            (a, b) <- runP2 ([1, 2, 3, 4, 5] :: [Int])
+            fromRight undefined a `shouldBe` [1, 2, 3]
+            rest <- Stream.toList b
+            rest `shouldBe` [4, 5]
+        it "(<= 3) for [1, 2, 3]" $ do
+            (a, b) <- runP2 ([1, 2, 3] :: [Int])
+            fromRight undefined a `shouldBe` [1, 2, 3]
+            rest <- Stream.toList b
+            rest `shouldBe` []
+
+{-# NOINLINE fusionBreaker #-}
+fusionBreaker :: a -> a
+fusionBreaker = id
+
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 -- Main
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