[Git][ghc/ghc][master] 2 commits: Add test for T18574

[Marge Bot]

 Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC


Commits:
0bb02873 by Sylvain Henry at 2020-10-01T18:34:53-04:00
Add test for T18574

- - - - -
e393f213 by Sylvain Henry at 2020-10-01T18:34:53-04:00
Allow fusion with catMaybes (#18574)

Metric Decrease:
   T18574

- - - - -


5 changed files:

- libraries/base/Data/Maybe.hs
- libraries/base/changelog.md
- + testsuite/tests/perf/should_run/T18574.hs
- + testsuite/tests/perf/should_run/T18574.stdout
- testsuite/tests/perf/should_run/all.T


Changes:

=====================================
libraries/base/Data/Maybe.hs
=====================================
@@ -256,8 +256,8 @@ listToMaybe = foldr (const . Just) Nothing
 -- >>> catMaybes $ [readMaybe x :: Maybe Int | x <- ["1", "Foo", "3"] ]
 -- [1,3]
 --
-catMaybes              :: [Maybe a] -> [a]
-catMaybes ls = [x | Just x <- ls]
+catMaybes :: [Maybe a] -> [a]
+catMaybes = mapMaybe id -- use mapMaybe to allow fusion (#18574)
 
 -- | The 'mapMaybe' function is a version of 'map' which can throw
 -- out elements.  In particular, the functional argument returns


=====================================
libraries/base/changelog.md
=====================================
@@ -32,6 +32,9 @@
   * Add `MonadFix` and `MonadZip` instances for `Complex`
 
   * Add `Ix` instances for tuples of size 6 through 15
+
+  * `catMaybes` is now implemented using `mapMaybe`, so that it is both a "good
+    consumer" and "good producer" for list-fusion (#18574)
    
 ## 4.14.0.0 *TBA*
   * Bundled with GHC 8.10.1


=====================================
testsuite/tests/perf/should_run/T18574.hs
=====================================
@@ -0,0 +1,7 @@
+import Data.Maybe
+
+main :: IO ()
+main = print $ even $ sum $ catMaybes $ map xx ([1..100000000] :: [Int])
+  where
+    xx n | even n    = Just n
+         | otherwise = Nothing


=====================================
testsuite/tests/perf/should_run/T18574.stdout
=====================================
@@ -0,0 +1 @@
+True


=====================================
testsuite/tests/perf/should_run/all.T
=====================================
@@ -379,3 +379,9 @@ test('UniqLoop',
       ],
      compile_and_run,
      ['-O -package ghc'])
+
+test('T18574',
+    [collect_stats('bytes allocated', 5), only_ways(['normal'])],
+    compile_and_run,
+    ['-O'])
+



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/5c32655fffd0d8862f3ba693351f1e612daa0b6b...e393f213f5ccff4fd6034d5294e51aa5a2720890

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/5c32655fffd0d8862f3ba693351f1e612daa0b6b...e393f213f5ccff4fd6034d5294e51aa5a2720890
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