Unboxing and streamlining Map maps

* Use an unboxed-sum version of `Maybe` to implement `mapMaybeWithKey`. This potentially (I suspect usually) allows all the `Maybe`s to be erased. * Comprehensive rewrite rules for both strict and lazy versions of `map`, `mapWithKey`, `mapMaybeWithKey`, and `filterWithKey` quickly get out of hand. Following `unordered-containers`, tame the mess by implementing both lazy and strict mapping functions in terms of versions that use unboxed results. Rewrite rules on these underlying functions will then apply uniformly. One concern: I found it a bit tricky to get the unfoldings I wanted; lots of things had to be marked `INLINABLE` explicitly.
haskell · Nov 20, 2022 · f287120 · f287120
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diff --git a/containers-tests/containers-tests.cabal b/containers-tests/containers-tests.cabal
@@ -104,6 +104,8 @@ library
     Utils.Containers.Internal.BitQueue
     Utils.Containers.Internal.BitUtil
     Utils.Containers.Internal.StrictPair
+    Utils.Containers.Internal.UnboxedMaybe
+    Utils.Containers.Internal.UnboxedSolo
   if impl(ghc >= 8.6.0)
     exposed-modules:
       Utils.NoThunks

diff --git a/containers/containers.cabal b/containers/containers.cabal
@@ -69,6 +69,8 @@ Library
         Utils.Containers.Internal.BitUtil
         Utils.Containers.Internal.BitQueue
         Utils.Containers.Internal.StrictPair
+        Utils.Containers.Internal.UnboxedMaybe
+        Utils.Containers.Internal.UnboxedSolo
 
     other-modules:
         Prelude

diff --git a/containers/src/Data/Map/Internal.hs b/containers/src/Data/Map/Internal.hs
@@ -3,6 +3,7 @@
 {-# LANGUAGE PatternGuards #-}
 #if defined(__GLASGOW_HASKELL__)
 {-# LANGUAGE DeriveLift #-}
+{-# LANGUAGE UnboxedTuples #-}
 {-# LANGUAGE RoleAnnotations #-}
 {-# LANGUAGE StandaloneDeriving #-}
 {-# LANGUAGE Trustworthy #-}
@@ -236,7 +237,9 @@ module Data.Map.Internal (
     -- * Traversal
     -- ** Map
     , map
+    , mapU
     , mapWithKey
+    , mapWithKeyU
     , traverseWithKey
     , traverseMaybeWithKey
     , mapAccum
@@ -301,6 +304,7 @@ module Data.Map.Internal (
 
     , mapMaybe
     , mapMaybeWithKey
+    , mapMaybeWithKeyU
     , mapEither
     , mapEitherWithKey
 
@@ -407,6 +411,8 @@ import Data.Data
 import qualified Control.Category as Category
 import Data.Coerce
 #endif
+import Utils.Containers.Internal.UnboxedMaybe
+import Utils.Containers.Internal.UnboxedSolo
 
 
 {--------------------------------------------------------------------
@@ -2849,6 +2855,7 @@ isProperSubmapOfBy f t1 t2
 filter :: (a -> Bool) -> Map k a -> Map k a
 filter p m
   = filterWithKey (\_ x -> p x) m
+{-# INLINE filter #-}
 
 -- | \(O(n)\). Filter all keys\/values that satisfy the predicate.
 --
@@ -2863,6 +2870,32 @@ filterWithKey p t@(Bin _ kx x l r)
   | otherwise = link2 pl pr
   where !pl = filterWithKey p l
         !pr = filterWithKey p r
+{-# NOINLINE [1] filterWithKey #-}
+
+{-# RULES
+"filterWK/filterWK" forall p q m. filterWithKey p (filterWithKey q m) =
+  filterWithKey (\k x -> q k x && p k x) m
+"filterWK/mapU" forall p f m. filterWithKey p (mapU f m) =
+  mapMaybeWithKeyU (\k x -> case f x of
+    SoloU y
+      | p k y -> JustU y
+      | otherwise -> NothingU) m
+"filterWK/mapWK#" forall p f m. filterWithKey p (mapWithKeyU f m) =
+  mapMaybeWithKeyU (\k x -> case f k x of
+    SoloU y
+      | p k y -> JustU y
+      | otherwise -> NothingU) m
+"mapU/filterWK" forall f p m. mapU f (filterWithKey p m) =
+  mapMaybeWithKeyU (\k x ->
+    if p k x
+    then case f x of SoloU y -> JustU y
+    else NothingU) m
+"mapWK#/filterWK" forall f p m. mapWithKeyU f (filterWithKey p m) =
+  mapMaybeWithKeyU (\k x ->
+    if p k x
+    then case f k x of SoloU y -> JustU y
+    else NothingU) m
+ #-}
 
 -- | \(O(n)\). Filter keys and values using an 'Applicative'
 -- predicate.
@@ -2977,17 +3010,54 @@ partitionWithKey p0 t0 = toPair $ go p0 t0
 
 mapMaybe :: (a -> Maybe b) -> Map k a -> Map k b
 mapMaybe f = mapMaybeWithKey (\_ x -> f x)
+{-# INLINE mapMaybe #-}
 
 -- | \(O(n)\). Map keys\/values and collect the 'Just' results.
 --
 -- > let f k _ = if k < 5 then Just ("key : " ++ (show k)) else Nothing
 -- > mapMaybeWithKey f (fromList [(5,"a"), (3,"b")]) == singleton 3 "key : 3"
 
 mapMaybeWithKey :: (k -> a -> Maybe b) -> Map k a -> Map k b
-mapMaybeWithKey _ Tip = Tip
-mapMaybeWithKey f (Bin _ kx x l r) = case f kx x of
-  Just y  -> link kx y (mapMaybeWithKey f l) (mapMaybeWithKey f r)
-  Nothing -> link2 (mapMaybeWithKey f l) (mapMaybeWithKey f r)
+mapMaybeWithKey f = \m ->
+  mapMaybeWithKeyU (\k x -> toMaybeU (f k x)) m
+{-# INLINE mapMaybeWithKey #-}
+
+mapMaybeWithKeyU :: (k -> a -> MaybeU b) -> Map k a -> Map k b
+mapMaybeWithKeyU _ Tip = Tip
+mapMaybeWithKeyU f (Bin _ kx x l r) = case f kx x of
+  JustU y  -> link kx y (mapMaybeWithKeyU f l) (mapMaybeWithKeyU f r)
+  NothingU -> link2 (mapMaybeWithKeyU f l) (mapMaybeWithKeyU f r)
+{-# NOINLINE [1] mapMaybeWithKeyU #-}
+
+{-# RULES
+"mapMaybeWK#/mapU" forall f g m. mapMaybeWithKeyU f (mapU g m) =
+  mapMaybeWithKeyU (\k x -> case g x of SoloU y -> f k y) m
+"mapU/mapMaybeWK#" forall f g m. mapU f (mapMaybeWithKeyU g m) =
+  mapMaybeWithKeyU
+    (\k x -> case g k x of
+               NothingU -> NothingU
+               JustU y -> case f y of SoloU z -> JustU z) m
+"mapMaybeWK#/mapWK#" forall f g m. mapMaybeWithKeyU f (mapWithKeyU g m) =
+  mapMaybeWithKeyU (\k x -> case g k x of SoloU y -> f k y) m
+"mapWK#/mapMaybeWK#" forall f g m. mapWithKeyU f (mapMaybeWithKeyU g m) =
+  mapMaybeWithKeyU
+    (\k x -> case g k x of
+               NothingU -> NothingU
+               JustU y -> case f k y of SoloU z -> JustU z) m
+"mapMaybeWK#/mapMaybeWK#" forall f g m. mapMaybeWithKeyU f (mapMaybeWithKeyU g m) =
+  mapMaybeWithKeyU
+    (\k x -> case g k x of
+               NothingU -> NothingU
+               JustU y -> f k y) m
+"mapMaybeWK#/filterWK" forall f p m. mapMaybeWithKeyU f (filterWithKey p m) =
+  mapMaybeWithKeyU (\k x -> if p k x then f k x else NothingU) m
+"filterWK/mapMaybeWK#" forall p f m. filterWithKey p (mapMaybeWithKeyU f m) =
+  mapMaybeWithKeyU (\k x -> case f k x of
+    NothingU -> NothingU
+    JustU y
+      | p k y -> JustU y
+      | otherwise -> NothingU) m
+ #-}
 
 -- | \(O(n)\). Traverse keys\/values and collect the 'Just' results.
 --
@@ -3045,17 +3115,38 @@ mapEitherWithKey f0 t0 = toPair $ go f0 t0
 -- > map (++ "x") (fromList [(5,"a"), (3,"b")]) == fromList [(3, "bx"), (5, "ax")]
 
 map :: (a -> b) -> Map k a -> Map k b
-map f = go where
+-- We define map using mapU solely to reduce the number of rewrite
+-- rules we need.
+map f = mapU (\x -> SoloU (f x))
+-- We delay inlinability of map to support map/coerce. While a
+-- mapU/coerce rule seems to work when everything is done just so,
+-- it feels too brittle to me for now (GHC 9.4).
+#ifdef __GLASGOW_HASKELL__
+{-# INLINABLE [1] map #-}
+#endif
+
+mapU :: (a -> SoloU b) -> Map k a -> Map k b
+mapU f = go where
   go Tip = Tip
-  go (Bin sx kx x l r) = Bin sx kx (f x) (go l) (go r)
--- We use a `go` function to allow `map` to inline. This makes
--- a big difference if someone uses `map (const x) m` instead
--- of `x <$ m`; it doesn't seem to do any harm.
+  go (Bin sx kx x l r)
+    | SoloU y <- f x
+    = Bin sx kx y (go l) (go r)
+#if defined (__GLASGOW_HASKELL__) && (__GLASGOW_HASKELL__ >= 806) && (__GLASGOW_HASKELL__ < 810)
+  -- Something goes wrong checking SoloU completeness
+  -- in these versions
+  go _ = error "impossible"
+#endif
+
+-- We use a `go` function to allow `mapU` to inline. Without this,
+-- we'd slow down both strict and lazy map, which wouldn't be great.
+-- This also lets us avoid a custom implementation of <$
 
+-- We don't let mapU inline until phase 0 because we need a step
+-- after map inlines.
 #ifdef __GLASGOW_HASKELL__
-{-# NOINLINE [1] map #-}
+{-# NOINLINE [0] mapU #-}
 {-# RULES
-"map/map" forall f g xs . map f (map g xs) = map (f . g) xs
+"mapU/mapU" forall f g xs . mapU f (mapU g xs) = mapU (\x -> case g x of SoloU y -> f y) xs
 "map/coerce" map coerce = coerce
  #-}
 #endif
@@ -3066,21 +3157,35 @@ map f = go where
 -- > mapWithKey f (fromList [(5,"a"), (3,"b")]) == fromList [(3, "3:b"), (5, "5:a")]
 
 mapWithKey :: (k -> a -> b) -> Map k a -> Map k b
-mapWithKey _ Tip = Tip
-mapWithKey f (Bin sx kx x l r) = Bin sx kx (f kx x) (mapWithKey f l) (mapWithKey f r)
+mapWithKey f = mapWithKeyU (\k a -> SoloU (f k a))
+#ifdef __GLASGOW_HASKELL__
+{-# INLINABLE mapWithKey #-}
+#endif
+
+-- | A version of 'mapWithKey' that takes a function producing a unary
+-- unboxed tuple.
+mapWithKeyU :: (k -> a -> SoloU b) -> Map k a -> Map k b
+mapWithKeyU f = go where
+  go Tip = Tip
+  go (Bin sx kx x l r)
+    | SoloU y <- f kx x
+    = Bin sx kx y (go l) (go r)
+#if defined (__GLASGOW_HASKELL__) && (__GLASGOW_HASKELL__ >= 806) && (__GLASGOW_HASKELL__ < 810)
+  -- Something goes wrong checking SoloU completeness
+  -- in these versions
+  go _ = error "impossible"
+#endif
 
 #ifdef __GLASGOW_HASKELL__
-{-# NOINLINE [1] mapWithKey #-}
+{-# NOINLINE [1] mapWithKeyU #-}
 {-# RULES
-"mapWithKey/mapWithKey" forall f g xs . mapWithKey f (mapWithKey g xs) =
-  mapWithKey (\k a -> f k (g k a)) xs
-"mapWithKey/map" forall f g xs . mapWithKey f (map g xs) =
-  mapWithKey (\k a -> f k (g a)) xs
-"map/mapWithKey" forall f g xs . map f (mapWithKey g xs) =
-  mapWithKey (\k a -> f (g k a)) xs
+"mapWK#/mapWK#" forall f g xs. mapWithKeyU f (mapWithKeyU g xs) = mapWithKeyU (\k x -> case g k x of SoloU y -> f k y) xs
+"mapWK#/mapU" forall f g xs. mapWithKeyU f (mapU g xs) = mapWithKeyU (\k x -> case g x of SoloU y -> f k y) xs
+"mapU/mapWK#" forall f g xs. mapU f (mapWithKeyU g xs) = mapWithKeyU (\k x -> case g k x of SoloU y -> f y) xs
  #-}
 #endif
 
+
 -- | \(O(n)\).
 -- @'traverseWithKey' f m == 'fromList' <$> 'traverse' (\(k, v) -> (,) k <$> f k v) ('toList' m)@
 -- That is, behaves exactly like a regular 'traverse' except that the traversing
@@ -4195,9 +4300,10 @@ instance (Ord k, Read k) => Read1 (Map k) where
 --------------------------------------------------------------------}
 instance Functor (Map k) where
   fmap f m  = map f m
+  a <$ m = map (const a) m
 #ifdef __GLASGOW_HASKELL__
-  _ <$ Tip = Tip
-  a <$ (Bin sx kx _ l r) = Bin sx kx a (a <$ l) (a <$ r)
+  {-# INLINABLE fmap #-}
+  {-# INLINABLE (<$) #-}
 #endif
 
 -- | Traverses in order of increasing key.

diff --git a/containers/src/Data/Map/Strict/Internal.hs b/containers/src/Data/Map/Strict/Internal.hs
@@ -1,7 +1,10 @@
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE BangPatterns #-}
 #if defined(__GLASGOW_HASKELL__)
+{-# LANGUAGE MagicHash #-}
+{-# LANGUAGE PatternSynonyms #-}
 {-# LANGUAGE Trustworthy #-}
+{-# LANGUAGE UnboxedTuples #-}
 #endif
 {-# OPTIONS_HADDOCK not-home #-}
 
@@ -420,6 +423,8 @@ import Data.Semigroup (Arg (..))
 import qualified Data.Set.Internal as Set
 import qualified Data.Map.Internal as L
 import Utils.Containers.Internal.StrictPair
+import Utils.Containers.Internal.UnboxedMaybe (pattern NothingU, pattern JustU)
+import Utils.Containers.Internal.UnboxedSolo (pattern SoloU)
 
 import Data.Bits (shiftL, shiftR)
 #ifdef __GLASGOW_HASKELL__
@@ -1271,17 +1276,19 @@ mergeWithKey f g1 g2 = go
 
 mapMaybe :: (a -> Maybe b) -> Map k a -> Map k b
 mapMaybe f = mapMaybeWithKey (\_ x -> f x)
+{-# INLINABLE mapMaybe #-}
 
 -- | \(O(n)\). Map keys\/values and collect the 'Just' results.
 --
 -- > let f k _ = if k < 5 then Just ("key : " ++ (show k)) else Nothing
 -- > mapMaybeWithKey f (fromList [(5,"a"), (3,"b")]) == singleton 3 "key : 3"
 
 mapMaybeWithKey :: (k -> a -> Maybe b) -> Map k a -> Map k b
-mapMaybeWithKey _ Tip = Tip
-mapMaybeWithKey f (Bin _ kx x l r) = case f kx x of
-  Just y  -> y `seq` link kx y (mapMaybeWithKey f l) (mapMaybeWithKey f r)
-  Nothing -> link2 (mapMaybeWithKey f l) (mapMaybeWithKey f r)
+mapMaybeWithKey f = \m ->
+  L.mapMaybeWithKeyU (\k x -> case f k x of
+    Nothing -> NothingU
+    Just !a -> JustU a) m
+{-# INLINABLE mapMaybeWithKey #-}
 
 -- | \(O(n)\). Traverse keys\/values and collect the 'Just' results.
 --
@@ -1340,19 +1347,9 @@ mapEitherWithKey f0 t0 = toPair $ go f0 t0
 -- > map (++ "x") (fromList [(5,"a"), (3,"b")]) == fromList [(3, "bx"), (5, "ax")]
 
 map :: (a -> b) -> Map k a -> Map k b
-map f = go
-  where
-    go Tip = Tip
-    go (Bin sx kx x l r) = let !x' = f x in Bin sx kx x' (go l) (go r)
--- We use `go` to let `map` inline. This is important if `f` is a constant
--- function.
-
+map f = L.mapU (\x -> let !y = f x in SoloU y)
 #ifdef __GLASGOW_HASKELL__
-{-# NOINLINE [1] map #-}
-{-# RULES
-"map/map" forall f g xs . map f (map g xs) = map (\x -> f $! g x) xs
-"map/mapL" forall f g xs . map f (L.map g xs) = map (\x -> f (g x)) xs
- #-}
+{-# INLINABLE map #-}
 #endif
 
 -- | \(O(n)\). Map a function over all values in the map.
@@ -1361,27 +1358,9 @@ map f = go
 -- > mapWithKey f (fromList [(5,"a"), (3,"b")]) == fromList [(3, "3:b"), (5, "5:a")]
 
 mapWithKey :: (k -> a -> b) -> Map k a -> Map k b
-mapWithKey _ Tip = Tip
-mapWithKey f (Bin sx kx x l r) =
-  let x' = f kx x
-  in x' `seq` Bin sx kx x' (mapWithKey f l) (mapWithKey f r)
-
+mapWithKey f = L.mapWithKeyU (\k x -> let !y = f k x in SoloU y)
 #ifdef __GLASGOW_HASKELL__
-{-# NOINLINE [1] mapWithKey #-}
-{-# RULES
-"mapWithKey/mapWithKey" forall f g xs . mapWithKey f (mapWithKey g xs) =
-  mapWithKey (\k a -> f k $! g k a) xs
-"mapWithKey/mapWithKeyL" forall f g xs . mapWithKey f (L.mapWithKey g xs) =
-  mapWithKey (\k a -> f k (g k a)) xs
-"mapWithKey/map" forall f g xs . mapWithKey f (map g xs) =
-  mapWithKey (\k a -> f k $! g a) xs
-"mapWithKey/mapL" forall f g xs . mapWithKey f (L.map g xs) =
-  mapWithKey (\k a -> f k (g a)) xs
-"map/mapWithKey" forall f g xs . map f (mapWithKey g xs) =
-  mapWithKey (\k a -> f $! g k a) xs
-"map/mapWithKeyL" forall f g xs . map f (L.mapWithKey g xs) =
-  mapWithKey (\k a -> f (g k a)) xs
- #-}
+{-# INLINABLE mapWithKey #-}
 #endif
 
 -- | \(O(n)\).