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.

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

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

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 +3115,41 @@ 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
+#ifdef __GLASGOW_HASKELL__
+-- 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).
+{-# INLINABLE [1] map #-}
+#else
 map 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.
+#endif
 
 #ifdef __GLASGOW_HASKELL__
-{-# NOINLINE [1] map #-}
+mapU :: (a -> SoloU b) -> Map k a -> Map k b
+mapU f = go where
+  go Tip = Tip
+  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.
+{-# 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 +3160,38 @@ 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
+#ifdef __GLASGOW_HASKELL__
+mapWithKey f = mapWithKeyU (\k a -> SoloU (f k a))
+{-# INLINABLE mapWithKey #-}
+#else
 mapWithKey _ Tip = Tip
 mapWithKey f (Bin sx kx x l r) = Bin sx kx (f kx x) (mapWithKey f l) (mapWithKey f r)
+#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 <$> f k v) ('toList' m)@
 -- That is, behaves exactly like a regular 'traverse' except that the traversing
@@ -4195,10 +4306,12 @@ instance (Ord k, Read k) => Read1 (Map k) where
 --------------------------------------------------------------------}
 instance Functor (Map k) where
   fmap f m  = map f m
-#ifdef __GLASGOW_HASKELL__
-  _ <$ Tip = Tip
-  a <$ (Bin sx kx _ l r) = Bin sx kx a (a <$ l) (a <$ r)
-#endif
+  {-# INLINABLE fmap #-}
+  a <$ m = map (const a) m
+  -- For some reason, we need an explicit INLINE or INLINABLE pragma to
+  -- get the unfolding to use map rather than expanding into a recursive
+  -- function that RULES will never match. Hmm....
+  {-# INLINABLE (<$) #-}
 
 -- | Traverses in order of increasing key.
 instance Traversable (Map k) where

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 +1347,16 @@ 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
+#ifdef __GLASGOW_HASKELL__
+map f = L.mapU (\x -> let !y = f x in SoloU y)
+{-# INLINABLE map #-}
+#else
 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.
-
-#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
- #-}
 #endif
 
 -- | \(O(n)\). Map a function over all values in the map.
@@ -1361,27 +1365,14 @@ 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
+#ifdef __GLASGOW_HASKELL__
+mapWithKey f = L.mapWithKeyU (\k x -> let !y = f k x in SoloU y)
+{-# INLINABLE mapWithKey #-}
+#else
 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)
-
-#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
- #-}
 #endif
 
 -- | \(O(n)\).