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Avoid thunks in IntSet minView and maxView by oheliav · Pull Request #1277 · haskell/containers · GitHub
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Avoid thunks in IntSet minView and maxView #1277

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Avoid thunks in IntSet minView and maxView #1277

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f2bdca6
Avoid thunks in IntSet minView and maxView
oheliav Nov 8, 2026
b7a9b77
Follow IntMap's design for IntSet minView and maxView
oheliav Nov 11, 2026
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38 changes: 26 additions & 12 deletions containers/src/Data/IntSet/Internal.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -1164,26 +1164,40 @@ splitMember x t =
-- | \(O(\min(n,W))\). Retrieves the maximal key of the set, and the set
-- stripped of that element, or 'Nothing' if passed an empty set.
maxView :: IntSet -> Maybe (Key, IntSet)
maxView t =
case t of Nil -> Nothing
Bin p l r | signBranch p -> case go l of (result, l') -> Just (result, binCheckL p l' r)
_ -> Just (go t)
maxView t = case t of
Nil -> Nothing
_ -> Just $ case maxViewSure t of result :*: t' -> (result, t')
{-# INLINE maxView #-}

maxViewSure :: IntSet -> StrictPair Key IntSet

@meooow25 meooow25 Nov 11, 2026

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Looks good overall, but instead of StrictPair we should have View type like for IntMap:

containers/containers/src/Data/IntMap/Internal.hs

Line 2393 in c9c0c88

data View a = View {-# UNPACK #-} !Key a !(IntMap a)

This makes a difference for GHC <9.4, documented here:

containers/containers/src/Data/IntSet/Internal.hs

Lines 1811 to 1815 in c9c0c88

-- This type allows GHC to return unboxed ints from leftmostTipSure, as
-- $wleftmostTipSure :: IntSet -> (# Int#, Word# #)
-- On a modern enough GHC (>=9.4) this is unnecessary, we could use StrictPair
-- instead and get the same Core.
data Tip' = Tip' {-# UNPACK #-} !Int {-# UNPACK #-} !BitMap

maxViewSure t =
case t of Nil -> error "maxViewSure Nil"
Bin p l r | signBranch p -> case go l of (result :*: l') -> result :*: binCheckL p l' r
_ -> go t
where
go (Bin p l r) = case go r of (result, r') -> (result, binCheckR p l r')
go (Tip kx bm) = case highestBitSet bm of bi -> (kx + bi, tip kx (bm .&. complement (bitmapOfSuffix bi)))
go (Bin p l r) = case go r of (result :*: r') -> result :*: binCheckR p l r'
go (Tip kx bm) = case highestBitSet bm of bi -> (kx + bi) :*: tip kx (bm .&. complement (bitmapOfSuffix bi))
go Nil = error "maxView Nil"
{-# NOINLINE maxViewSure #-}

-- | \(O(\min(n,W))\). Retrieves the minimal key of the set, and the set
-- stripped of that element, or 'Nothing' if passed an empty set.
minView :: IntSet -> Maybe (Key, IntSet)
minView t =
case t of Nil -> Nothing
Bin p l r | signBranch p -> case go r of (result, r') -> Just (result, binCheckR p l r')
_ -> Just (go t)
minView t = case t of
Nil -> Nothing
_ -> Just $ case minViewSure t of result :*: t' -> (result, t')
{-# INLINE minView #-}

minViewSure :: IntSet -> StrictPair Key IntSet
minViewSure t =
case t of Nil -> error "minViewSure Nil"
Bin p l r | signBranch p -> case go r of (result :*: r') -> result :*: binCheckR p l r'
_ -> go t
where
go (Bin p l r) = case go l of (result, l') -> (result, binCheckL p l' r)
go (Tip kx bm) = case lowestBitSet bm of bi -> (kx + bi, tip kx (bm .&. complement (bitmapOfSuffix bi)))
go (Bin p l r) = case go l of (result :*: l') -> result :*: binCheckL p l' r
go (Tip kx bm) = case lowestBitSet bm of bi -> (kx + bi) :*: tip kx (bm .&. complement (bitmapOfSuffix bi))
go Nil = error "minView Nil"
{-# NOINLINE minViewSure #-}

-- | \(O(\min(n,W))\). Delete and find the minimal element.
--
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