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Prevent Lazy.uncons from allocating a thunk #559

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Nov 25, 2022
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Prevent Lazy.uncons from allocating a thunk #559

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10 changes: 7 additions & 3 deletions Data/ByteString/Lazy.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -353,9 +353,13 @@ head (Chunk c _) = S.unsafeHead c
-- if it is empty.
uncons :: ByteString -> Maybe (Word8, ByteString)
uncons Empty = Nothing
uncons (Chunk c cs)
= Just (S.unsafeHead c,
if S.length c == 1 then cs else Chunk (S.unsafeTail c) cs)
uncons (Chunk c cs) = case S.length c of
-- Don't move this test inside of the Just or (,).
-- We don't want to allocate a thunk to put inside of the tuple!
-- And if "let !tl = ... in Just (..., tl)" seems more appealing,
-- remember that this function must remain lazy in cs.
1 -> Just (S.unsafeHead c, cs)
_ -> Just (S.unsafeHead c, Chunk (S.unsafeTail c) cs)
{-# INLINE uncons #-}

-- | /O(1)/ Extract the elements after the head of a ByteString, which must be
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