diff options
Diffstat (limited to 'libraries/base/Data')
| -rw-r--r-- | libraries/base/Data/Foldable.hs | 25 |
1 files changed, 14 insertions, 11 deletions
diff --git a/libraries/base/Data/Foldable.hs b/libraries/base/Data/Foldable.hs index 48acbe2d14..85557844b1 100644 --- a/libraries/base/Data/Foldable.hs +++ b/libraries/base/Data/Foldable.hs @@ -145,10 +145,12 @@ class Foldable t where -- >>> fold [Sum 1, Sum 3, Sum 5] -- Sum {getSum = 9} -- - -- Infinite structures never terminate: + -- Folds of infinite structures do not terminate when the Monoid's + -- `mappend` is strict: -- -- >>> fold (repeat Nothing) -- * Hangs forever * + -- fold :: Monoid m => t m -> m fold = foldMap id @@ -168,10 +170,16 @@ class Foldable t where -- >>> foldMap (replicate 3) [1, 2, 3] -- [1,1,1,2,2,2,3,3,3] -- - -- Infinite structures never terminate: + -- When a Monoid's `mappend` is lazy, 'foldMap' can produce a (possibly + -- unbounded) result even from an unbounded structure: + -- + -- >>> import qualified Data.ByteString.Lazy as L + -- >>> import qualified Data.ByteString.Builder as B + -- >>> let bld :: Int -> B.Builder; bld i = B.intDec i <> B.word8 0x20 + -- >>> let lbs = B.toLazyByteString $ foldMap bld [0..] + -- >>> L.take 64 lbs + -- "0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24" -- - -- >>> foldMap Sum [1..] - -- * Hangs forever * foldMap :: Monoid m => (a -> m) -> t a -> m {-# INLINE foldMap #-} -- This INLINE allows more list functions to fuse. See #9848. @@ -1304,11 +1312,6 @@ https://gitlab.haskell.org/ghc/ghc/-/issues/17867 for more context. -------------- --- In order to avoid having actual Unicode glyphs in the module source, --- the below numeric HTML entity codes are used: --- --- * ellipsis = … - -- $overview -- -- #overview# @@ -1420,8 +1423,8 @@ https://gitlab.haskell.org/ghc/ghc/-/issues/17867 for more context. -- last element is reached, by which point a deep stack of pending function -- applications may have been built up in memory. -- --- In finite structures for which right-to-left sequencing no less efficient as --- left-to-right sequencing, there is no inherent performance distinction +-- In finite structures for which right-to-left sequencing no less efficient +-- than left-to-right sequencing, there is no inherent performance distinction -- between left-associative and right-associative folds. If the structure's -- @Foldable@ instance takes advantage of this symmetry to also make strict -- right folds space-efficient and lazy left folds corecursive, one need only |