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| author | Michael Snoyman <michael@snoyman.com> | 2014-08-10 15:54:01 -0500 |
|---|---|---|
| committer | Austin Seipp <austin@well-typed.com> | 2014-08-10 15:54:02 -0500 |
| commit | cbfa107604f4cbfaf02bd633c1faa6ecb90c6dd7 (patch) | |
| tree | 2b876bbb54e61cd3d684936f3f76a1e4c5a92535 /libraries/base/Prelude.hs | |
| parent | 8d90ffae3a3c0d7c9ac8800ae91887aeaa9917d3 (diff) | |
| download | haskell-cbfa107604f4cbfaf02bd633c1faa6ecb90c6dd7.tar.gz | |
Improve seq documentation; part of trac issue #9390
Summary: Signed-off-by: Michael Snoyman <michael@snoyman.com>
Test Plan: Review documentation change
Reviewers: simonpj, austin
Reviewed By: austin
Subscribers: phaskell, hvr, simonmar, relrod, ezyang, carter
Differential Revision: https://phabricator.haskell.org/D136
GHC Trac Issues: #9390
Diffstat (limited to 'libraries/base/Prelude.hs')
| -rw-r--r-- | libraries/base/Prelude.hs | 7 |
1 files changed, 7 insertions, 0 deletions
diff --git a/libraries/base/Prelude.hs b/libraries/base/Prelude.hs index 6be784603c..9b1119e155 100644 --- a/libraries/base/Prelude.hs +++ b/libraries/base/Prelude.hs @@ -163,6 +163,13 @@ f $! x = let !vx = x in f vx -- see #2273 -- | The value of @'seq' a b@ is bottom if @a@ is bottom, and otherwise -- equal to @b@. 'seq' is usually introduced to improve performance by -- avoiding unneeded laziness. +-- +-- A note on evaluation order: the expression @seq a b@ does /not/ guarantee +-- that @a@ will be evaluated before @b@. The only guarantee given by @seq@ is +-- that the both @a@ and @b@ will be evaluated before @seq@ returns a value. In +-- particular, this means that @b@ may be evaluated before @a@. If you need to +-- guarantee a specific order of evaluation, you must use the function @pseq@ +-- from the parallel package. seq :: a -> b -> b seq _ y = y #endif |