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| author | Tristan Cacqueray <tdecacqu@redhat.com> | 2020-03-21 16:55:03 +0000 |
|---|---|---|
| committer | Marge Bot <ben+marge-bot@smart-cactus.org> | 2020-03-25 22:42:06 -0400 |
| commit | 1c5a15f7d257d0a2ded4850e38b6174965a7735b (patch) | |
| tree | ba37717e836fe681ad8ab9fc3dbcf8e7402ad150 | |
| parent | 7a04920b23376b2759b5049820400ddd40f7b89e (diff) | |
| download | haskell-1c5a15f7d257d0a2ded4850e38b6174965a7735b.tar.gz | |
Base: add Control.Applicative optional example
This change adds an optional example.
Tracking: #17929
| -rw-r--r-- | libraries/base/Control/Applicative.hs | 17 |
1 files changed, 17 insertions, 0 deletions
diff --git a/libraries/base/Control/Applicative.hs b/libraries/base/Control/Applicative.hs index 87394e078b..e898bf8c04 100644 --- a/libraries/base/Control/Applicative.hs +++ b/libraries/base/Control/Applicative.hs @@ -141,5 +141,22 @@ instance Alternative ZipList where -- extra functions -- | One or none. +-- +-- It is useful for modelling any computation that is allowed to fail. +-- +-- ==== __Examples__ +-- +-- Using the Control.Monad.Except as an Alternative, a couple of functions: +-- +-- >>> canFail = throwError "it failed" :: Except String Int +-- >>> final = return 42 :: Except String Int +-- +-- Can be combined by allowing the first function to fail: +-- +-- >>> runExcept $ canFail *> final +-- Left "it failed" +-- >>> runExcept $ optional canFail *> final +-- Right 42 + optional :: Alternative f => f a -> f (Maybe a) optional v = Just <$> v <|> pure Nothing |