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Warn against Data.Semigroup.Option and Data.Monoid.First/Last #128

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Warn against Data.Semigroup.Option and Data.Monoid.First/Last #128

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d37af28
Warn against using Data.Monoid.First, noting that it will be removed …
andrewthad Apr 25, 2018
d161ae6
document the plan for removing Data.Semigroup.Option
andrewthad Apr 25, 2018
034c7ca
minor improvement to wording of warning against First. Add warning ag…
andrewthad Apr 26, 2018
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10 changes: 10 additions & 0 deletions libraries/base/Data/Monoid.hs
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Expand Up @@ -88,6 +88,16 @@ import Data.Semigroup.Internal
--
-- >>> getFirst (First (Just "hello") <> First Nothing <> First (Just "world"))
-- Just "hello"
--
-- Use of this type is discouraged. Note the following equivalence:
--
-- > Data.Monoid.First x === Maybe (Data.Semigroup.First x)
--
-- In additional to being equivalent in the structural sense, the two
-- also have 'Monoid' instances that behave the same. This type will
-- be marked deprecated in GHC 8.8. It will be removed in GHC 8.10.
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Maybe shorten this to "be marked as deprecated in GHC 8.8, and removed in GHC 8.10".

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Good call. Done.

-- Users are advised to use the variant from "Data.Semigroup" and wrap
-- it in 'Maybe'.
newtype First a = First { getFirst :: Maybe a }
deriving ( Eq -- ^ @since 2.01
, Ord -- ^ @since 2.01
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7 changes: 6 additions & 1 deletion libraries/base/Data/Semigroup.hs
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Expand Up @@ -455,7 +455,12 @@ mtimesDefault n x
-- underlying 'Monoid'.
--
-- Ideally, this type would not exist at all and we would just fix the
-- 'Monoid' instance of 'Maybe'
-- 'Monoid' instance of 'Maybe'.
--
-- In GHC 8.4 and higher, the 'Monoid' instance for 'Maybe' has been
-- corrected to lift a 'Semigroup' instance instead of a 'Monoid'
-- instance. Consequently, this type is no longer useful. It will be
-- marked deprecated in GHC 8.8 and removed in GHC 8.10.
newtype Option a = Option { getOption :: Maybe a }
deriving ( Eq -- ^ @since 4.9.0.0
, Ord -- ^ @since 4.9.0.0
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