[PATCH] Add Functor, Applicative, Monad instances for First, Last

[Edward Kmett]

Looks good to me.

-Edward


On Sat, Jun 28, 2014 at 1:58 PM, Ben Gamari <bgamari.foss at gmail.com> wrote:

> This was proposed in 2011 [1] with no serious objections although wasn't
> implemented until it was again mentioned in 2014 [2].
>
> [1] http://www.haskell.org/pipermail/libraries/2011-January/015552.html
> [2] http://www.haskell.org/pipermail/libraries/2014-June/023228.html
> ---
>  libraries/base/Control/Applicative.hs | 11 ++++++++++-
>  libraries/base/Data/Monoid.hs         | 14 ++++++++++++++
>  2 files changed, 24 insertions(+), 1 deletion(-)
>
> diff --git a/libraries/base/Control/Applicative.hs
> b/libraries/base/Control/Applicative.hs
> index 4e77479..81ce513 100644
> --- a/libraries/base/Control/Applicative.hs
> +++ b/libraries/base/Control/Applicative.hs
> @@ -54,7 +54,7 @@ import Control.Monad (liftM, ap, MonadPlus(..))
>  import Control.Monad.ST.Safe (ST)
>  import qualified Control.Monad.ST.Lazy.Safe as Lazy (ST)
>  import Data.Functor ((<$>), (<$))
> -import Data.Monoid (Monoid(..))
> +import Data.Monoid (Monoid(..), First(..), Last(..))
>  import Data.Proxy
>
>  import Text.ParserCombinators.ReadP (ReadP)
> @@ -281,6 +281,15 @@ instance (ArrowZero a, ArrowPlus a) => Alternative
> (WrappedArrow a b) where
>      empty = WrapArrow zeroArrow
>      WrapArrow u <|> WrapArrow v = WrapArrow (u <+> v)
>
> +-- Added in base-4.8.0.0
> +instance Applicative First where
> +        pure x = First (Just x)
> +        First x <*> First y = First (x <*> y)
> +
> +instance Applicative Last where
> +        pure x = Last (Just x)
> +        Last x <*> Last y = Last (x <*> y)
> +
>  -- | Lists, but with an 'Applicative' functor based on zipping, so that
>  --
>  -- @f '<$>' 'ZipList' xs1 '<*>' ... '<*>' 'ZipList' xsn = 'ZipList'
> (zipWithn f xs1 ... xsn)@
> diff --git a/libraries/base/Data/Monoid.hs b/libraries/base/Data/Monoid.hs
> index b71176b..bb87243 100644
> --- a/libraries/base/Data/Monoid.hs
> +++ b/libraries/base/Data/Monoid.hs
> @@ -251,6 +251,13 @@ instance Monoid (First a) where
>          r@(First (Just _)) `mappend` _ = r
>          First Nothing `mappend` r = r
>
> +instance Functor First where
> +        fmap f (First x) = First (fmap f x)
> +
> +instance Monad First where
> +        return x = First (Just x)
> +        First x >>= m = First (x >>= getFirst . m)
> +
>  -- | Maybe monoid returning the rightmost non-Nothing value.
>  newtype Last a = Last { getLast :: Maybe a }
>          deriving (Eq, Ord, Read, Show, Generic, Generic1)
> @@ -260,6 +267,13 @@ instance Monoid (Last a) where
>          _ `mappend` r@(Last (Just _)) = r
>          r `mappend` Last Nothing = r
>
> +instance Functor Last where
> +        fmap f (Last x) = Last (fmap f x)
> +
> +instance Monad Last where
> +        return x = Last (Just x)
> +        Last x >>= m = Last (x >>= getLast . m)
> +
>  {-
>  {--------------------------------------------------------------------
>    Testing
> --
> 1.9.1
>
>
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