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| -rw-r--r-- | libraries/base/Data/Monoid.hs | 26 |
1 files changed, 26 insertions, 0 deletions
diff --git a/libraries/base/Data/Monoid.hs b/libraries/base/Data/Monoid.hs index cf55b2150c..413072262b 100644 --- a/libraries/base/Data/Monoid.hs +++ b/libraries/base/Data/Monoid.hs @@ -233,6 +233,32 @@ instance (Applicative f, Bounded a) => Bounded (Ap f a) where minBound = pure minBound maxBound = pure maxBound +-- Note that even if the underlying 'Num' and 'Applicative' instances are +-- lawful, for most 'Applicative's, this instance will not be lawful. It's +-- particularly tempting to use this with lists, but then these laws all fail +-- to hold: +-- +-- Commutativity: +-- +-- >>> Ap [10,20] + Ap [1,2] +-- Ap {getAp = [11,12,21,22]} +-- >>> Ap [1,2] + Ap [10,20] +-- Ap {getAp = [11,21,12,22]} +-- +-- Additive inverse: +-- +-- >>> Ap [] + negate (Ap []) +-- Ap {getAp = []} +-- >>> fromInteger 0 :: Ap [] Int +-- Ap {getAp = [0]} +-- +-- Distributivity: +-- +-- >>> Ap [1,2] * (3 + 4) +-- Ap {getAp = [7,14]} +-- >>> (Ap [1,2] * 3) + (Ap [1,2] * 4) +-- Ap {getAp = [7,11,10,14]} +-- -- | @since 4.12.0.0 instance (Applicative f, Num a) => Num (Ap f a) where (+) = liftA2 (+) |