Maybe doesn't satisfy the monoid properties

[Andrés Sicard-Ramírez]

The Monoid instance

instance Monoid a => Monoid (Maybe a) where
  mempty = Nothing

  Nothing `mappend` _       = Nothing
  _       `mappend` Nothing = Nothing
  Just x1 `mappend` Just x2 = Just (x1 `mappend` x2)

defined in https://github.com/meiersi/strict-base-types/blob/master/src/Data/Maybe/Strict.hs doesn't satisfy the monoid properties. Is it related to the strictness feature?

[Simon Meier]

Hmm, that wouldn't be good. Could you give a concrete counter-example?

Andrés Sicard-Ramírez [e-mail] schrieb am Do., 28. Jan.
2016 3:32 PM:

The Monoid instance

instance Monoid a => Monoid (Maybe a) where
mempty = Nothing

Nothing mappend _ = Nothing
_ mappend Nothing = Nothing
Just x1 mappend Just x2 = Just (x1 mappend x2)

defined in
https://github.com/meiersi/strict-base-types/blob/master/src/Data/Maybe/Strict.hs
doesn't satisfy the monoid properties. Is it related to the strictness
feature?

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[Andrés Sicard-Ramírez]

Could you give a concrete counter-example?

No. But I don't see any problem by fixing the two first equations by

Nothing `mappend` m       = m
m       `mappend` Nothing = m