Re: [Haskell-cafe] monoids induced by Applicative/Alternative/Monad/MonadPlus?

22 Aug 2013


      Or, if there are no such definitions, where would be a good place to add 
them?

Petr

Dne 08/20/2013 06:55 PM, Petr Pudlák napsal(a):
...
Dear Haskellers,
are these monoids defined somewhere?
|import  Control.Applicative
import  Data.Monoid
newtype  AppMonoid  m a =AppMonoid  (m  a)
instance  (Monoid  a,Applicative  m) =>Monoid  (AppMonoid  m a)where
     mempty =AppMonoid  $ pure mempty
     mappend (AppMonoid  x) (AppMonoid  y) =AppMonoid  $ mappend <$> x <*> y
-- With the () monoid for `a` this becames the monoid of effects.
newtype  AltMonoid  m a =AltMonoid  (m  a)
instance  Alternative  m =>Monoid  (AltMonoid  m a)where
     mempty =AltMonoid  empty
     mappend (AltMonoid  x) (AltMonoid  y) =AltMonoid  $ x <|> y|
(and similarly for Monad/MonadPlus, until they become subclasses of 
Applicative?)
Best regards,
Petr