On 13-08-22 04:04 PM, Petr Pudlák wrote:
Or, if there are no such definitions, where would be a good place to add
them?
If they are to be added to the base libraries, the Data.Monoid module
would be my choice.
I did wish I had the AppMonoid instance on several occasions, when
using various parser combinator libraries that don't support this
reasonable instance of Monoid:
> numericLiteral = optionalMonoid (string "+" <|> string "-")
> <> some digit
> <> optionalMonoid (string "." <> some digit)
The problem is, the AppMonoid newtype would not help in that situation
unless it also implemented Applicative and Alternative class, as well as
the parsing primitives. Without the latter, the above code would look
like this:
> numericLiteral = optionalMonoid
(AppMonoid (string "+" <|> string "-"))
> <> some (AppMonoid digit)
> <> optionalMonoid (AppMonoid (string ".")
> <> some (AppMonoid digit))
The point of the above is that I don't think there is enough
justification for these newtypes. The Applicative and Alternative
instances are typically used because of the primitives they come with,
and newtype wrappings like AppMonoid and AltMonoid can't support those
easily. Unless ekmett adds the appropriate instances to his parsers
package, they would be too clumsy to use.
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
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