Haskell记录的Monoid，Applicative，Monad等的场分布

Question

Is there a package that does roughly the following:

Given a record:

data R = R { a :: TypeA,
             b :: TypeB,
             c :: TypeC }

derives a lifted record:

data R_L f = R_L { a_L :: f TypeA,
                   b_L :: f TypeB,
                   c_L :: f TypeC }

and offers a couple of instances and functions similar to:

instance (Monoid (f TypeA), Monoid (f TypeB), Monoid (f TypeC))
         => (Monoid (R_L f)) where
  mempty = R_L mempty mempty mempty
  mplus a b = ...fieldwise mplus...

sequenceR :: (Monad m) => R_L m -> m R
sequenceR = ... run fields, sum results ...
sequenceRA :: (Applicative m) => R_L m -> m R
sequenceRA x = R <$> a_L x <*> b_L x <*> c_L x

and probably others. Is there a package that provides this functionality and when not, which of the mechanisms (TH? Generics?) is best to use to implement it?

Answer 1

The monoid part is possible with generic-deriving , which offers an alternative GMonoid , of which generics are automatically an instance.

{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE FlexibleInstances, FlexibleContexts, DefaultSignatures #-}

import Data.Monoid
import qualified Generics.Deriving.Monoid as M

data R_L f = R_L { a :: f [()],
                   b :: f String,
                   c :: f () } deriving (Generic)

Now you can do the following:

*Main> let x = R_L (Just [()]) Nothing (Just ()) `M.gmappenddefault` R_L (Just [()]) (Just "foo") (Just ())
*Main> a x
Just [(),()]
*Main> b x
Just "foo"
*Main> c x
Just ()

(I am still figuring out the generic Show instance.)

An ordinary Monoid instance can be created as follows (thought, that might be called boilerplate again...):

instance (Monoid (f [()]), Monoid (f String), Monoid (f ())) => Monoid (R_L f) where
    mempty = M.memptydefault
    mappend = M.mappenddefault

In the package, there are also other derived instances for Functor , Traversable and Foldable .

Monad and Applicative can maybe be modelled with generics similar to the existing instances, if you change the kind of your type ; although, maybe not in the way you like, since, I think, they would only make sense as a product functor of the fields' types, and not as you propose.

Answer 2

In the true open source spirit I wrote my own library:

http://hackage.haskell.org/package/fieldwise

https://github.com/gracjan/fieldwise