[Haskell-cafe] Re: Polyvariadic functions operating with a monoid
Kevin Jardine
kevinjardine at gmail.com
Wed Oct 13 01:36:54 EDT 2010
I have turned the code into a library and put it up on github here:
http://github.com/kevinjardine/polyToMonoid
The library includes two versions of the function: ptm does not
require a termination function but does not allow partial evaluation
either. ctm is more composable (returning a function that consumes the
next parameter) and requires a termination function trm to return the
result.
The source includes thorough Haddock friendly comments with examples.
My plan is to upload it to Hackage later this week but I wondered if
anyone had any comments before I do.
Kevin
On Oct 11, 11:08 am, Kevin Jardine <kevinjard... at gmail.com> wrote:
> It also appears that we need type families to reconstruct the original
> Haskell list system using polyToMonoid.
>
> instance (a ~ a') => Monoidable a [a'] where
> toMonoid a = [a]
>
> testList = putStrLn $ show $ polyToMonoid (mempty :: [a]) "a" "b" "c"
>
> Given this instance of Monoidable, you can put any number of values
> after
> polyToMonoid (mempty :: [a]) as long as they are exactly the same
> type.
>
> In other words, this acts exactly like the usual Haskell list, going
> back to my original point that polyToMonoid is a sort of generalised
> list or "a function that takes a bunch of values that can be stuck
> together in some way".
>
> I am a bit surprised that the (a ~ a') is needed, but Haskell will
> not compile this code with the more usual
>
> instance Monoidable a [a] where
> toMonoid a = [a]
>
> Kevin
>
> On Oct 11, 9:54 am, Kevin Jardine <kevinjard... at gmail.com> wrote:
>
> > Hi Oleg,
>
> > I've found that if I also add two other slightly scary sounding
> > extensions: OverlappingInstances and IncoherentInstances, then I can
> > eliminate the unwrap function *and* use your type families trick to
> > avoid the outer type annotation.
>
> > My latest code is here:
>
> > {-# LANGUAGE TypeSynonymInstances, FlexibleInstances,
> > MultiParamTypeClasses, TypeFamilies #-}
> > {-# LANGUAGE OverlappingInstances, IncoherentInstances #-}
> > module PolyTest where
>
> > import Data.Monoid
>
> > class Monoid m => Monoidable a m where
> > toMonoid :: a -> m
>
> > squish :: Monoidable a m => m -> a -> m
> > squish m a = (m `mappend` (toMonoid a))
>
> > class Monoid m => PolyVariadic m r where
> > polyToMonoid :: m -> r
>
> > instance (Monoid m', m' ~ m) => PolyVariadic m m' where
> > polyToMonoid acc = acc
>
> > instance (Monoidable a m, PolyVariadic m r) => PolyVariadic m (a->r)
> > where
> > polyToMonoid acc = \a -> polyToMonoid (squish acc a)
>
> > Here are three examples. The resulting notation is short enough now
> > that I am no longer tempted to use CPP.
>
> > All you need to do is to specify the type for mempty. And even this
> > can be skipped if you want to put in the specific mempty value
> > (although I think that the type annotation is often better if slightly
> > longer as it documents clearly what monoid the result is being mapped
> > into).
>
> > -- [String] example
> > instance Show a => Monoidable a [String] where
> > toMonoid a = [show a]
>
> > testStringList = putStrLn $ show $ polyToMonoid (mempty :: [String])
> > True () (Just (5::Int))
>
> > -- String example
> > instance Show a => Monoidable a String where
> > toMonoid a = show a
>
> > testString = putStrLn $ polyToMonoid (mempty :: String) True () (Just
> > (5::Int))
>
> > -- product example
>
> > instance Monoid Double where
> > mappend = (*)
> > mempty = (1.0) :: Double
>
> > instance Monoidable Int Double where
> > toMonoid = fromIntegral
>
> > instance Monoidable Double Double where
> > toMonoid = id
>
> > testProduct = putStrLn $ show $ polyToMonoid (mempty :: Double) (5 ::
> > Int) (2.3 :: Double) (3 :: Int) (8 :: Int)
>
> > main = do
> > testStringList
> > testString
> > testProduct
>
> > $ runhaskell PolyTest.hs
> > ["True","()","Just 5"]
> > True()Just 5
> > 276.0
>
> > Kevin
>
> > On Oct 11, 2:39 am, o... at okmij.org wrote:
>
> > > Sorry, I'm still catching up. I'm replying to first few messages.
>
> > > > instance Show a => Monoidable a [String] where
> > > > toMonoid a = [show a]
>
> > > > main = putStrLn $ unwrap $ polyToMonoid [] True () (Just (5::Int))
> > > > fails to compile.
>
> > > The error message points to the first problem:
>
> > > > No instances for (Monoidable Bool [a],
> > > > Monoidable () [a],
> > > > ...
>
> > > The presence of the type variable 'a' means that the type checker
> > > doesn't know list of what elements you want (in other words, the
> > > context is not specific enough to instantiate the type variable
> > > a). Thus, we need to explicitly tell that we wish a list of strings:
>
> > > > test3 = putStrLn $ unwrap $polyToMonoid ([]::[String]) True () (Just (5::Int))
>
> > > Now we get a different error, which points to the real problem this
> > > time: the expression `unwrap ....' appears as an argument to
> > > putStrLn. That means that we are required to produce a String as a
> > > monoid. Yet we specified ([]::[String]) as mempty, which is unsuitable
> > > as mempty for the String monoid. If we desire the [String] monoid as
> > > the result, we need to change the context. For example,
>
> > > > test3 = mapM_ putStrLn $ unwrap $
> > > > polyToMonoid ([]::[String]) True () (Just (5::Int))
> > > > Another example that also fails to compile (but I cannot see why):
> > > > main = putStrLn $ show $ unwrap $ polyToMonoid (0::Int) (1::Int)
> > > > (2::Int) (3::Int)
> > > > No instance for (PolyVariadic Int (WMonoid m))
> > > > arising from a use of `polyToMonoid'
>
> > > The error message is informative, mentioning the type variable,
> > > m. Whenever that happens, we know that we put a bounded polymorphic
> > > expression in the context that is not specific enough. We need some
> > > type annotations. In our case, the function 'show' can show values of
> > > many types. The type checker does not know that we wish an Int monoid
> > > specifically. So, we have to specialize the show function:
>
> > > > test4 = putStrLn $ (show :: Int -> String) $
> > > > unwrap $ polyToMonoid (0::Int) (1::Int) (2::Int) (3::Int)
>
> > > At this point one may wonder if this is all worth it. There are too
> > > many annotations. Fortunately, if you are not afraid of one more
> > > extension, the annotations can be avoided. Your example would be
> > > accepted as it was written, see test3 and test4 below.
>
> > > > {-# LANGUAGE TypeSynonymInstances #-}
> > > > {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, TypeFamilies #-}
>
> > > > module M where
>
> > > > import Data.Monoid
>
> > > > newtype WMonoid m = WMonoid{unwrap :: m}
>
> > > > class Monoid m => Monoidable a m where
> > > > toMonoid :: a -> m
>
> > > > class Monoid m => PolyVariadic m p where
> > > > polyToMonoid :: m -> p
>
> > > > instance (Monoid m', m' ~ m) => PolyVariadic m (WMonoid m') where
> > > > polyToMonoid acc = WMonoid acc
>
> > > > instance (Monoidable a m, PolyVariadic m r) => PolyVariadic m (a->r) where
> > > > polyToMonoid acc = \a -> polyToMonoid (acc `mappend` toMonoid a)
>
> > > > instance Show a => Monoidable a String where
> > > > toMonoid = show
>
> > > > instance Show a => Monoidable a [String] where
> > > > toMonoid a = [show a]
>
> > > > test2 = putStrLn $ unwrap $ polyToMonoid "" True () (Just (5::Int))
>
> > > > test3 = mapM_ putStrLn $ unwrap $ polyToMonoid [] True () (Just (5::Int))
>
> > > > instance Monoid Int where
> > > > mappend = (+)
> > > > mempty = 0
>
> > > > instance Monoidable Int Int where
> > > > toMonoid = id
>
> > > > test4 = putStrLn $ show $
> > > > unwrap $ polyToMonoid (0::Int) (1::Int) (2::Int) (3::Int)
>
> > > P.S. Indeed, "polyToMonoid' = unwrap . polyToMonoid" does not do what
> > > one wishes to. One should regard `unwrap' as a sort of terminator of
> > > the argument list.
>
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