# [Haskell-cafe] Re: Polyvariadic functions operating with a monoid

Kevin Jardine kevinjardine at gmail.com
Sun Oct 10 07:28:01 EDT 2010

```For anyone who's interested, the code I have now is:

{-# LANGUAGE TypeSynonymInstances, FlexibleInstances,
MultiParamTypeClasses #-}
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 => 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)

and three example uses are:

-- [String] example
instance Show a => Monoidable a [String] where
toMonoid a = [show a]

testStringList = putStrLn \$ show \$ ((polyToMonoid [""] True () (Just
(5::Int))) :: [String])

-- String example
instance Show a => Monoidable a String where
toMonoid a = show a

testString = putStrLn \$ ((polyToMonoid "" True () (Just (5::Int))) ::
String)

-- sum example

instance Monoid Int where
mappend = (+)
mempty = 0

instance Monoidable Int Int where
toMonoid = id

testSum = putStrLn \$ show \$ ((polyToMonoid (0::Int) (1::Int) (2::Int)
(3::Int)) :: Int)

main = do
testStringList
testString
testSum

\$ runhaskell PolyTest.hs
["","True","()","Just 5"]
True()Just 5
6

This removes the unwrap and I don't mind the need for the outer type
cast.

I do wonder if there is a need for the first (dummy) parameter to
communicate the type as well as this seems redundant given the outer
type cast but I can't find a way to remove it.

It appears that GHC needs to be told the type both coming and going so
to speak for this to work consistently.

Any suggestions for improvement welcome!

Kevin

On Oct 10, 11:12 am, Kevin Jardine <kevinjard... at gmail.com> wrote:
> OK, upon further investigation, the problem is that GHC cannot in
> general infer the return type of polyToMonoid despite the hint it is
> given (the type signature of the first parameter).
>
> If I write:
>
> main = putStrLn \$ show \$ unwrap \$ ((polyToMonoid [""] True (Just
> (5::Int))) :: WMonoid [String])
>
> or
>
> main = putStrLn \$ show \$ unwrap \$ ((polyToMonoid (0::Int) (1::Int)
> (2::Int) (3::Int)) :: WMonoid Int)
>
> the code compiles and returns the expected result.
>
> Kevin
>
> On Oct 10, 8:58 am, Kevin Jardine <kevinjard... at gmail.com> wrote:
>
> > And in fact in both cases, it appears that GHC is trying to derive the
> > *wrong* instances of PolyVariadic.
>
> > It should be deriving:
>
> > PolyVariadic Int (WMonoid Int)
>
> > not
>
> > PolyVariadic Int (WMonoid m)
>
> > and
>
> > PolyVariadic [String] (WMonoid [String])
>
> > not
>
> > PolyVariadic [String] (WMonoid String)
>
> > specifically, GHC is attempting to derive PolyVariadic with the wrong
> > version of WMonoid in each case.
>
> > I'm using GHC 6.12.3
>
> > Perhaps the new GHC 7 type system would work better?
>
> > Kevin
>
> > On Oct 10, 8:26 am, Kevin Jardine <kevinjard... at gmail.com> wrote:
>
> > > Hi Brandon,
>
> > > True, when I replace [] with [""], I get a different error message:
>
> > >  No instance for (PolyVariadic [[Char]] (WMonoid String))
>
> > > which now looks a bit like the Int example. In both cases, GHC appears
> > > to be unable to derive the appropriate instance of PolyVariadic. Why
> > > this is so, but worked for Oleg's specific example. is still not clear
> > > to me.
>
> > > Kevin
>
> > > On Oct 9, 11:51 pm, Brandon S Allbery KF8NH <allb... at ece.cmu.edu>
> > > wrote:
>
> > > > -----BEGIN PGP SIGNED MESSAGE-----
> > > > Hash: SHA1
>
> > > > On 10/9/10 10:25 , Kevin Jardine wrote:
>
> > > > > instance Show a => Monoidable a [String] where
> > > > >     toMonoid a = [show a]
>
> > > > > main = putStrLn \$ unwrap \$ polyToMonoid [] True () (Just (5::Int))
>
> > > > > fails to compile.
>
> > > > > Why would that be? My understanding is that all lists are
> > > > > automatically monoids.
>
> > > > I *think* the problem here is that Oleg specifically pointed out that the
> > > > first parameter to polyToMonoid must specify the type of the monoid.  []
> > > > tells you it's a list, therefore a monoid, but it doesn't say enough to
> > > > allow the [String] instance to be chosen.  (No, the fact that you only
> > > > declared an instance for [String] isn't really enough.)
>
> > > > - --
> > > > brandon s. allbery     [linux,solaris,freebsd,perl]      allb... at kf8nh.com
> > > > system administrator  [openafs,heimdal,too many hats]  allb... at ece.cmu.edu
> > > > electrical and computer engineering, carnegie mellon university      KF8NH
> > > > -----BEGIN PGP SIGNATURE-----
> > > > Version: GnuPG v2.0.10 (Darwin)
> > > > Comment: Using GnuPG with Mozilla -http://enigmail.mozdev.org/
>
> > > > iEYEARECAAYFAkyw49wACgkQIn7hlCsL25VZygCfVETk+3AZ3gKoBy4pZ7j8g4Km
> > > > WXgAnjrbO9rEl2HnQtGQ31EyRuhWzI4r
> > > > =YMDw
> > > > -----END PGP SIGNATURE-----
> > > > _______________________________________________
> > > > Haskell-Cafe mailing list
> > > > Haskell-C... at haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe
>
> > > _______________________________________________
> > > Haskell-Cafe mailing list
> > > Haskell-C... at haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe
>
> > _______________________________________________
> > Haskell-Cafe mailing list
> > Haskell-C... at haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe
>
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-C... at haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe
```

More information about the Haskell-Cafe mailing list