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

[Kevin Jardine]

Another example that also fails to compile (but I cannot see why):

import Data.PolyToMonoid
import Data.Monoid

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

instance Monoidable Int Int where
    toMonoid = id

main = putStrLn $ show $ unwrap $ polyToMonoid (0::Int) (1::Int)
(2::Int) (3::Int)

In this case, I was expecting a sumOf function.

This gives me:

No instance for (PolyVariadic Int (WMonoid m))
      arising from a use of `polyToMonoid'

Any further suggestions?

On Oct 9, 4:25 pm, Kevin Jardine <kevinjard... at gmail.com> wrote:
> Oleg,
>
> Another puzzle is that:
>
> instance Show a => Monoidable a String where
>     toMonoid a = show a
>
> main = putStrLn $ unwrap $ polyToMonoid "" True () (Just (5::Int))
>
> works just fine, but
>
> 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.
>
> Kevin
>
> On Oct 9, 2:28 pm, Kevin Jardine <kevinjard... at gmail.com> wrote:
>
> > Hi Bartek,
>
> > Yes, it compiles, but when I try to use polyToMonoid', it turns out
> > that this function is no longer polyvariadic, unlike the original
> > polyToMonoid .
>
> > This may be what Luke meant when he wrote "you lose composability".
>
> > Even with the extra unwrap function I think that this is pretty cool,
> > but I would ideally like to hide the unwrap.
>
> > Kevin
>
> > On Oct 9, 1:50 pm, Bartek Æwik³owski <paczesi... at gmail.com> wrote:
>
> > > Hello Kevin,
>
> > > 2010/10/9 Kevin Jardine <kevinjard... at gmail.com>:
>
> > > > I was attempting to turn this into a small library and wanted to avoid
> > > > exporting unwrap.
>
> > > > I defined:
>
> > > > polyToMonoid' = unwrap . polyToMonoid
>
> > > If you disable MonomorphismRestriction this definition typechecks just
> > > fine. Alternatively, you can ask ghci about the type of "unwrap .
> > > polyToMonoid" and paste that into the type sig.
>
> > > regards,
> > > Bartek Æwik³owski
> > > _______________________________________________
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