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

[Kevin Jardine]

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-----
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> > 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
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