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

[Kevin Jardine]

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:
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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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