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...@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...@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...@kf8nh.com > > system administrator [openafs,heimdal,too many hats] allb...@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...@haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe > > _______________________________________________ > Haskell-Cafe mailing list > haskell-c...@haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
