-----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-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
