Illegal type synonym family application in instance (Was: Breakage with 6.10)

[Niklas Broberg]

On 10/11/08, David Menendez <dave at zednenem.com> wrote:
> On Fri, Oct 10, 2008 at 8:40 PM, Niklas Broberg
>  <niklas.broberg at gmail.com> wrote:
>  > src\HSX\XMLGenerator.hs:71:0
>  >    Illegal type synonym family application in instance: XML m
>  >    In the instance declaration for `EmbedAsChild m (XML m)´
>  > ---------------
>  >
>  > Could someone help me point out the problem here? The relevant code is:
>  >
>  > instance XMLGen m => EmbedAsChild m (XML m) where
>  >  asChild = return . return . xmlToChild
>  >
>  > class XMLGen m => EmbedAsChild m c where
>  >  asChild :: c -> GenChildList m
>  >
>  > class Monad m => XMLGen m where
>  >  type XML m
>  >  ....
>  >
>  > This works fine with 6.8.3, so what's new in 6.10, and what would I do
>  > to solve it?
>
>
> I'm guessing there was a bug in 6.8.3 that allowed this. (The
>  implementation of type families is present but not supported in 6.8,
>  presumably because of problems like this.)
>
>  I don't have 6.10, so I can't test anything, but you might try
>  rewriting the EmbedAsChild instances like so:
>
>     instance (XMLGen m, XML m ~ x) => EmbedAsChild m x where ...

Thanks a lot David, that's indeed what I needed.

I'm not sure I see why the style I used previously was illegal though,
it seemed perfectly natural to me. And it works that way for
`EmbedAsChild m (Child m)´, where `Child m´ is a data type family and
not a synonym, so why not for a synonym too? But hey, as long as
there's a way to do what I want. :-)

Cheers,

/Niklas