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

Niklas Broberg:

On 10/11/08, David Menendez wrote:

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On Fri, Oct 10, 2008 at 8:40 PM, Niklas Broberg wrote:

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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. :-)

As suggested, it was a bug in 6.8.3 that you could make a class instance where the head involved a type synonym family. We cannot allow synonym families in class instances heads as it is impossible to check for overlap of such instances; eg, consider type family F a type instance F Bool = Int class C a instance C Int instance C (F a) Now a context (C (F Bool)) would match both instances. This is especially bad, as the type instance for F Bool may be defined in a different module as the instances for C; so, it is even in principle impossible to check for such overlap. The situation is different for data families as they are data types and not type synonyms. Moreover, instance (XMLGen m, XML m ~ x) => EmbedAsChild m x where ... is fine as it clearly overlaps with any other instance of EmbedAsChild. I hope that clarifies the situation. Manuel