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

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Re[2]: [Haskell-cafe] I do not...

Niklas Broberg

11 Oct 2008 11 Oct '08

12:40 a.m.

dons:

...

A breakdown of the remaing causes for DependencyFailed, [...] 4 hsx-0.4.4

--------------- src/hsx$ runhaskell Setup build [snip warnings] 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? Btw, I also have problems with the haskell-src-exts that imports Data.Generics.Instances (to generate Data and Typeable instances). Where would these have moved to in the new base? And how would I make the code work with both 6.8.3 and 6.10? Thanks, /Niklas

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Niklas Broberg

11 Oct 11 Oct

12:45 a.m.

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

...

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 ....

Eh, reading that again I realize there's a bit code needed to understand the above. So here's *really* the relevant code: ---------------- class Monad m => XMLGen m where type XML m data Child m xmlToChild :: XML m -> Child m [....] class XMLGen m => EmbedAsChild m c where asChild :: c -> GenChildList m instance XMLGen m => EmbedAsChild m (XML m) where asChild = return . return . xmlToChild ---------------- ... and just a note that GenChildList is a type synonym for a certain monad returning a list, hence the double returns. :-) Cheers, /Niklas

Duncan Coutts

1:30 a.m.

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

On Sat, 2008-10-11 at 02:40 +0200, Niklas Broberg wrote:

...

Btw, I also have problems with the haskell-src-exts that imports Data.Generics.Instances (to generate Data and Typeable instances). Where would these have moved to in the new base? And how would I make the code work with both 6.8.3 and 6.10?

By having it use base-3 rather than 4. Duncan

David Menendez

1:53 a.m.

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

On Fri, Oct 10, 2008 at 8:40 PM, Niklas Broberg 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 ... Alternatively, you can make separate instances for every type in the range of XML. -- Dave Menendez http://www.eyrie.org/~zednenem/

Niklas Broberg

2:03 a.m.

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

On 10/11/08, David Menendez wrote:

...

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

Manuel M T Chakravarty

15 Oct 15 Oct

6:43 a.m.

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

Niklas Broberg:

...

On 10/11/08, David Menendez wrote:

...
On Fri, Oct 10, 2008 at 8:40 PM, Niklas Broberg 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. :-)

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

Niklas Broberg

11 Oct 11 Oct

7:02 p.m.

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

On 10/11/08, Niklas Broberg wrote:

...

dons:

...
A breakdown of the remaing causes for DependencyFailed, [...] 4 hsx-0.4.4

New version uploaded that works with both 6.8.3 and 6.10 rc1 (through dark cpp magic). I doubt I need to show this trick to anyone else since I seem to have been the only one brave/foolish enough to depend on this quirk of type families in 6.8. I'm proud to say I'm off the list now anyway. :-) Cheers, /Niklas

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David Menendez
Duncan Coutts
Manuel M T Chakravarty
Niklas Broberg