darcs hsx patch, add EmbedAsAttr instance for (Attr a (XMLGenT m v))

[Jeremy Shaw]

Hello,

Sometimes it would be nice to write code like this:

page =
...
<a href=fooURL>foo</a>

fooURL = XMLGenT m String
fooURL = return "http://example.org/foo"


Where the attribute value is in the XMLGenT monad. (This is very
useful when using web-routes).

But, we can't because there is no suitable instance. This patch adds
an instance,

> instance (EmbedAsAttr m (Attr a v), TypeCastM m1 m) => EmbedAsAttr
m (Attr a (XMLGenT m1 v))

(Thanks goes to Sergey Mironov for figuring this out).

- jeremy

[Thomas Hartman]

nice!

>  fooURL = XMLGenT m String

I assume :: was meant instead of =

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[Niklas Broberg]

> But, we can't because there is no suitable instance. This patch adds an
> instance,
>
>> instance (EmbedAsAttr m (Attr a v), TypeCastM m1 m) => EmbedAsAttr m (Attr
>> a (XMLGenT m1 v))

I thought this instance was already included, it's a clear oversight
that it isn't. Patch applied, thanks!

Cheers,

/Niklas

[Jeremy Shaw]

Sweet!

You have a similar feeling instance,

instance (XMLGen m, EmbedAsAttr m a) => EmbedAsAttr m (XMLGenT m a)
where
asAttr ma = ma >>= asAttr

But that is not quite the same. But is close enough to perhaps may you
think you covered this issue :)

- jeremy

[Niklas Broberg]

> You have a similar feeling instance,
>
> instance (XMLGen m, EmbedAsAttr m a) => EmbedAsAttr m (XMLGenT m a) where
>  asAttr ma = ma >>= asAttr
>
> But that is not quite the same. But is close enough to perhaps may you think
> you covered this issue :)

I think you're spot on. :)

/Niklas

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