On Mon, Nov 1, 2010 at 6:09 PM, Christopher Done wrote:

On 1 November 2010 17:53, Paolino wrote:

...

I'd like to have a template haskell function that take some constraints and a class name and write an empty class from those and relative empty instance to simulate typeclass synonyms.

As I've never written TH and couldn't find a easily adaptable code around, I ask here for the code, or some hints on how to arrive there.

I took Justin Bailey's haskelldb-th library as a TH example to work from and rewrote one TH function to try my hand at it, it's quite easy to follow with a simple example:

http://hpaste.org/paste/41035/demo

Maybe this is enough example to get you going. The rest you can find syntax parts from the TH Haddock documentation.

A useful FYI: the API docs are (almost) completely devoid of comments, but if you click to see the source, it does have some additional information in comments there, just not Haddock-formatted.

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-- Work is punishment for failing to procrastinate effectively.

I think I've got something nice in the end. http://hpaste.org/41042/classsynonymhs example: class ( ParteDi (Servizio a) s , Read a , Eq a , Show a , Integer `ParteDi` s ) => SClass s a $(classSynonym ''SClass) ghci ":i SClass" command is printing some strange type variables but it compiles paolino 2010/11/1 Gábor Lehel

On Mon, Nov 1, 2010 at 6:09 PM, Christopher Done wrote:

...

On 1 November 2010 17:53, Paolino wrote:

...

I'd like to have a template haskell function that take some constraints and a class name and write an empty class from those and relative empty instance to simulate typeclass synonyms.

As I've never written TH and couldn't find a easily adaptable code around, I ask here for the code, or some hints on how to arrive there.

I took Justin Bailey's haskelldb-th library as a TH example to work from and rewrote one TH function to try my hand at it, it's quite easy to follow with a simple example:

http://hpaste.org/paste/41035/demo

Maybe this is enough example to get you going. The rest you can find syntax parts from the TH Haddock documentation.

A useful FYI: the API docs are (almost) completely devoid of comments, but if you click to see the source, it does have some additional information in comments there, just not Haddock-formatted.

...

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

-- Work is punishment for failing to procrastinate effectively.

It's first time I use TH. It would be nice to point out the motivations for using it. If everything TH does is doable without it, the point of using it is write less code, eliminating some necessary and automatically computable code. But I guess there is some more ..... paolino 2010/11/2 Antoine Latter

2010/11/1 Paolino :

...

I think I've got something nice in the end.

http://hpaste.org/41042/classsynonymhs

example:

class ( ParteDi (Servizio a) s , Read a , Eq a , Show a , Integer `ParteDi` s ) => SClass s a

$(classSynonym ''SClass)

ghci ":i SClass" command is printing some strange type variables but it compiles

Template Haskell might be overkill for this. In the past, I've done:

...

class (Eq b, Show b, MyClass b, MyOtherClass b) => MySynonym b instance (Eq b, Show b, MyClass b, MyOtherClass b) => MySynonym b

I think this requires a couple of GHC extensions, but TemplateHaskell is an extension as well. Maybe there are pitfalls with this approach.

Antoine