[Haskell-cafe] template haskell for typeclass synonyms

[Gábor Lehel]

Well, Template Haskell is what you go to when what you want -can't- be
reasonably expressed with standard GHC Haskell. It's something of a
last resort (at least in my case). Typeclass synonyms can be faked
reasonably well with UndecidableInstances, but if you want to, for
example, automatically generate nontrivial instances of some class for
user-provided types, Template Haskell is frequently what you need to
use.

2010/11/2 Paolino <paolo.veronelli at gmail.com>:
> 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 <aslatter at gmail.com>
>>
>> 2010/11/1 Paolino <paolo.veronelli at gmail.com>:
>> > 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
>
>



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