Re: [Haskell-cafe] template haskell for typeclass synonyms

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 :

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

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2010/11/1 Paolino :

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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

-- Work is punishment for failing to procrastinate effectively.