[Haskell-cafe] Is Template Haskell a suitable macro language?

[Jacques Carette]

Magnus Jonsson wrote:
> I have the same problem too when using Haskell. The more I try to 
> enforce static guarantees the more I get lots of datatypes that are 
> similar except for one or two constructors. The best way I have found 
> to avoid this is to simply give up on some of the static guarantees 
> and just use one datatype that contains all the constructors. Less 
> static guarantees but also less needless type coaxing between 90% 
> similar types. I haven't tried using macros.
In Ocaml, you can frequently use polymorphic variants to get the same 
effect.

Which means that if you are willing to do enough type-class-hackery, it 
should, in principle, be possible to do the same in Haskell.  But it 
sure isn't as convenient!

This all points to some clear need for more ``flavours'' of polymorphism 
being needed (in Haskell), to be able to express *in the type system* 
what TH allows you to say "outside".

Jacques