Data.Generics vs. Template Haskell

[Vyacheslav Akhmechet]

---------- Forwarded message ----------
From: Vyacheslav Akhmechet <coffeemug at gmail.com>
Date: Aug 24, 2006 5:35 PM
Subject: Re: Data.Generics vs. Template Haskell
To: Bulat Ziganshin <Bulat.Ziganshin at gmail.com>


Ok, so there are two aspects that I can see here. One such aspect is
ability to "reflect" on types (at runtime or compile time). An example
of that is creating an automatic mapping from relational databases to
haskell newtypes.

Another aspect is being able to get an a representation of Haskell
code AST in terms of Haskell datatypes in order to be able to process
Haskell ASTs in Haskell.

As far as I can see the first aspect is a subset of the second because
all Haskell types are known at compile time. If this is the case, why
the need for two libraries (I am not counting DriFT because it looks
like it hasn't been maintained for a long time)? Or am I missing
something?

Additionally, which one is more maintained and has the most potential
to become standard (whether de facto or real) in the future?

On 8/24/06, Bulat Ziganshin <bulat.ziganshin at gmail.com> wrote:
> Hello Vyacheslav,
>
> Thursday, August 24, 2006, 11:51:46 PM, you wrote:
>
> > I am trying to figure out where these two libraries stand in relation
> > to each other and which one is preferred to do generic programming in
> > Haskell. I understand that their goals often intersect but couldn't
> > find any good comparisons. Could someone point me in the right
> > direction?
>
> search for "generics" on hswiki, you should find a lot of papers. in
> particular, there is a new paper that compares many different
> approaches to generic programming. in particular, TH is not g.p.
> tools, it's just a universal Haskell code generator whcih can be used
> to solve particular tasks in this area. but to solve general problem
> of defining traversal function what has some general case and a nu,ber
> of type-specific cases TH is not very appropriate
>
> TH (and DriFT) are widely used to generate instances like the haskell
> compiler itself generates instances for Eq/Show/... and TH,
> unlike many g.p. tools does it at compile-time. using TH, you also get
> access to field/constructor names
>
> summary: if your task fits into the features provided by g.p. tools,
> you will gain a much time using them. if your task need compile-time
> code generation (so that generic program will be no slower than
> hand-written one) or you need some advanced features which your g.p.
> tool doesn't provide - welcome to TH with all its programming
> oddities. development of TH routines for g.p. is an order of magnitude
> slower (at least!) than doing the same with specialized tool
>
>
> --
> Best regards,
>  Bulat                            mailto:Bulat.Ziganshin at gmail.com
>
>