Re: [Haskell-cafe] advice: instantiating/duplicating modules
Dave Bayer wrote: I've since done some experiments with Template Haskell, and I see that Arie Peterson has suggested how you could proceed. However, are you sure that you can't find a way to get this to work in vanilla Haskell without extensions? Or, for that matter, are you sure there isn't a way to get functional dependencies to work? (I felt pretty dumb for a little while before I found one for my problem, although as usual the issues are clear in hindsight.) Yeah, a class/phantom type approach now seems more natural. At the time, I had just learned about Template Haskell, and this problem seemed like a nice nail :-). The code works for me, so I never bothered to rewrite it. Greetings, Arie ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] advice: instantiating/duplicating modules
I wrote a combination reader/writer monad (a la the RWS monad in the mtl) and I find myself wanting to use multiple instances of it in the same stack of transformers. The functional dependencies prevent this from working out. The class is called MonadRW and the transformer is called RWT. I find myself wishing I could import the same module twice, but instead of having two names for the same entity, I want the definitions proper of the module to be duplicated: I want two separate MonadRW classes and two separate RWT transformers. Since the module integrates the MonadRW and RWT transformer with the mtl, I would then only need to lift the instances of the instantiated MonadRW classes through the other RWTs. I'm rather unfamiliar with Template Haskell, but it sounds like it might fit the bill. Although, if I recall correctly, instances and type declarations splicing are yet to be implemented in GHC? Does anyone have any advice how to do this? I'm envisioning some preprocessing. I'd like to know if someone has developed a tool for this already or if this path has been attempted. Thanks for your time, Nick PS - If this doesn't work out, I'm aware that I could use type-indexed products and coproducts to pull it off, but I think that would drastically reduce the number of people who could understand/maintain the code without having to learn such cool stuff. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] advice: instantiating/duplicating modules
Hello Nicolas, Friday, June 29, 2007, 9:07:38 PM, you wrote: I'm rather unfamiliar with Template Haskell, but it sounds like it might fit the bill. Although, if I recall correctly, instances and type declarations splicing are yet to be implemented in GHC? instances - definitely not. i've used TH to generate Show instances http://www.haskell.org/bz/th3.htm http://www.haskell.org/bz/thdoc.htm -- Best regards, Bulatmailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] advice: instantiating/duplicating modules
On Jun 29, 2007, at 10:07 AM, Nicolas Frisby wrote:
I wrote a combination reader/writer monad (a la the RWS monad in the
mtl) and I find myself wanting to use multiple instances of it in the
same stack of transformers. The functional dependencies prevent this
from working out.
I found myself in a situation implementing two very similar
algorithms where I had to really puzzle out how to get functional
dependencies to work, and I ended up writing a class (I abridge):
class (Num b, Real c) = Elem a b c | a - b, a - c where
qr :: a - b - b - QR c
and then declaring instances like
data Rlist a = Rlist
instance Integral a = Elem (Rlist a) a (Ratio a) where
qr w x y = Yes (x % y) False
When I wanted this version of qr, I'd call qr Rlist. Rlist is a
dummy class parameter to get the functional dependencies to work, it
does nothing besides select a version of my code. I couldn't get the
functional dependencies to work out any other way, so I accepted this.
Later, I realized that Haskell had named records, and I went back and
rewrote stuff as follows:
data (Num b, Real c) = Elem b c = Elem {
qr :: b - b - Rem c }
rlist :: Integral a = Elem a (Ratio a)
rlist = Elem {
qr = (\x y - Just (x % y, False)) }
Now all I had to do was change the case from qr Rlist to qr rlist
and the rest of my code worked exactly as before. (I love how often
this sort of thing happens in Haskell.) I went from thinking that
passing around a bunch of functions was kludgey, to thinking that a
beginner like me using multi-parameter type classes unnecessarily was
obfuscation. In any case, it didn't matter what I thought, the code
either way was virtually identical.
I've since done some experiments with Template Haskell, and I see
that Arie Peterson has suggested how you could proceed. However, are
you sure that you can't find a way to get this to work in vanilla
Haskell without extensions? Or, for that matter, are you sure there
isn't a way to get functional dependencies to work? (I felt pretty
dumb for a little while before I found one for my problem, although
as usual the issues are clear in hindsight.)
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