From hpacheco at gmail.com  Sun Jul  8 12:01:21 2007
From: hpacheco at gmail.com (Hugo Pacheco)
Date: Sun Jul  8 11:54:59 2007
Subject: [Template-haskell] Newbie question - class dependencies
Message-ID: <7b92c2840707080901g3158e75cq49a2c87376265a9e@mail.gmail.com>

Hi,
I'm wondering if I can (ab)use from TH in order to generalize a
context-dependant function into a generic function without that context, as
long as I now that the arguments I'm applying the function to belong to the
class.

I guess this would work similarly to an automatic instance generation
example.

My example would be:

module Main where

myprint :: a -> String
myprint = show

main = do
    putStr $ myprint 1
    putStr $ myprint '2'

My ideia would be to stage the computation of *myprint* until it is called,
where the *Show* instance can be satisfied.

hugo
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From igloo at earth.li  Sun Jul  8 12:24:15 2007
From: igloo at earth.li (Ian Lynagh)
Date: Sun Jul  8 12:17:52 2007
Subject: [Template-haskell] Newbie question - class dependencies
In-Reply-To: <7b92c2840707080901g3158e75cq49a2c87376265a9e@mail.gmail.com>
References: <7b92c2840707080901g3158e75cq49a2c87376265a9e@mail.gmail.com>
Message-ID: <20070708162415.GA30332@matrix.chaos.earth.li>

On Sun, Jul 08, 2007 at 05:01:21PM +0100, Hugo Pacheco wrote:
> 
> module Main where
> 
> myprint :: a -> String
> myprint = show
> 
> main = do
>    putStr $ myprint 1
>    putStr $ myprint '2'
> 
> My ideia would be to stage the computation of *myprint* until it is called,
> where the *Show* instance can be satisfied.

Do you mean that you want to have

    myprint :: Integer -> String
    myprint = show

generated when GHC notices that myprint is called with an Integer type?
Is so, then no, TH, can't do that.

What you can do is to write a function

    mkMyPrint :: Type -> Q [Dec]

which you could then call like so:

    $( mkMyPrint ''Integer )

but you have to do so by hand.


Thanks
Ian

From hpacheco at gmail.com  Sun Jul  8 13:10:43 2007
From: hpacheco at gmail.com (Hugo Pacheco)
Date: Sun Jul  8 13:04:21 2007
Subject: [Template-haskell] Newbie question - class dependencies
In-Reply-To: <20070708162415.GA30332@matrix.chaos.earth.li>
References: <7b92c2840707080901g3158e75cq49a2c87376265a9e@mail.gmail.com>
	<20070708162415.GA30332@matrix.chaos.earth.li>
Message-ID: <7b92c2840707081010i32287f55w2cba9f313b47d9e0@mail.gmail.com>

What I do need is to delay the evaluation of myprint, so I can replace it
with the specialized version, but you said it isn't possible.
Thanks anyway,
hugo
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