Re: default instance for IsString

24 Apr 2012

      On Tue, Apr 24, 2012 at 1:08 PM, Erik Hesselink  wrote:
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On Tue, Apr 24, 2012 at 10:55, Michael Snoyman  wrote:
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On Tue, Apr 24, 2012 at 11:36 AM, Erik Hesselink  wrote:
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On Tue, Apr 24, 2012 at 08:32, Michael Snoyman  wrote:
...
Here's a theoretically simple solution to the problem. How about
adding a new method to the IsString typeclass:
   isValidString :: String -> Bool
If you're going with this approach, why not evaluate the conversion
from String immediately? For either case you have to know the
monomorphic type, and converting at compile time is more efficient as
well. But we're getting pretty close to Template Haskell here.
I could be mistaken, but I think that would be much harder to
implement at the GHC level. GHC would then be responsible for taking a
compile-time value and having it available at runtime (i.e., lifting
in TH parlance). Of course, I'm no expert on GHC at all, so if someone
who actually knows what they're talking about says that this concern
is baseless, I agree that your approach is better.
But GHC already has all the infrastructure for this, right? You can do
exactly this with TH.
Erik
Yes, absolutely. The issue is that TH can be too heavy for both the
library author and user:

* For the author, you now have to deal with generating some `Q Exp`
instead of just producing your data with normal Haskell code.
* For the user, you need to replace "foo" with [qqname|foo|].

There's also quite a bit of TH hatred out there, but I'm definitely
not in that camp. Nonetheless, I *do* think it would be nice to avoid
TH in this case if possible.

Michael