default instance for IsString

Erik Hesselink hesselink at
Tue Apr 24 12:08:41 CEST 2012

On Tue, Apr 24, 2012 at 10:55, Michael Snoyman <michael at> wrote:
> On Tue, Apr 24, 2012 at 11:36 AM, Erik Hesselink <hesselink at> wrote:
>> On Tue, Apr 24, 2012 at 08:32, Michael Snoyman <michael at> 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.


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