default instance for IsString

Simon Marlow marlowsd at
Tue Apr 24 15:21:43 CEST 2012

On 24/04/2012 11:08, Erik Hesselink wrote:
> 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.

No, Michael is right.  The library writer would need to provide

   fromString :: String -> Q Exp

since there's no way to take an aribtrary value and convert it into 
something we can compile.


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