On Wed, Feb 29, 2012 at 11:05 PM, AntC
I repeat: nobody is using a "type-level string". You (or someone) is making it up.
It isn't clear where that idea came from.
On Thu, Sep 15, 2011 at 7:51 AM, Simon Peyton-Jones
Yes, it would, and of course any impl of TDNR would need an internal constraint similar to your Select. Â In my original proposal I was hiding that, but it has to be there in the implementation. Â But you are right that making it explicit might be a good thing. Â Esp with Julien's new kind stuff (coming soon) we should be able to say
class Select (rec :: *) (fld :: String) where type ResTy rec fld:: * get :: rec -> ResTy rec fld
data T = MkT { x,y :: Int } instance Select T "x" where get (MkT {x = v}) = v
Oh.
On Mon, Jan 2, 2012 at 4:38 AM, Simon Peyton-Jones
It seems to me that there's only one essential missing language feature, which is appropriately-kinded type-level strings (and, ideally, the ability to reflect these strings back down to the value level). <snip> Specifically
* Allow String as a new kind
* Now you can define classes or types with kinds like
MyCls :: String -> a -> Constraint
T :: String -> *
* Provide type-level string literals, so that “foo” :: String
Huh. You may want to call your type-level-things-that-identify-fields strings, labels, fieldLabels, or rumbledethumps, but surely that's not the point of interest here? /g -- "Would you be so kind as to remove the apricots from the mashed potatoes?"