I agree with Henning, and moreover with wren's point that Data.Set is an often-unsatisfactory data structure for interning in general. Map can do the job more obviously and more comprehensively, at the cost of a few more pointers -- and when has Haskell ever been about trading clarity for fewer pointers? I don't think I'll formally vote, since I haven't written any Haskell in months, but I think the addition of this at-first-glance seemingly-useless function adds surprise and a little bit of headscratching to the user interface as a whole, to the extent that I feel like it has a "weirdness cost" even if most users can just ignore it. It also complicates (in my mind) the semantic story of Data.Set, from something purely about a collection of elements and a membership test to something which has specific opinions on which of two notionally "equal" things are most appropriate in a given situation. I think the cost of turning Data.Set from a simple thing into a complicated thing is a subtle one that should not totally be overlooked -- API simplicity is a virtue in itself. Add to that Henning and wren's evidence, which I interpret as saying that this function isn't quite the *perfect* tool for seemingly *any* job, and it doesn't seem worth the cognitive / maintenance cost to me. (I perhaps do not expect these considerations to be very strong or very important, but I think it would be unwise to ignore them altogether.) On Tue, Jun 28, 2016 at 11:19:16AM +0200, Henning Thielemann wrote:
On Mon, 27 Jun 2016, Nicolas Godbout wrote:
Example use case: In a parser, the memory footprint can be reduced by collapsing all equal strings to a single instance of each string. To achieve this, one needs a way to get a previously seen string (internally, a pointer) equal to a newly parsed string. Amazingly, this is very difficult with the current "containers" library interface. One current option is to use a Map instead, e.g., 'Map String String' which stores twice as many pointers as necessary.
I think I'd prefer the (Map String String) variant to an obscure Set lookup function. I wonder whether you will later use the Map anyway, as the compiler grows and you need to attach more data to tokens. In order to make your intent clearer you might define
newtype SharedToken = SharedToken String
and use (Map String SharedToken). _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries