+1 When I previously searched for (and failed to find) this function in order to implement an intern table, I remember trying 'lookup', 'lookupEQ' and 'find' in approximately that order, so that's my order if preference for its name. 'lookupInterned' or similar could work for me too if you want to scare off beginners although I don't see a strong need myself. Certainly a mention of interning in the haddocks would help. WHAT It is proposed to add a ‘lookup' function on 'Set' in the "containers" package. Feedback during the next two weeks is welcome. The function
lookup :: Ord a => a -> Set a -> Maybe a
is almost indentical to the 'member' function but, in addition, returns the value stored in the set. WHY The point of this proposal is to facilitate program-wide data sharing. The 'lookup' function gives access to a pointer to an object already stored in a Set and equal to a given argument. The 'lookup' function is a natural extension to the current 'lookupLT', 'lookupGT', 'lookupLE' and 'lookupGE' functions, with obvious semantics. 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. HOW The git pull request at https://github.com/haskell/containers/pull/291 contains the straight-forward implementation of the 'lookup’ function on 'Set', with test cases, as a patch against the current containers master branch. Salutations, Nicolas. _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries