Thanks Konrad. I certainly do not want to add this to fmap. However, I think there are lots of situations where mapping keys is well-defined. For example, you have a tree of data from XML or JSON, and you want to replace the keys (strings) with instances of their actual type.
I guess given the ambiguity that names like "mapkeys" might be misleading. Other suggestions? Stu > > On 30.05.2009, at 17:11, Stuart Halloway wrote: > >> clojure.contrib.generic.functor.fmap will return a map with values >> updated by a function. What if I wany *keys* updated by a function? >> Does this exist yet? If not, what should I name it for inclusion in >> contrib? > > Speaking strictly from an implementation point of view, you could add > a type tag to your map that identifies it as a "map for which fmap > acts on keys", and implement the multimethod fmap for that type. > > However, transforming keys is a much less well-defined operation than > transforming values. You can consider a map a function from keys to > values; fmap then returns the composition of that function with an > arbitrary function f. That's always well-defined. A transformation of > keys can lead to ambiguities: what if two different keys are > transformed to the same new key, but the associated values differ? > For this reason, the operation you want does not satisfy the > conditions for fmap. > > Finally, I don't know of any other module that implements such a > function, perhaps because of the ambiguity problem. > > Konrad. > > > > --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en -~----------~----~----~----~------~----~------~--~---
