This is a place where reducers and transducers diverge. Transducers don't have support for automatic kv reducing (there are some tricky details as to why this was possible in reducers but not as easy in transducers, which I mostly don't remember at this point). That is an area of possible future extension.
On Friday, May 19, 2017 at 12:49:09 AM UTC-5, Tianxiang Xiong wrote: > > I think this is one of those cases where I need to see an example to > understand. > > From what I can tell, the `(fn [f1] (rfn ...))` argument to `folder` is a > reducing-function-transformer--i.e. transducer, except there are some > differences in things like order of application in composition. I *don't* see > this 3-arity case for the transducer version of `clojure.core/map`. > > In my mind, the following are equivalent > > (r/reducer [1 2 3] (map f)) > > (r/map f [1 2 3]) > > So the following should be equivalent: > > ;; Reducer > (fn [f1] > (rfn [f1 k] > ([ret k v] > (f1 ret (f k v))))) > > ;; macroexpands to > (fn [f1] > (fn > ([] (f1)) > ([ret v] (f1 ret (f v))) > ([ret k v] (f1 ret (f k v))))) > > ;; and should be equivalent to the transducer (map f): > (fn [rf] > (fn > ([] (rf)) > ([result] (rf result)) > ([result input] > (rf result (f input))) > ([result input & inputs] > (rf result (apply f input inputs))))) > > > Yet they are clearly *not* the same: there is nothing in the transducer > about `[ret k v]`. So I must be missing something fundamental about the > relationship between reducers and transducers. > > > On Thursday, May 18, 2017 at 9:34:54 PM UTC-7, Alex Miller wrote: >> >> Reducers combine functionally, so they all have to support it to create >> any composite reducer that contains map. >> >> On Thursday, May 18, 2017 at 10:11:23 PM UTC-5, Tianxiang Xiong wrote: >>> >>> Would that ever be the case for `r/map`? Or does it only apply to >>> certain other reducers? >>> >>> On Thursday, May 18, 2017 at 4:11:39 PM UTC-7, Alex Miller wrote: >>>> >>>> The 3 arity is used when reducing over a map, like reduce-kv. Reducers >>>> do this automatically which varies from the core reduce. >>> >>> -- 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 --- You received this message because you are subscribed to the Google Groups "Clojure" group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
