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.
>>
>>
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