How to write monoid protocol in Clojure?

问题

The following does not work, for obvious reasons.

(defprotocol Monoid
  (mappend [a b])
  (mzero []))

mzero has zero arguments, and zero argument methods are not allowed (or do not make sense) in protocols. In Haskell or Scala, where the dispatch is type-based rather than value-based, this is not a problem.

What would be the correct way to conceptualize and write Monoid protocol in Clojure?

回答1:

looking at the source, the way that this is implemented in the new reducers library is not as a procotol but an overloaded function. a no-args call is mzero; two args call is mappend.

more exactly, monoid takes two arguments - op and ctor and returns a function which, when called with no arguments, evaluates ctor, and when called with two, delegates to op.

this is consistent with how zero is handled in a fold, for example - reduce (fold) will evaluate the function being folded with no args to find the zero, if necessary.

i feel a bit ashamed to show something so unexciting, but i don't see how you can do better within clojure. thanks for the explanations/education in the comments.

来源：https://stackoverflow.com/questions/10767793/how-to-write-monoid-protocol-in-clojure