On Tuesday, June 2, 2015 at 2:00:17 PM UTC-4, piast...@gmail.com wrote: > > I don't know how to read this: > > > http://www.leonardoborges.com/writings/2012/12/02/monads-in-small-bites-part-ii-applicative-functors/ > > (defmulti pure (fn [f _] f))(defmethod pure List [_ v] "Wraps value v in a > list" (List. [v])) > > > Is this a List followed by a vector of 2 elements, or this a List object > whose constructor is being called with 2 arguments? > > List [_ v] > > > Also, am I correct to say that "_" is used instead of "f" purely to > communicate intent to the next human developer who looks at this? It can > not be indicating a type or a signature to the JVM, yes? >
The (List. [v]) line is calling a class constructor, for some class named "List", with a single argument, which is a one-element vector containing the second argument to a multimethod whose first argument is being used solely to decide dispatch. The _ is a valid Clojure identifier, but one that is conventionally used to name an argument that is being ignored rather than used. So the dispatch function is ignoring the second argument and using the first, and the one method of the multimethod we see here is ignoring the first argument and using the second. You may sometimes see more than one _ in one place, e.g. [_ _ _ x y _ z], which technically has each _ produce a binding that shadows the preceding _ binding, but again if the convention is being followed none of those parameters are actually being used anyway. -- 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.
