Following James's description, I would image constantly's implementation to look something like the following.
(defn constantly [value] #(identity value)) If you haven't seen the # macro before, the form below is equivalent. (defn constantly [value] (fn [] (identity value))) Making use of Chris Houser's fine repl-utils library, you can easily view the source at the REPL. user=> (clojure.contrib.repl-utils/source constantly) (defn constantly "Returns a function that takes any number of arguments and returns x." [x] (fn [& args] x)) Seems I wasn't too far off, the canonical version allows any number of args to be passed, but still just returns the value given at the time of declaration. I think it's safe to view constantly as a function wrapper around 'identity'. user=> (clojure.contrib.repl-utils/source identity) (defn identity "Returns its argument." [x] x) That is, I would view constantly to be the same as identity except it returns a function which can be passed around, rather than the value itself. Finally, you could just as well do '#(identity the-value)' and get the same result. user=> (= (#(identity "ab")) ((constantly "ab"))) true Sorry if I beat this to death; some of it was for my own learning and benefit. On Jan 25, 2:08 pm, wubbie <sunj...@gmail.com> wrote: > thanks James, > I'll have a look. > > -sun > > On Jan 25, 2:00 pm, James Reeves <weavejes...@googlemail.com> wrote: > > > On Jan 25, 5:34 pm, wubbie <sunj...@gmail.com> wrote: > > > > What's the typical usage of fn constantly ? > > > When you need a function that constantly returns the same result :) > > > That probably doesn't tell you any more than you know already, so I'll > > give you a real world use. My rnd-utils library has a function > > generating random strings that match a regular expression. It does > > this by converting a regex into a sequence of functions, and then > > concatenating the results into a string. > > > So a regex like "ab[def]" would be converted into two functions: > > [(constantly "ab") (rnd-choice "def")] > > > This can then be turned into a string using: (apply str (map apply > > function-sequence)) > > > - James --~--~---------~--~----~------------~-------~--~----~ 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 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 -~----------~----~----~----~------~----~------~--~---
