you didn't beat it to death. I learned something. I also got confused. You identity implementation makes perfect sense. But even with your explanation, I'm still lost as to how the contrib one works. Lucky for me you brought this up so I can challenge myself to try to understand what's going on. Thanks.
On Sun, Jan 25, 2009 at 7:06 PM, rzeze...@gmail.com <rzeze...@gmail.com>wrote: > > 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 -~----------~----~----~----~------~----~------~--~---
