On 1/20/14, 12:38 PM, Andy Smith wrote: > Hi, > > (let bindings form) is a special form. As I understand it, let can be > reformulated in terms of functions e.g. > > (let [x 2] (* x 20)) equivalent to ((fn [x] (* x 20)) 2) > (let [x 3 y (* x x)] (- y x)) equivalent to ((fn [x] ((fn [x y] (- y x)) x > (* x x))) 3) > > So if we can always reformulate in this was, why cant let be implemented as > a macro rather than a special form? > > Where have I gone wrong in my understanding here? > > Andy >
Clojure has made several choices which make implementing let via macro expanding to function application a nonstarter: 1. Clojure is compiled. There is no interpreter for clojure, all clojure code is compiled to bytecode before executing, even at the repl. So the compiler needs to be fast for interactive work and cannot spend a lot of time in optimizing passes typically used to avoid function call overhead for every local name binding. 2. Debuggers There are a number of nice debugging tool kits for Java/the JVM. They operating by reading metadata (like local names) from class files. Because clojure maps let bound locals to the same JVM construct as Java, you can re-use Java debuggers for Clojure. 3. JVM interop The JVM does not have TCO built in, so in order to provide general constant space tail calls you must do extensive code transforms(breaks 1 and 2) or use your own calling convention(method call and trampoline, instead of just a method call) which would complicate interop. Clojure instead provides less general construct loop/recur for constant space iteration. loop/recur is local to a function body, so if let macro expanded to a fn call, you would not be able to recur from inside a let. So while technically possible (there are scheme impls on the jvm that may) Clojure does not expand let in to a fn call -- And what is good, Phaedrus, And what is not good— Need we ask anyone to tell us these things? -- -- 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/groups/opt_out.
