I've been coding in Clojure since mid-2008, I have a Lisp background but coded mainly in Java for a number of years (2000-2010)
I still goof from time to time with parenthesis. Just yesterday, I screwed up some expression imbrication in a module and it took me at least 30 mns to figure it out. Ok it was at the end of a very long day but I made a stupid mistake, I kept hammering on the problem endlessly. It was not a compilation problem, the logic was just wrong. I relied on the editor automatic original indentation and had not re-indent after changing some of the code. That happens from time to time. The best thing is to sit back, take a deep breath and look at in a detached way. Most of the time it blows up in your face and you just bang your head on your desk repeating yourself that you are an idiot. For me the fac compilation error is clear because I searched immediately for a value binding named fac since fac here was not a fn call. There are no binding named fac only an fn. At that point it's pretty obvious that a call to fac is missing hence the missing parenthesis set around fac (- n 1). That comes from practice. I do not see how you can get wired to it otherwise. I rarely now type code to call a function without the required parenthesis. The advantage here is that most of the time you have less code lines to deal with compared to its Java equivalent. Luc P. On Tue, 2 Aug 2011 15:19:05 -0700 Sean Corfield <seancorfi...@gmail.com> wrote: > On Tue, Aug 2, 2011 at 12:11 AM, recurve7 <dan.m...@gmail.com> wrote: > > > user=> (defn fac [n] (if (= n 1) 1 (* n fac (- n 1)))) > > #'user/fac > > user=> (fac 3) > > java.lang.ClassCastException: user$fac cannot be cast to > > java.lang.Number (NO_SOURCE_FILE:0) > > > > Let's assume you'd put the code in a source file and run that. Now > you'll get: > > java.lang.ClassCastException: user$fac cannot be cast to > java.lang.Number (fac.clj:1) > > Where line 1 of fac.clj is: > > (defn fac [n] (if (= n 1) 1 (* n fac (- n 1)))) > > I'm not sure the positional information is all that helpful when > there's only one line of code so let's look at the exception itself: > > user$fac cannot be cast to java.lang.Number > > The REPL told us we defined #'user/fac so that's our user$fac piece. > We're using it in (* n fac (- n 1)) so that must be where the > compiler is expecting a number instead of fac. What is that > expression? It has a function * and three arguments: n, fac, (- n 1). > n and (- n 1) are numbers but fac is not (it's a Clojure function - > user$fac). > > So what's wrong? It looks like parentheses are missing in that > expression and it should be a nested call to fac instead of a use of > the value of fac: > > (* n (fac (- n 1))) > > I've been monitoring the Java Ranch forum on Clojure and omitting > parentheses seems to be a fairly common mistake folks make when > coming from Java. I guess that the parentheses being in the > unfamiliar position before the function makes it easy to write func > args instead of (func args), especially if args is already an > expression like (- n 1)? > > Given the prefix nature of Lisp syntax, when confronted with (func > arg1 arg2 arg3), as long as func can take three arguments, the > compiler is only going to be able to give a type mismatch error (at > best) if any of those arguments can't be converted to an obvious type > (and Clojure's general lack of compile-time checking means you'll get > runtime casting errors). > > Coming from a Java background, that's doubly hard to understand > because a) the type checking is done at runtime and b) many functions > can take an arbitrary number of arguments. It's really not a flaw > with Clojure's compiler, nor something that it can do much about, but > it gets much easier to figure out once you've spent more time with > the language. -- Luc P. ================ The rabid Muppet -- 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
