2011/9/4 julianrz <julia...@yahoo.com> > Hello All, > I am new to Clojure. Surprised why this code does not work: > > user=> (filter #(%) [1 2 3]) > ClassCastException java.lang.Long cannot be cast to clojure.lang.IFn > > Here my intent behind #(%) is to define a lambda function returning > its argument. Since Clojure defines truth on any type, it should be > acceptable as filter function, and the result should be the original > array > > Some narrowing down. What's #(%) really good for? >
Hello, #(%) is a macro which expands to (fn* [p1#] (p1#)) #(%) could only be 'useful' if % is a callable without arguments. What you seem to be after is the builtin function named identity : identity returns its argument : Clojure> (filter identity [1 true false nil]) (1 true) Clojure> HTH, -- Laurent > > user=> #(%) > #<user$eval48$fn__49 user$eval48$fn__49@5e2c17f7> > > so it is a function > > user=> (#(%)) > ArityException Wrong number of args (0) passed to: user$eval26$fn > > Ok, here not enough arguments supplied, fair enough. Let's fix that: > user=> (#(%) 1) > ClassCastException java.lang.Long cannot be cast to clojure.lang.IFn > > Same problem as the first example. So Clojure understands this is a > function, knows it takes some arguments, but cannot call it? What's > wrong? The following works: > > (user=> (filter (fn [a] a) [1 2 3]) > (1 2 3) > > So apparently it has something to do with the fact that #() is > shorthand. So in my case, it produces a function which cannot be > used... In fact, it seems to just yield the constant, not a function. > Ask me, it should work (auto-wrap the constant in a function) or the > expression should be illegal. I think it deserves better > diagnostics... > > Also, in Practical Clojure book, it says: "The shothand function form > #(* %1 %2) is actually identical to the longer form (fn [x y] (* x y)) > before it is even seen by the compiler." If you literally apply this > rule, you will get > > ((fn[x] (1)) 1) > > which throws same exception, not surprisingly, since it tries to > evaluate 1 as a fucntion. What it should have done is transform the > expression into > > ((fn[x] 1) 1) > > which works fine... Special-case it? > > What do you think? > > Julian > > -- > 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 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
