Hi 2009/9/26 gerryx...@gmail.com <gerryx...@gmail.com>: > > (defn float2 [f a b] > (f (float a ) (float b))) > > (float2 + 1 2) => 3.0 > > (defmacro mfloat2 [f a b] > (f (float a) (float b))) > (mfloat2 + 1 2 ) => 2.0 ??? macro expend to last expression in > list,right?
This is because (mfloat2 + 1 2) expands to: + (float 1) (float 2) Notice that there are no parentheses around the whole thing. i.e. this is not a function call. And if you evaluate that, you get: user=> + (float 1) (float 2) #<core$_PLUS___4323 clojure.core$_plus___4...@b2ee9a> 1.0 2.0 Only the last value is used. Also: user=> (mfloat2 + (+ 0 1) (+ 1 1)) java.lang.ClassCastException: clojure.lang.PersistentList (NO_SOURCE_FILE:0) > (defmacro m2float2 [f a b] > `(~f (float ~a) (float ~b))) > (mfloat2 + 1 2) => 3.0 This expands as follows: user=> (macroexpand-1 '(m2float2 + 1 2)) (+ (clojure.core/float 1) (clojure.core/float 2)) so it does the right thing. And passing in expressions instead of literal numbers works too: user=> (m2float2 + (+ 0 1) (+ 1 1)) 3.0 > (defmacro m3float2 [f a b] > (list f (float a ) (float b))) > (m3float2 + 1 2) => 3.0 This one expands as follows: user=> (macroexpand-1 '(m3float2 + 1 2)) (+ 1.0 2.0) and you get the same error as for the first one if you try to pass in something that is not a literal number: user=> (m3float2 + (+ 0 1) (+ 1 1)) java.lang.ClassCastException: clojure.lang.PersistentList (NO_SOURCE_FILE:0) > functions of macro only not evaluating args ? A macro is basically a function that takes some code and produces other code before the compiler compiles the program. I am not an expert on macros, so maybe someone else here could give you a proper answer. > what's the difference between macro and call by name in other > language? I don't know what "call by name" is. A book that is often recommended for helping to understand lisp macros is "On Lisp" by Paul Graham. This is a book about Common Lisp macros, but they are quite similar to Clojure macros. The book is downloadable from here: http://www.paulgraham.com/onlisptext.html -- Michael Wood <esiot...@gmail.com> --~--~---------~--~----~------------~-------~--~----~ 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 -~----------~----~----~----~------~----~------~--~---
