Changeset 8729 in project

Timestamp:

02/23/08 23:24:12 (12 years ago)

Author:

mfredrickson

Message:

Adding macro examples showing a task that would be hard with Ruby's eval.

File:

• wiki/chicken-for-ruby-programmers (modified) (1 diff)

wiki/chicken-for-ruby-programmers

@@ -745 +745 @@
 your program, which is usually the best time to do it.
 
-  TODO
-
+Scheme macros ''rewrite'' your code during compile time. They can
+range from simple to complex, with some macros defining entire
+"sublanguages" embedded in Scheme.
+
+Some people call Rails' {{acts_as_foo}} functions macros. This 
+description is not wrong, as these functions do ''rewrite'' your
+classes in a similar way to Scheme macros, but they are not quite
+as powerful.
+
+Here is a simple example of a task that is easy in Scheme, but 
+much, much harder using Ruby's eval. Say you were debugging a
+program and found yourself printing out variables at certain 
+points in the execution, along with the name of the variable 
+so you could tell what you were looking at. 
+
+<enscript highlight=scheme>
+  (print "myvar: " myvar)
+</enscript>
+
+You decide that repeatedly typing the variable name twice (once to
+indicate which variable, once to get the value) is a waste of time.
+Using a macro, you can quickly and easily abstract away the common
+syntax into one place.
+
+<enscript highlight=scheme>
+  (define-macro (ez-debug var)
+    (let ((name (symbol->string var)))
+      `(print ,name ": " ,var)))
+    
+  (define myvar '(this is a list))
+
+  (ez-debug myvar)
+</enscript>
+
+This simply wouldn't be possible with a function. By the time a
+procedure is called, syntactic information like variables names
+has been optimized away. 
+
+