I think this is a very good explanation (though my opinion doesn't carry much weight, I'll admit :-).
One thing, IMHO, is you could explain why a special form (such as IF) cannot be written using a macro. I remember reading about that in a book or a tutorial and I think it would be apt here too. Cheers Vijay On 6/22/06, Larry Clapp <[EMAIL PROTECTED]> wrote: > *** What's the difference between a function, a macro, and a special > form? > > Functions, Macros, and Special Forms > ------------------------------------ > > When a function is called, all of its arguments are evaluated. A > function can be FUNCALL'ed or APPLY'ed. Anybody can write a function: > the programmer or the compiler writer. > > When a macro is called, none of its arguments are evaluated, but are > instead passed directly to the macro as lists (or whatever). A macro > cannot be FUNCALL'ed or APPLY'ed. Anybody can write a macro: the > programmer or the compiler writer. > > When a special form is called, the evaluation of its arguments depends > on the special form in question. For example, IF evaluates its first > argument, and depending in its outcome, then evaluates either the > second or third of its arguments. LET does something different. > PROGN does something different still. Only the compiler writer can > write a special form, and the list of special forms is pre-defined and > fixed; see section 3.1.2.1.2.1 Special Forms in the CLHS. Special > forms can also do other strange and interesting things that neither > functions nor macros can do, like EVAL-WHEN or THE. Each special form > defines its own semantics. > > Functions & Macros > ------------------ > > "Variables abstract over values, functions abstract over behavior, > macros abstract over syntax." > -- Joe Marshall, 23 Mar 2004, [EMAIL PROTECTED] > > Abstracting behavior: Instead of writing the same code over and over, > you write a function and call it. You've abstracted the behavior of > the code into a function. > > Say you have a window with a button. You want another window to pop > up when the user presses the button. You write a function to make the > window pop up, and when the user presses the button, you call the > function. You can call the function from other places, and the same > window will pop up. > > Abstracting syntax: Instead of writing the same *looking* code over > and over, you write a macro and call it. You've abstracted the syntax > of the code, or the way the code looks, into a macro. > > Say you find yourself saying this a lot: > > (let ((x (do-something))) > (if x > (do-something-with x) > (let ((y (do-something-else))) > (if y > (do-something-with y) > ...)))) > > and you think, "Too much repetition! Too much boilerplate! There's > got to be a better way!" And there is. You write a WHEN-LET macro, > and your code changes to > > (when-let ((x (do-something)) > (do-something-with x)) > ((y (do-something-else)) > (do-something-with y)) > (...)) > > No boilerplate, no repetition. You've added new syntax. You can use > the same syntax somewhere else, and your code will behave the same > way. > > -- L > > > _______________________________________________ > cl-faq mailing list > cl-faq@lispniks.com > http://www.lispniks.com/mailman/listinfo/cl-faq > _______________________________________________ cl-faq mailing list cl-faq@lispniks.com http://www.lispniks.com/mailman/listinfo/cl-faq
