Steven D'Aprano <[EMAIL PROTECTED]> writes:
> Now, if you want to tell me that, despite all the talk, Lisp coders don't
> actually create new syntax or mini-languages all that often, that they
> just use macros as functions, then the question becomes: why do you need
> macros then if you are just using them as functions? Why not use functions?
Macros let you write what amounts to functions that don't evaluate
their arguments. Think of the endless clpy wars over the ternary
conditional operator. You want to write something like
def ternary(test, iftrue, iffalse):
if test: return iftrue
else iffalse
but because of side effects, you don't want
a = cond(test, f(x), g(x))
to evaluate both f and g. That is trivial to do with a macro but
can't be done with a function. Macros can be used tastefully and
effectively in Lisp programs, though not everyone limits their usage
like that.
Haskell uses a different approach: 1) all arguments are unevaluated
unless the program tries to actually use the values. 2) there are no
side effects anyway. This may be why Haskell's current lack of macros
apparently isn't a big problem.
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