From: Brian Candler <B.Candler@...>
Date: 2003-04-13T02:19:11+09:00
Subject: Re: The "!" and "?" characters.

On Sun, Apr 13, 2003 at 01:04:53AM +0900, Ryan Pavlik wrote:
> I wonder at this... Lisp has macros, and they don't make it harder
> to read.

Perhaps because it is impossible to make Lisp harder to read :-))

> Actually, Lisp-style macros applied to ruby would be somewhat
> more like a simple eval, where you might go (in pseodoruby):
> 
>     defmacro c_for(st1, st2, st3)
>         #{st1}
>         while(#{st2})
>             yield
>             #{st3}
>         end
>     end
...
>     c_for(i = 0, i < 200, i += 1) {
>         ...
>     }

I'm not sure if that's the best example, because you can do that anyway if
st1/st2/st3 are proc objects:

    def c_for(st1, st2, st3)
        st1.call
        while(st2.call)
            yield
            st3.call
        end
    end

    i = nil
    c_for(proc {i = 0}, proc {i < 10}, proc {i += 1}) do
        puts i
    end

I'm not a Lisp hacker (I've tried and failed there), but are Lisp macros
more powerful than that?

Passing in a proc object means that it's pre-parsed, oven-basted and
ready-to-run. Furthermore you can pass them around as first-class objects:

    def make_c_for(st1, st2, st3)
        proc {
            st1.call
            while(st2.call)
                yield 
                st3.call
            end
        }
    end

    i = nil
    myloop = make_c_for (proc {i = 0}, proc {i < 10}, proc {i += 1}) do
       puts i
    end

    myloop.call

This kind of function composition was something I saw quite a lot of when
trying to learn Lisp.

Regards,

Brian.