Re: What evil would happen?

["Chad R. Meiners" <crmeiners@hotmail.com>]

"Wojtek Narczynski" <wojtek@power.com.pl> wrote in message
news:5ad0dd8a.0307131407.4955979e@posting.google.com...
> "Chad R. Meiners" <crmeiners@hotmail.com> wrote in message
news:<bes4uv$1o29$1@msunews.cl.msu.edu>...
>
> > > "Functions" with side effects can only inject their side effect into
> > > where they have been hardcoded to.
> > This is wrong.  Function have the capability to randomly introduce side
> > effects just like any other section of code.
>
> Have you read the whole thread?
I read all the root nodes, but I was responding to the single sentence which
is incorrect.
>
> > In my experience, functions are more useful
> > when they are pure.  Sure there are times when a function cannot be
pure,
> > but sub-routine with a side-effect should be made a procedure in most
cases.
>
> I agree that functions should be pure. But procedures cannot return an
> unconstrained value. Again, this is what this thread is about.

However, procedure's can return parse trees.

> > I suggest that if you want to return composites from
> > functions you should return a record type.
> >
> > type Composite(StringSize : Natural) is record
> >      User_Name : String(1..StringSize);
> >      Result    : Boolean;
> > end record;
> >
> > function Parse_User_Name(Item : String) return Composite;
> >
> > However, the following might also be a suitable procedure for the task
>
> Yes, this works, but causes code explosion, and memory copying (when
> gathering all the parsed pieces together). This is what you can neatly
> do in SML with tagged union types, by the way :-)

You are prematurely trying to optimize.  Furthermore,  returning records
does not automatically cause code explosion or extra memory copying.  I have
used similar techniques when writing recursive decent parsers.  The code is
both readable and concise (it is also fast enough for any test I cared to
give it).  If you are going to write any sort of language parser, I
recommended browsing the source for GNAT's parser.  It is probably overkill
for what you want but you can get a lot of good ideas from it.  The way it
represents tokens is inspired. ;-)

-CRM