Path: news.cs.au.dk!not-for-mail
From: Peter Andersen <datpete@mjolner.com>
Newsgroups: comp.lang.beta
Subject: Re: IO-redirection?
Date: 26 Jun 2000 13:26:09 -0000
Organization: University of Aarhus, Department of Computer Science (DAIMI)
Lines: 87
Approved: mailtonews@cs.au.dk
Distribution: world
Message-ID: <20000626132609.18947.qmail@noatun.mjolner.dk>
Reply-To: Peter Andersen <datpete@mjolner.com>
NNTP-Posting-Host: daimi.cs.au.dk
X-Trace: xinwen.cs.au.dk 962026034 555361 255.255.255.255 (26 Jun 2000 13:27:14 GMT)
X-Complaints-To: news@cs.au.dk
NNTP-Posting-Date: 26 Jun 2000 13:27:14 GMT
Xref: news.cs.au.dk comp.lang.beta:12489

Atle writes:

> Hello, honorable members of the Beta community!
> 
> I have a small, quick problem, I should know this, but somehow my intuition doesn't ssem to work.
> I want to do the 
> printON(FILE* strm)
> {
> 	fprintf(strm, "This and that\n");
> 	... and so on ..
> }
> 
> I thought this was the way
> 
> printON:
> (#
> 	strm: ##STREAM;
> 
> 	'This and that'->strm.putLine;
> 	... and so on ...
> 
> #)
> 
> this compiles, but
> 
> 	screen##->printON;
> 
> does not?
> 
> I know I'm doing something ver wrong here, but .. what?

First of all you are missing a specification of the enter list
of printON:

   enter ##strm

BUT:

the notation ##STREAM means a "pattern reference" to the pattern
stream. In the application "strm.putLine" this would mean
"instantiate a new object corresponding to the pattern referenced,
and then invoke the putLine method of that object".
And the syntax screen##->printON would mean: "Obtain the pattern
information (i.e. type information) of the screen object, assign this
pattern information to the enter parameter of printON, and invoke
the do-part of printON".

This would probably work, but it is inefficient to first extract the
type information of the screen-object and then instantiate an instance
of it (which will immediately become garbage after the printON has
completed). Instead you should use *object references* as in:

printON:
  (#
     strm: ^STREAM;
  enter strm[] (* stream to print on *)
  do 'This and that'->strm.putLine;
     ... and so on ...
  #)

....

do screen[] -> printON

The the existing screen object will be used, and the only objects
instantiated would be the printON and putLine objects.

For an overview of the BETA constructs you may also wish to
consult the "BETA Terminology" and "BETA Quick Reference Card"
which can be found in your "doc/beta-intro" directory.
For the latest versions see

http://www.mjolner.com/mjolner-system/documentation/tutorials.html

which refers these documents.

Sincerely,

Peter Andersen, Mjolner Informatics