TITLE

Shell Style Redirection

VERSION

  Maintainer: David Nicol <perl6rfc@davidnicol.com>
  Date: 8 Aug 2000
  Last Modified: 5 Sep 2000
  Mailing List: perl6-language-io@perl.org
  Number: 66
  Version: 2
  Status: Developing

ABSTRACT

The use redirect pragma or a new redirect keyword is introduced to allow redirection of streams of any kind into each other.

The keyword may not be really needed, as when does it make any sense to do a lt or gt comparison with a file handle?

SUMMARY

	sub callfritz{

	   local STDIN < $InputData;

	   local STDOUT > PREVIOUSLY_OPENED_HANDLE;

	   eval `cat fritz.pl`;

	};

is proposed as an alternative to doing the same thing with a variety of open2 calls.

DESCRIPTION

As an alternative to the Bourne shell style open syntax described in `perldoc -f open`, this proposal overloads the less than and greater than operators in order that subsequent statements, particularly external routines that will be looking to a file descriptor table for their file handles, will get from and give to where we want them to.

The redirection is affected by the scoping operators like any other variable which alters the situation.

It will also provide another way to capture STDERR from within backticks.

	sub ToolErrorsFirst{

		# place to hold the output
		my $tooloutput;	

		# place to hold the errors
		my $errors;

		# use redirect: very similar to shell redirection
		my use redirect STDOUT > $tooloutput ;   

		# alternately, FILE > $scalar is obviously a redirect
		# (if we accept the overload of >)
		# so no additional keywords are required
		my  STDERR > $errors ;   

		$tooloutput = `tool @_`;
		
		return $errors . $tooloutput;

	};

Currently I do this kind of thing by using the file system as temporary storage.

another use of the angle brackets would be as a single-character print operator, similar to << in C++ streams.

	print $abc;
	print OUT $z;	# one way to do it

	< $abc;
	OUT < $z;	# another way to do it

Left-angle could differ from print by returning the file handle, instead of a success code, making C++ like constructions possible:

	< $this < "and" < $that; 	# same as print "${this}and$that"

IMPLEMENTATION

We need overloading based on type. The < operator will be like print when there is a file handle on the left side, it will be like assigning from <FH> when there is a file handle on the right, and it will be like the Bourne shell duplicate open when there are file handles on both sides.

Setting up a way to trap the standard error from a forked process and load it into a scalar -- will that be difficult?

MIGRATION

Code that compares file handles, or compares file handles with scalars, will break.

This file is part of the Perl 6 Archive