Output redirection (REDIRECT-OUTPUT)

Reference

MAE/4

Author

M. Anton Ertl (anton@mips.complang.tuwien.ac.at)

Problem

Many words exist for convenient output to the user output device (e.g., ., .S, F.). Replicating this functionality for output to files is very cumbersome and a lot of work.

Proposal

redirect-output ( file-id xt -- ) file-ext

Set the user output device to file-id, EXECUTE xt, restore the old user output device. If an exception is THROWn during the execution of xt, the old user output device is restored, and the exception is THROWn onwards.

Typical Use

( r ) report-file @ ['] f. redirect-output

Remarks

The syntax is CATCH-like to ensure proper restoration under all circumstances (including THROWs). A more convenient syntax should be found for CATCH as well as redirect-output.

Michael Gassanenko's point about output to memory is a good one. Either we address this by adding words for creating fids for memory buffers, or we change this proposal to take an xt ( c-addr u -- ) instead of a fid.

Why not simply have a variable for the output file, similar to BASE? Providing varying bases through a variable BASE was a mistake; I am sure no Forth programmer will have trouble reciting a story where BASE lead to problems. A similar design mistake for output redirection would cause more trouble (e.g., some bug causes a THROW while output is redirected => the user does not even know that something happened).

STDOUT is used in Gforth for the default OUTFILE-ID (i.e., the standard output at the start of the system).

Experience

none

Comments

Michael L. Gassanenko:

xt... It smells LISP, and I do not like LISP smell in Forth.

maybe,

 redirect-output N>R ... NR> restore-output
	

would be better than

: aux14 ." xt=" MYVAR @ U. ;
...  ['] aux14 redirect-output
	

What about redirecting output to strings? (IMO, it would be more useful)

What hapens if the memory buffer being the current output device overflows?

Peter Knaggs:

Why not define a STDIN and STDOUT words witch provide the standard fild-id for terminal I/O and redefine all I/O words to be file based. Thus redirecting output would simply be a case of changing the output file id.

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