Source code position in files

[Other proposals]


Some applications (e.g., the parser generator Gray) need to remember the current position in the source code (e.g., to refer to the position in error messages). If the source code is in blocks, the source position is available through BLK and >IN, but for source code in files there is currently no way to get at the file name and line number.


The string specified by c-addr u is the filename of the file currently being processed by the text interpreter. This string can be accessed during the whole lifetime of the system. An ambiguous condition exists if the innermost instance of the text interpreter is not processing input from a file, or if it was invoked with INCLUDE-FILE.
u is the line number of the line currently being processed by the text interpreter. The first line of the file has line number 1. An ambiguous condition exists if the innermost instance of the text interpreter is not processing input from a file.

Typical Use

sourcefilename filename 2! sourceline# line# !
filename 2@ type line# @ .


Starting the line count with 1 is a convention used in all editors and compilers I know in Unix.

The requirement to have the name available forever avoids the need for programs to make their own copies of the file name (possibly many times); a system implementing NEEDS or REQUIRED usually stores the file names for the whole session anyway.


Gforth and iForth have had these words since 1995 (I don't know the lifetime of the filename in iForth). I have not needed them often, but when I needed them, they were hard to replace (Marcel Hendrix replaced them in Gray by saving the current input buffer, once for every node (typically several per line, and hundreds or thousands in a real-world grammar); the result uses lots of memory, and is not as convenient to use as a version that can report line numbers. The uses I remember are Gray and assert.fs (assertions are much more useful if you know easily which assertion failed). AFAIK Marcel Hendrix used these words in his profiling utility.

Gforth relocates the filenames on SAVESYSTEM (transferring them from ALLOCATEd storage to ALLOTed storage). This has led to using a more complex interface instead of SOURCEFILENAME in one case, because the result of SOURCEFILENAME would not be valid across sessions: LOADFILENAME# @ ( -- u ) LOADFILENAME#>STR ( u1 -- addr u ); I would probably use different names and avoid the @ if I intended this to be used by others.


Michael Gassenenko (in <>):
There is a probability that a system will die from
memory overflow after executing INCLUDED n times.

For example, one person insisted that it must be 
possible to invoke a Norton-Commander-like shell 
from Forth, and to load Forth source files by
pressing Enter on them. I attached the Volkov Commander
(a free NC clone), and associated the action-on-enter
- write the name of the file on which Enter is pressed
the file zz.out
- place the code of 'F10' to the keyboard buffer to
force an exit from the Volkov Commander.
(Note: the file zz.out is cleared before the commander invokation). 

Nobody needs tens of the strings "zz.out" in the dictionary.
But how can one desable their creation?
The system could store the name only once.

Michael Gassanenko also suggested implementing SOURCELINE# in ANS Forth based on:

FILE-LINE# ( ud fid -- n ior )

Rewind the file specified by fid.
While the value returned by FILE-POSITION is less than ud,
perform the functionality of READ-LINE.
If no error condition was encountered,
return n, the number of encountered system-defined 
end-of-line delimiters, and ior equal to zero.
If an error has been encountered, ior is non-zero
and n is undefined.
If the end-of-file has been encountered before
the value returned by FILE-POSITION has become greater or
equal to ud, n is the number of lines in the file
(whether the last line not followed by the end-of line delimiter
is counted, is implementation-dependent).

Note. When an end-of-line delimiter is 
encountered, READ-LINE returns u2<u1 and true.


The user is allowed to do such things as
; how the system would know what has happened?

Anton Ertl