5.13.10 Main Differences from Standard REXX
Now, as the functionality has been explained, let me point out the
main conceptual differences from standard REXX; they are:

[Current position.]
     ARexx does not differ between a current read and write
     position, but uses a common current position for both reading
     and writing.  Further, this current position (which it is
     called in this documentation) can be set to any byte within
     the file, and to the end-of-file position. Note that the
     current position is zero-based.

[Indirect naming.]
     The stream I/O operations in ARexx do not get a parameter
     which is the name of the file. Instead, ARexx uses an
     indirect naming scheme. The OPEN() built-in function binds a
     REXX stream name for a file to a named file in the AmigaDOS
     operating system; and later, only the REXX stream name is
     used in other stream I/O functions operating on that file.

[Special stream names.]
     There are two special file names in ARexx: STDOUT and STDIN,
     which refer to the standard input file and standard output
     file. With respect to the indirect naming scheme, these are
     not file names, but names for open streams; i.e. they can be
     used in stream I/O operations other than OPEN(). For some
     reason, is it possible to close STDIN but not STDOUT.

[NOTREADY not supported.]
     ARexx has no NOTREADY condition. Instead, you must detect
     errors by calling EOF() and checking the return codes from
     each I/O operations.

[Other things missing.]
     In ARexx, all files must be explicitly opened. There is no
     way to reposition line-wise, except for reading lines and
     keeping a count yourself.

Of course, ARexx also has a lot of functionality which is not part
of standard REXX, like relative repositioning, explicit opening,
an end-of-file indicator, etc. But this functionality is
descriptive above in the descriptions of extended built-in
functions, and it is of less interest here.

When an ARexx script has opened a file in Write mode, other ARexx
scripts are not allowed to access that file. However, if the file
is opened in Read or Append mode, then other ARexx scripts can
open the file too, and the same state of the contents of the file
is seen by all scripts.

Note that it is difficult to translate between using standard REXX
stream I/O and ARexx stream I/O. In particular, the main problem
(other than missing functionality in one of the systems) is the
processing of end-of-lines. In standard REXX, the end-of-file is
detected by checking whether there is more data left, while in
ARexx one checks whether the end-of-file has been read. The
following is a common standard REXX idiom:

while lines(‘file’)>0   /* for each line available */
     say linein(‘file’)   /* process it */
end

In ARexx this becomes:

     tmp = readln(‘file’)    /* attempt to read first line */
     do until eof(‘file’)    /* if EOF was not seen */
          say tmp              /* process line */
          tmp = readln(‘file’) /* attempt to read next line */
     end

It is hard to mechanically translate between them,

because of the lack of an EOF() built-in function in standard
REXX, and the lack of a LINES() built-in function in ARexx.

Note that in the ARexx example, an improperly terminated last line
is not read as an independent line, since READLN() searches for an
end-of-line character sequence. Thus, in the last invocation tmp
is set to the last unterminated line, but EOF() returns true too.
To make this different, make the UNTIL subterm of the DO loop
check for the expression EOF(‘file’) && TMP<>“.

The limit of 1000 characters for READLN() means that a generic
line reading routine in ARexx must be similar to this:

     readline: procedure
          parse arg filename
          line = ‘’
          do until length(tmpline)<1000
               tmpline = readln(filename)
               line = line || tmpline
          end
          return line

This routine calls READLN() until it returns a line that is
shorter than 1000 characters. Note that end-of-file checking is
ignored, since READLN() returns an empty string a the end-of-
stream.


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