Introduction - Hello ISO World
==============================

First the traditional "Hello world" program. This requires only two
procedures from the Simple Text I/O module STextIO: WriteString to output
the greeting string, and WriteLn to output an end-of-line mark.

MODULE HelloISO;
(*
 * The traditional minimal program - using the ISO Standard I/O library.
 *)

IMPORT STextIO;	(* 'Simple' - ie stdin & stdout - text I/O *)

BEGIN
  STextIO.WriteString ("Hello ISO World");
  STextIO.WriteLn;
END HelloISO.

Note that output from STextIO goes to the standard output device (or channel,
to use the ISO Standard term) stdout, which is by default the terminal screen
but can be redirected by operating system command line redirection (">" or
">>") to a file. (The I/O library also offers redirection from the program -
see StdChans later.)



Copying files - 1 - stdin to stdout
===================================

Now let us move on to a program which also uses input - a simple copy file
utility. We will develop several alternative versions in order to try out
various I/O procedures. The copy program will also serve as a useful partial
solution to any problem which reads a file, processes the data, and writes an
output file.

First, the obvious strategy: repetitively read a character, write that
character, until end of file. This translates into a Modula LOOP loop
(this form of loop is appropriate since the read must precede the termination
test, which depends on the result of the read attempt, and the write follows
a successful read - thus this is the classic "n&1/2-times" loop, with the
termination test in the middle). The loop uses STextIO.ReadChar to read the
characters, SIOResult.ReadResult to check the result of the read, and
STextIO.WriteChar to write the characters. The complication is that the
Standard treats an end-of-line mark as a special entity, not a normal
character; thus when ReadChar attempts to read an end-of-line mark, the read
fails - ReadResult returns a special status, and STextIO.SkipLine must be
used to consume the end-of-line mark. If this is not done, the read process
will never get past the end-of-line mark - ReadChar will continue to return
the endOfLine result. [Note 1]

MODULE Copy1;
(*
 * Copy a file - version 1 - STextIO.ReadChar/WriteChar
 *)

IMPORT STextIO, SIOResult;
IMPORT ProgArgs;

VAR
  ch : CHAR;

BEGIN
  LOOP
    STextIO.ReadChar (ch);
    CASE SIOResult.ReadResult() OF
    | SIOResult.endOfInput : EXIT;
    | SIOResult.endOfLine  : STextIO.SkipLine;
                             STextIO.WriteLn;
    | SIOResult.allRight   : STextIO.WriteChar (ch);
    ELSE
      ProgArgs.Assert (FALSE, "Strange read result");
    END;
  END (* LOOP *) ;
END Copy1.

As explained by its definition, SkipLine in fact skips any number of
characters up to and including the next end-of-line mark. Thus it can be
called at any time to discard the rest of the current line of input. As it is
used by Copy1, there is always only the end-of-line mark to be skipped.
Note too that every line is terminated by an end-of-line mark, including the
last. Copy1 terminates by reading all the normal characters of the last line,
then attempting to read the end-of-line mark and failing with the endOfLine
result, consuming the end-of-line mark with SkipLine, then attempting to read
the first character of a further line and failing with the endOfInput result.

As noted above, Copy1 can be used to copy a source file to a destination file
(instead of just copying keyboard input (stdin) to display output (stdout))
by operating system command line redirection:

	Copy1 < inputFile > outputFile



Copying files - 2 - named files
===============================

Alternatively, the program can explicitly read from and write to specified
files. In this case, the TextIO module is used instead of STextIO. All the
facilities of STextIO are duplicated in TextIO, with an extra first
parameter, which specifies the file from or to which the I/O occurs (the
Standard refers to them as "channels", emphasising that the source or
destination of I/O may not be a disk file - it could be a communications
line, or some other specialised device, or even the keyboard and display).
Thus the body of Copy1 is simply modified to produce Copy2:

MODULE Copy2;
(*
 * Copy a file - version 2 - TextIO.ReadChar/WriteChar
 *)

IMPORT StreamFile, TextIO, IOResult;
IMPORT ProgArgs;

VAR
  in, out : StreamFile.ChanId;
  ch : CHAR;

BEGIN
  ... more needed here ...
  LOOP
    TextIO.ReadChar (in, ch);
    CASE IOResult.ReadResult(in) OF
    | IOResult.endOfInput : EXIT;
    | IOResult.endOfLine  : TextIO.SkipLine (in);
                            TextIO.WriteLn (out);
    | IOResult.allRight   : TextIO.WriteChar (out, ch);
    ELSE
      ProgArgs.Assert (FALSE, "Strange read result");
    END;
  END (* LOOP *) ;
  ... more needed here ...
END Copy2.

Note that to declare the input and output channels, we must import the
channel identifier type from module StreamFile, and that SIOResult is also
replaced by IOResult, which specifies which channel's results are needed.

Now, how to initialise and finalise the in and out channels? (not needed for
STextIO, since stdin and stdout are always available and cleaned up at end of
program). We use the StreamFile module's Open procedure, specifying the
channel to be connected to the file, the name of the file, and a set of flags
indicating whether the file is to be opened for reading or writing, etc. At
the end of the program, we use StreamFile.Close to ensure that the final
output is actually written to the file and the access to the files is closed.

MODULE Copy2;
(*
 * Copy a file - version 2 - TextIO.ReadChar/WriteChar
 *)

IMPORT StreamFile, StdDevice, TextIO, IOResult;
IMPORT ProgArgs;

VAR
  in, out : StreamFile.ChanId;
  result : StreamFile.OpenResults;
  success : BOOLEAN;
  ch : CHAR;

BEGIN
  (* Open the input and output files *)
  StreamFile.Open (in, "copy2.in", StreamFile.read, result);
  ProgArgs.Assert (result = StreamFile.opened, "Unable to open source file");
  StdDevice.Delete ("copy2.out", success);
  StreamFile.Open (out, "copy2.out", StreamFile.write, result);
  ProgArgs.Assert (result = StreamFile.opened, "Unable to open output file");

  (* Copy the data *)
  LOOP
    TextIO.ReadChar (in, ch);
    CASE IOResult.ReadResult(in) OF
    | IOResult.endOfInput : EXIT;
    | IOResult.endOfLine  : TextIO.SkipLine (in);
                            TextIO.WriteLn (out);
    | IOResult.allRight   : TextIO.WriteChar (out, ch);
    ELSE
      ProgArgs.Assert (FALSE, "Strange read result");
    END;
  END (* LOOP *) ;

  (* Tidy up *)
  StreamFile.Close (in);
  StreamFile.Close (out);
END Copy2.

Note that Open (..., read, ...) requires that the file already exist (ie, the
"old" flag is implied). Also, the result of the Open attempt is returned in a
parameter of type OpenResults.
An Open (..., Write, ...) will succeed whether or not the file already
exists. If it does not exist, it is created empty. If it does exist, the
program output will simply overwrite whatever is previously in the file,
leaving trailing old contents if the number of characters written is less
than the old size of the file. Since we do not want this "update" behaviour,
we first call the Delete procedure of module StdDevice to ensure any old file
is gone; we ignore the result BOOLEAN from Delete since the failure may have
been due to the fact that the file did not exist.

But Copy2 is rather inflexible - it always reads from "copy2.in" and writes
to "copy2.out"; contrast this with Copy1, which could copy any files by
simply changing the command line redirection. To make Copy2 more useful, the
file names must be string variables (ARRAYs OF CHAR):
  
  inFileName, outFileName : ARRAY [1..fileNameLength] OF CHAR;
  ...
  StreamFile.Open (in, inFileName, StreamFile.read, result);
  ...
  StreamFile.Open (out, outFileName, StreamFile.write, result);

Now, how to obtain values for these string variables? To approximate command
line redirection, we would need to supply the file names as command line
arguments, and use the ProgArgs facilities for parsing command lines:

  (* Argument processing - get required arguments or give usage message *)
  IF ProgArgs.ArgNumber() = 3 THEN
    (* Program name plus two file name arguments *)
    ProgArgs.GetArg (1, inFileName);
    ProgArgs.GetArg (2, outFileName);
  ELSE
    TextIO.WriteString (StdChans.StdErrChan(), "Usage: copy2 inputFile outputFile");
    TextIO.WriteLn (StdChans.StdErrChan());
    HALT;
  END;

Copy2 would now be invoked by

	Copy2 inputFile outputFile

Alternatively, the file names might be returned by a call to some graphical
user interface procedure, which allowed the user to select them from a pick
list; a further possibility is for the program to issue text prompts and
read the file names from stdin:

PROCEDURE AskFileName (prompt : ARRAY OF CHAR; VAR fileName : ARRAY OF CHAR);
(*
 * Prompt on standard output and read a filename from standard input.
 * The prompt is "File name for <prompt>: "
 *)
BEGIN
  STextIO.WriteString ("File Name for ");
  STextIO.WriteString (prompt);
  STextIO.WriteString (": ");
  STextIO.ReadToken (fileName);
  STextIO.SkipLine;
END AskFileName;

Why ReadToken, not the expected (and available) ReadString? As detailed
later, ReadToken skips any leading white space and returns the next non-blank
group of characters; ReadString just reads as many characters as it can,
including leading, embedded and trailing white space, and possibly causing
problems with the Open processing.
Note too that this style of obtaining file names should be used with care: by
requiring user interaction at run time, it prevents the program being used in
redirected or pipelined environments, as is often done by command scripts
which automate processing or assemble tools to perform more complex tasks
(Unix 'filters').



Copying files - variations on the copy loop
===========================================

Now we turn to some variations on the basic copy loop. The variations will
use STextIO procedures, so that the copy source is stdin and the destination
stdout. This style is recommended as the simplest and most flexible when only
one input and one output file is required. However, TextIO equivalents exist
in all cases, simply dropping the "S" module prefix and adding file
parameters and Open and Close calls.

First variation - ReadRestLine
------------------------------

The STextIO procedure ReadRestLine will read an entire line into a string
variable, which can then be output in its entirety by WriteString. The only
problem is if a line is longer than the string array - ReadRestLine always
reads an entire line (or from wherever previous reads had left the input
stream), discarding anything which will not fit in the array, and setting the
ReadResult to outOfRange if some was discarded. The following program uses a
large array to make the problem unlikely, and reports any truncations. Since
ReadRestLine always leaves the input stream positioned before an end-of-line
mark, we can move the SkipLine/WriteLn to the end of the loop - but note that
the ReadResult = endOfLine CASE branch must be retained with no action: when
an empty line is encountered, ReadRestLine will return the empty string in
the array, and ReadResult endOfLine - the CASE statement must avoid an
invalid CASE selector error, and allow the end-of-line to be processed by the
code after the CASE statement.

  LOOP
    STextIO.ReadRestLine (line);
    CASE SIOResult.ReadResult() OF
    | SIOResult.endOfInput : EXIT;
    | SIOResult.endOfLine  : (* handled below *)
    | SIOResult.outOfRange : TextIO.WriteString (StdChans.StdErrChan(),
                                                 "Line truncated");
                             STextIO.WriteString (line);
    | SIOResult.allRight   : STextIO.WriteString (line);
    ELSE
      ProgArgs.Assert (FALSE, "Strange SIOResult");
    END;
    STextIO.SkipLine;
    STextIO.WriteLn;
  END (* LOOP *) ;

This version also uses output to the standard error channel stderr. The
module StdChans exports function procedures which return the channel ids of
stdin, stdout, and stderr, for use with TextIO procedures. Recall that output
to stderr is distinct from that to stdout: while both default to the display,
if stdout is redirected to a file, stderr will still appear on the screen
(unless separately redirected to the same or a different file - impossible on
DOS); thus stderr should be used for error messages which it is important
that the user see.

Second variation - ReadString
-----------------------------

The next version uses ReadString. This procedure reads up to the capacity of
the string passed as a parameter, or to end-of-line, whichever comes first.
Thus, compared with ReadRestLine, it does not discard any part of the current
line left when the string is filled. A subsequent ReadString will simply pick
up from where the last left off, so that the file is copied in chunks of
whatever size string variable is used. The end-of-line processing moves back
to the appropriate branch of the read result test:

  LOOP
    STextIO.ReadString (string);
    CASE SIOResult.ReadResult() OF
    | SIOResult.endOfInput : EXIT;
    | SIOResult.endOfLine  : STextIO.SkipLine;
                             STextIO.WriteLn;
    | SIOResult.allRight   : STextIO.WriteString (string);
    ELSE
      ProgArgs.Assert (FALSE, "Strange SIOResult");
    END;
  END (* LOOP *) ;

Third variation - ReadToken
---------------------------

Finally, the ReadToken procedure will skip leading spaces and tabs, and then
read as many non-space characters before the next end-of-line as will fit in
the string parameter; any remaining characters on the line are left for the
next read, as for ReadString. Thus ReadToken can be used to copy the file in
token chunks, discarding any spaces or tabs which separate them. If we add a
single space between tokens on a line, the output file has been reduced to
the minimum number of spaces (provided the string variable used is large
enough to avoid splitting any tokens).

  LOOP
    STextIO.ReadToken (string);
    CASE SIOResult.ReadResult() OF
    | SIOResult.endOfInput : EXIT;
    | SIOResult.endOfLine  : STextIO.SkipLine;
                             STextIO.WriteLn (out);
		             first := TRUE;
    | SIOResult.allRight   : IF NOT first THEN STextIO.WriteChar (' '); END;
                             STextIO.WriteString (string);
			     first := FALSE;
    ELSE
      ProgArgs.Assert (FALSE, "Strange SIOResult");
    END;
  END (* LOOP *) ;

Fourth variation - Raw I/O
--------------------------

Of course, if the aim is only to copy a file, we need not use text I/O
facilities at all. Module SRawIO may be used to read and write single bytes
or groups of bytes without any interpretation as special values like
end-of-line; this style of copy will also handle binary files like
executable images, etc. Unfortunately, the RawIO.Read and .Write procedures
read and write fixed amounts (the size of the buffer variable passed as a
parameter); RawIO.Read returns the wrongFormat ReadResult on the incomplete
buffer at end of file, but with no way of knowing how many bytes were read,
and RawIO.Write has no length parameter to allow an incomplete buffer to be
written. Thus they seem to be restricted to single-byte copies:

  LOOP
    (*
     * Raw IO copy - use a single-byte CHAR buffer if the length is not known
     *)
    SRawIO.Read (char);
    CASE SIOResult.ReadResult(in) OF
    | SIOResult.endOfInput : EXIT;
    | SIOResult.wrongFormat: ProgArgs.Assert (FALSE, "wrongFormat");
    | SIOResult.allRight : SRawIO.Write (char); END;
    ELSE
      ProgArgs.Assert (FALSE, "Strange SIOResult");
    END;
  END (* LOOP *) ;

or to copying files known to comprise fixed-length records:

  LOOP
    (*
     * Raw IO copy - file comprises fixed length records
     *)
    SRawIO.Read (record);
    CASE SIOResult.ReadResult(in) OF
    | SIOResult.endOfInput  : EXIT;
    | SIOResult.wrongFormat : ProgArgs.Assert (FALSE, "wrongFormat");
    | SIOResult.allRight    : SRawIO.Write (record); END;
    ELSE
      ProgArgs.Assert (FALSE, "Strange SIOResult");
    END;
  END (* LOOP *) ;

It is possible to combine the efficiency of reading and writing many bytes at
a time with detection of variable lengths - by using the lower-level IOChan
module's RawRead and RawWrite procedures. The ugly aspect of this is that the
buffer address must be passed; for a dynamically-allocated buffer, the
pointer to the buffer holds the address, but for static or automatic buffers
('normal' program variables) we must use SYSTEM.ADR to determine the address:

  LOOP
    (*
     * Raw IO at IOChan level.
     *)
    IOChan.RawRead (StdChans.StdInChan(), SYSTEM.ADR(buffer), bufferLength,
                    locsRead);
    CASE SIOResult.ReadResult() OF
    | SIOResult.endOfInput : EXIT;
    | SIOResult.allRight   : IOChan.RawWrite (StdChans.StdOutChan(),
                                              SYSTEM.ADR(buffer), locsRead);
    ELSE
      ProgArgs.Assert (FALSE, "Strange IOResult");
    END;
  END;


Other modules
=============

The following is a list of all modules; those already mentioned above are
marked * , and should cover most simple usage; others include a brief summary.

ARGDEVIC
	Low-level module providing device-dependant argument access for
	ProgramArgs - not implemented yet
CHANCONS
	Defines Open flags and results - imported and renamed by StreamFile,
	RndFile, SeqFile, TermFile.
IOCHAN	
	Low-level module providing device-independent Look, Skip,
	TextRead/Write, RawRead/Write and miscellaneous facilities for
	TextIO, WholeIO, RealIO, RawIO, IOResult
IOCONSTS
	Defines ReadResults - imported and renamed by IOResult.
IOLINK
	Low-level module providing channel <-> device facilities.
IORESULT *
LONGIO
	I/O for LONGREAL type - in gpm LONGREAL = REAL
PROGRAMA
	Argument handling - replacement for ProgArgs - not implemented yet
	Use ProgArgs
RAWIO	*
REALIO
	ReadReal and WriteFloat/Eng/Fixed/Real (latter Fixed if it will fit,
	else Float) - ie formatting flexibility as RealStr
RNDFILE
	Access to files for random access - Open, Close, positioning
SEQFILE
	Access to files for rewindable sequential access - Open, Close,
	ReRead, ReWrite
SIORESUL *
SLONGIO
	As for RealIO, but LONGREAL type. On gpm implementations, LONGREAL =
	REAL.
SRAWIO	*
SREALIO	*
STDCHANS *
STDDEVIC
	Low-level module providing device-dependent access to Unix/DOS
	streams (files, and devices such as buffered keyboard, display)
STEXTIO	*
STREAMFI *
SWHOLEIO *
TERMDEVI
	Low-level module providing device-dependent access to Unix/DOS
	terminal devices, giving control over buffering & echoing
TERMFILE
	Access to terminal devices - Open, Close, buffering & echoing mode.
TEXTIO	*
WHOLEIO	*