  
  [1X10 [33X[0;0YStreams[133X[101X
  
  [33X[0;0Y[13XStreams[113X  provide  flexible  access  to [5XGAP[105X's input and output processing. An
  [13Xinput  stream[113X  takes  characters  from  some source and delivers them to [5XGAP[105X
  which  [13Xreads[113X  them  from  the stream. When an input stream has delivered all
  characters it is at [10Xend-of-stream[110X. An [13Xoutput stream[113X receives characters from
  [5XGAP[105X which [13Xwrites[113X them to the stream, and delivers them to some destination.[133X
  
  [33X[0;0YA major use of streams is to provide efficient and flexible access to files.
  Files  can  be  read  and  written  using [2XRead[102X ([14X9.8-1[114X) and [2XAppendTo[102X ([14X9.8-3[114X),
  however  the  former only allows a complete file to be read as [5XGAP[105X input and
  the  latter  imposes  a high time penalty if many small pieces of output are
  written  to  a  large file. Streams allow input files in other formats to be
  read  and  processed, and files to be built up efficiently from small pieces
  of  output. Streams may also be used for other purposes, for example to read
  from and print to [5XGAP[105X strings, or to read input directly from the user.[133X
  
  [33X[0;0YAny  stream  is  either  a  [13Xtext  stream[113X,  which  translates the [10Xend-of-line[110X
  character  ([10X\n[110X) to or from the system's representation of [10Xend-of-line[110X (e.g.,
  [13Xnew-line[113X  under  UNIX  and  [13Xcarriage-return[113X-[13Xnew-line[113X under DOS), or a [13Xbinary
  stream[113X,  which  does not translate the [10Xend-of-line[110X character. The processing
  of other unprintable characters by text streams is undefined. Binary streams
  pass them unchanged.[133X
  
  [33X[0;0YWhereas  it  is  cheap  to  append  to  a  stream, streams do consume system
  resources,  and  only a limited number can be open at any time, therefore it
  is  necessary  to  close  a  stream  as  soon  as possible using [2XCloseStream[102X
  ([14X10.2-1[114X).  If  creating  a stream failed then [2XLastSystemError[102X ([14X9.1-1[114X) can be
  used to get information about the failure.[133X
  
  
  [1X10.1 [33X[0;0YCategories for Streams and the StreamsFamily[133X[101X
  
  [1X10.1-1 IsStream[101X
  
  [33X[1;0Y[29X[2XIsStream[102X( [3Xobj[103X ) [32X Category[133X
  
  [33X[0;0YStreams  are  [5XGAP[105X  objects  and  all  open  streams, input, output, text and
  binary, lie in this category.[133X
  
  [1X10.1-2 IsClosedStream[101X
  
  [33X[1;0Y[29X[2XIsClosedStream[102X( [3Xobj[103X ) [32X Category[133X
  
  [33X[0;0YWhen  a  stream  is  closed, its type changes to lie in [2XIsClosedStream[102X. This
  category is used to install methods that trap accesses to closed streams.[133X
  
  [1X10.1-3 IsInputStream[101X
  
  [33X[1;0Y[29X[2XIsInputStream[102X( [3Xobj[103X ) [32X Category[133X
  
  [33X[0;0YAll input streams lie in this category, and support input operations such as
  [2XReadByte[102X ([14X10.3-3[114X) (see [14X10.3[114X)[133X
  
  [1X10.1-4 IsInputTextStream[101X
  
  [33X[1;0Y[29X[2XIsInputTextStream[102X( [3Xobj[103X ) [32X Category[133X
  
  [33X[0;0YAll  [13Xtext[113X  input  streams  lie  in  this  category.  They translate new-line
  characters read.[133X
  
  [1X10.1-5 IsInputTextNone[101X
  
  [33X[1;0Y[29X[2XIsInputTextNone[102X( [3Xobj[103X ) [32X Category[133X
  
  [33X[0;0YIt  is  convenient to use a category to distinguish dummy streams (see [14X10.9[114X)
  from others. Other distinctions are usually made using representations[133X
  
  [1X10.1-6 IsOutputStream[101X
  
  [33X[1;0Y[29X[2XIsOutputStream[102X( [3Xobj[103X ) [32X Category[133X
  
  [33X[0;0YAll output streams lie in this category and support basic operations such as
  [2XWriteByte[102X ([14X10.4-1[114X) (see Section [14X10.4[114X).[133X
  
  [1X10.1-7 IsOutputTextStream[101X
  
  [33X[1;0Y[29X[2XIsOutputTextStream[102X( [3Xobj[103X ) [32X Category[133X
  
  [33X[0;0YAll  [13Xtext[113X  output  streams  lie  in  this  category  and  translate new-line
  characters on output.[133X
  
  [1X10.1-8 IsOutputTextNone[101X
  
  [33X[1;0Y[29X[2XIsOutputTextNone[102X( [3Xobj[103X ) [32X Category[133X
  
  [33X[0;0YIt  is  convenient to use a category to distinguish dummy streams (see [14X10.9[114X)
  from others. Other distinctions are usually made using representations[133X
  
  [1X10.1-9 StreamsFamily[101X
  
  [33X[1;0Y[29X[2XStreamsFamily[102X [32X family[133X
  
  [33X[0;0YAll streams lie in the [2XStreamsFamily[102X.[133X
  
  
  [1X10.2 [33X[0;0YOperations applicable to All Streams[133X[101X
  
  [1X10.2-1 CloseStream[101X
  
  [33X[1;0Y[29X[2XCloseStream[102X( [3Xstream[103X ) [32X operation[133X
  
  [33X[0;0YIn  order  to  preserve  system  resources and to flush output streams every
  stream should be closed as soon as it is no longer used using [2XCloseStream[102X.[133X
  
  [33X[0;0YIt  is  an  error  to  try  to read characters from or write characters to a
  closed  stream.  Closing  a stream tells the [5XGAP[105X kernel and/or the operating
  system  kernel  that  the  file  is  no longer needed. This may be necessary
  because the [5XGAP[105X kernel and/or the operating system may impose a limit on how
  many streams may be open simultaneously.[133X
  
  [1X10.2-2 FileDescriptorOfStream[101X
  
  [33X[1;0Y[29X[2XFileDescriptorOfStream[102X( [3Xstream[103X ) [32X operation[133X
  
  [33X[0;0Yreturns  the  UNIX  file  descriptor  of the underlying file. This is mainly
  useful  for  the  [2XUNIXSelect[102X  ([14X10.2-3[114X) function call. This is as of now only
  available  on  UNIX-like  operating  systems  and  only for streams to local
  processes and local files.[133X
  
  [1X10.2-3 UNIXSelect[101X
  
  [33X[1;0Y[29X[2XUNIXSelect[102X( [3Xinlist[103X, [3Xoutlist[103X, [3Xexclist[103X, [3Xtimeoutsec[103X, [3Xtimeoutusec[103X ) [32X function[133X
  
  [33X[0;0Ymakes  the  UNIX  C-library function [10Xselect[110X accessible from [5XGAP[105X for streams.
  The functionality is as described in the man page (see UNIX file descriptors
  (integers)  for  streams.  They  can  be obtained via [2XFileDescriptorOfStream[102X
  ([14X10.2-2[114X)  for  streams  to  local processes and to local files. The argument
  [3Xtimeoutsec[103X  is a timeout in seconds as in the [10Xstruct timeval[110X on the C level.
  The  argument  [3Xtimeoutusec[103X is analogously in microseconds. The total timeout
  is the sum of both. If one of those timeout arguments is not a small integer
  then no timeout is applicable ([9Xfail[109X is allowed for the timeout arguments).[133X
  
  [33X[0;0YThe return value is the number of streams that are ready, this may be 0 if a
  timeout  was specified. All file descriptors in the three lists that are not
  yet ready are replaced by [9Xfail[109X in this function. So the lists are changed![133X
  
  [33X[0;0YThis  function  is only available if your operating system has [10Xselect[110X, which
  is detected during compilation of [5XGAP[105X.[133X
  
  
  [1X10.3 [33X[0;0YOperations for Input Streams[133X[101X
  
  [33X[0;0YTwo  operations normally used to read files: [2XRead[102X ([14X9.8-1[114X) and [2XReadAsFunction[102X
  ([14X9.8-2[114X)  can  also  be  used  to  read [5XGAP[105X input from a stream. The input is
  immediately  parsed  and  executed.  When reading from a stream [3Xstr[103X, the [5XGAP[105X
  kernel generates calls to [10XReadLine([3Xstr[103X[10X)[110X to supply text to the parser.[133X
  
  [33X[0;0YThree  further  operations: [2XReadByte[102X ([14X10.3-3[114X), [2XReadLine[102X ([14X10.3-4[114X) and [2XReadAll[102X
  ([14X10.3-5[114X),  support  reading  characters from an input stream without parsing
  them. This can be used to read data in any format and process it in [5XGAP[105X.[133X
  
  [33X[0;0YAdditional  operations for input streams support detection of end of stream,
  and  (for  those  streams  for which it is appropriate) random access to the
  data.[133X
  
  [1X10.3-1 Read[101X
  
  [33X[1;0Y[29X[2XRead[102X( [3Xinput-text-stream[103X ) [32X operation[133X
  
  [33X[0;0Yreads the input-text-stream as input until [10Xend-of-stream[110X occurs. See [14X9.8[114X for
  details.[133X
  
  [1X10.3-2 ReadAsFunction[101X
  
  [33X[1;0Y[29X[2XReadAsFunction[102X( [3Xinput-text-stream[103X ) [32X operation[133X
  
  [33X[0;0Yreads  the  input-text-stream as function and returns this function. See [14X9.8[114X
  for details.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27X# a function with local `a' does not change the global one[127X[104X
    [4X[25Xgap>[125X [27Xa := 1;;[127X[104X
    [4X[25Xgap>[125X [27Xi := InputTextString( "local a; a := 10; return a*10;" );;[127X[104X
    [4X[25Xgap>[125X [27XReadAsFunction(i)();[127X[104X
    [4X[28X100[128X[104X
    [4X[25Xgap>[125X [27Xa;[127X[104X
    [4X[28X1[128X[104X
    [4X[25Xgap>[125X [27X# reading it via `Read' does[127X[104X
    [4X[25Xgap>[125X [27Xi := InputTextString( "a := 10;" );;[127X[104X
    [4X[25Xgap>[125X [27XRead(i);[127X[104X
    [4X[25Xgap>[125X [27Xa;[127X[104X
    [4X[28X10[128X[104X
  [4X[32X[104X
  
  [1X10.3-3 ReadByte[101X
  
  [33X[1;0Y[29X[2XReadByte[102X( [3Xinput-stream[103X ) [32X operation[133X
  
  [33X[0;0Y[2XReadByte[102X  returns  one character (returned as integer) from the input stream
  [3Xinput-stream[103X.  [2XReadByte[102X  returns [9Xfail[109X if there is no character available, in
  particular if it is at the end of a file.[133X
  
  [33X[0;0YIf  [3Xinput-stream[103X is the input stream of a input/output process, [2XReadByte[102X may
  also return [9Xfail[109X if no byte is currently available.[133X
  
  [33X[0;0Y[2XReadByte[102X  is  the basic operation for input streams. If a [2XReadByte[102X method is
  installed  for  a user-defined type of stream which does not block, then all
  the  other  input stream operations will work (although possibly not at peak
  efficiency).[133X
  
  [33X[0;0Y[2XReadByte[102X  will  wait  (block) until a byte is available. For instance if the
  stream  is  a connection to another process, it will wait for the process to
  output a byte.[133X
  
  [1X10.3-4 ReadLine[101X
  
  [33X[1;0Y[29X[2XReadLine[102X( [3Xinput-stream[103X ) [32X operation[133X
  
  [33X[0;0Y[2XReadLine[102X  returns  one  line  (returned as string [13Xwith[113X the newline) from the
  input  stream  [3Xinput-stream[103X.  [2XReadLine[102X reads in the input until a newline is
  read or the end-of-stream is encountered.[133X
  
  [33X[0;0YIf  [3Xinput-stream[103X is the input stream of a input/output process, [2XReadLine[102X may
  also  return  [9Xfail[109X or return an incomplete line if the other process has not
  yet  written  any more. It will always wait (block) for at least one byte to
  be  available,  but  will then return as much input as is available, up to a
  limit of one line.[133X
  
  [33X[0;0YA  default  method  is  supplied  for  [2XReadLine[102X  which simply calls [2XReadByte[102X
  ([14X10.3-3[114X)  repeatedly.  This  is only safe for streams that cannot block. The
  kernel  uses  calls  to  [2XReadLine[102X to supply input to the parser when reading
  from a stream.[133X
  
  [1X10.3-5 ReadAll[101X
  
  [33X[1;0Y[29X[2XReadAll[102X( [3Xinput-stream[103X[, [3Xlimit[103X] ) [32X operation[133X
  
  [33X[0;0Y[2XReadAll[102X returns all characters as string from the input stream [3Xstream-in[103X. It
  waits (blocks) until at least one character is available from the stream, or
  until  there  is  evidence  that no characters will ever be available again.
  This last indicates that the stream is at end-of-stream. Otherwise, it reads
  as much input as it can from the stream without blocking further and returns
  it  to  the  user. If the stream is already at end of file, so that no bytes
  are available, [9Xfail[109X is returned. In the case of a file stream connected to a
  normal  file  (not  a  pseudo-tty  or  named pipe or similar), all the bytes
  should be immediately available and this function will read the remainder of
  the file.[133X
  
  [33X[0;0YWith  a  second argument, at most [3Xlimit[103X bytes will be returned. Depending on
  the  stream  a bounded number of additional bytes may have been read into an
  internal buffer.[133X
  
  [33X[0;0YA  default  method  is  supplied  for  [2XReadAll[102X  which  simply calls [2XReadLine[102X
  ([14X10.3-4[114X)  repeatedly.  This  is  only  really  safe for streams which cannot
  block. Other streams should install a method for [2XReadAll[102X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xi := InputTextString( "1Hallo\nYou\n1" );;[127X[104X
    [4X[25Xgap>[125X [27XReadByte(i);[127X[104X
    [4X[28X49[128X[104X
    [4X[25Xgap>[125X [27XCHAR_INT(last);[127X[104X
    [4X[28X'1'[128X[104X
    [4X[25Xgap>[125X [27XReadLine(i);[127X[104X
    [4X[28X"Hallo\n"[128X[104X
    [4X[25Xgap>[125X [27XReadLine(i);[127X[104X
    [4X[28X"You\n"[128X[104X
    [4X[25Xgap>[125X [27XReadLine(i);[127X[104X
    [4X[28X"1"[128X[104X
    [4X[25Xgap>[125X [27XReadLine(i);[127X[104X
    [4X[28Xfail[128X[104X
    [4X[25Xgap>[125X [27XReadAll(i);[127X[104X
    [4X[28X""[128X[104X
    [4X[25Xgap>[125X [27XRewindStream(i);;[127X[104X
    [4X[25Xgap>[125X [27XReadAll(i);[127X[104X
    [4X[28X"1Hallo\nYou\n1"[128X[104X
  [4X[32X[104X
  
  [1X10.3-6 IsEndOfStream[101X
  
  [33X[1;0Y[29X[2XIsEndOfStream[102X( [3Xinput-stream[103X ) [32X operation[133X
  
  [33X[0;0Y[2XIsEndOfStream[102X  returns  [9Xtrue[109X  if  the  input stream is at [13Xend-of-stream[113X, and
  [9Xfalse[109X otherwise. Note that [2XIsEndOfStream[102X might return [9Xfalse[109X even if the next
  [2XReadByte[102X ([14X10.3-3[114X) fails.[133X
  
  [1X10.3-7 PositionStream[101X
  
  [33X[1;0Y[29X[2XPositionStream[102X( [3Xinput-stream[103X ) [32X operation[133X
  
  [33X[0;0YSome input streams, such as string streams and file streams attached to disk
  files,   support   a  form  of  random  access  by  way  of  the  operations
  [2XPositionStream[102X,   [2XSeekPositionStream[102X  ([14X10.3-9[114X)  and  [2XRewindStream[102X  ([14X10.3-8[114X).
  [2XPositionStream[102X  returns a non-negative integer denoting the current position
  in  the  stream  (usually the number of characters [13Xbefore[113X the next one to be
  read.[133X
  
  [33X[0;0YIf  this  is  not  possible,  for  example  for  an input stream attached to
  standard input (normally the keyboard), then [9Xfail[109X is returned[133X
  
  [1X10.3-8 RewindStream[101X
  
  [33X[1;0Y[29X[2XRewindStream[102X( [3Xinput-stream[103X ) [32X operation[133X
  
  [33X[0;0Y[2XRewindStream[102X  attempts  to return an input stream to its starting condition,
  so  that  all  the same characters can be read again. It returns [9Xtrue[109X if the
  rewind succeeds and [9Xfail[109X otherwise[133X
  
  [33X[0;0YA default method implements RewindStream using [2XSeekPositionStream[102X ([14X10.3-9[114X).[133X
  
  [1X10.3-9 SeekPositionStream[101X
  
  [33X[1;0Y[29X[2XSeekPositionStream[102X( [3Xinput-stream[103X, [3Xpos[103X ) [32X operation[133X
  
  [33X[0;0Y[2XSeekPositionStream[102X attempts to rewind or wind forward an input stream to the
  specified position. This is not possible for all streams. It returns [9Xtrue[109X if
  the seek is successful and [9Xfail[109X otherwise.[133X
  
  
  [1X10.4 [33X[0;0YOperations for Output Streams[133X[101X
  
  [1X10.4-1 WriteByte[101X
  
  [33X[1;0Y[29X[2XWriteByte[102X( [3Xoutput-stream[103X, [3Xbyte[103X ) [32X operation[133X
  
  [33X[0;0Ywrites   the  next  character  (given  as  [13Xinteger[113X)  to  the  output  stream
  [3Xoutput-stream[103X.  The  function  returns  [9Xtrue[109X  if the write succeeds and [9Xfail[109X
  otherwise.[133X
  
  [33X[0;0Y[2XWriteByte[102X  is  the basic operation for output streams. If a [2XWriteByte[102X method
  is  installed  for  a user-defined type of stream, then all the other output
  stream operations will work (although possibly not at peak efficiency).[133X
  
  [1X10.4-2 WriteLine[101X
  
  [33X[1;0Y[29X[2XWriteLine[102X( [3Xoutput-stream[103X, [3Xstring[103X ) [32X operation[133X
  
  [33X[0;0Yappends  [3Xstring[103X  to  [3Xoutput-stream[103X. A final newline is written. The function
  returns [9Xtrue[109X if the write succeeds and [9Xfail[109X otherwise.[133X
  
  [33X[0;0YA  default  method is installed which implements [2XWriteLine[102X by repeated calls
  to [2XWriteByte[102X ([14X10.4-1[114X).[133X
  
  [1X10.4-3 WriteAll[101X
  
  [33X[1;0Y[29X[2XWriteAll[102X( [3Xoutput-stream[103X, [3Xstring[103X ) [32X operation[133X
  
  [33X[0;0Yappends  [3Xstring[103X  to [3Xoutput-stream[103X. No final newline is written. The function
  returns [9Xtrue[109X if the write succeeds and [9Xfail[109X otherwise. It will block as long
  as  necessary  for  the write operation to complete (for example for a child
  process to clear its input buffer )[133X
  
  [33X[0;0YA default method is installed which implements [2XWriteAll[102X by repeated calls to
  [2XWriteByte[102X ([14X10.4-1[114X).[133X
  
  [33X[0;0YWhen  printing  or appending to a stream (using [2XPrintTo[102X ([14X9.8-3[114X), or [2XAppendTo[102X
  ([14X9.8-3[114X)  or  when  logging  to  a  stream),  the  kernel generates a call to
  [2XWriteAll[102X for each line output.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xstr := "";; a := OutputTextString(str,true);;[127X[104X
    [4X[25Xgap>[125X [27XWriteByte(a,INT_CHAR('H'));[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27XWriteLine(a,"allo");[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27XWriteAll(a,"You\n");[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27XCloseStream(a);[127X[104X
    [4X[25Xgap>[125X [27XPrint(str);[127X[104X
    [4X[28XHallo[128X[104X
    [4X[28XYou[128X[104X
  [4X[32X[104X
  
  
  [1X10.4-4 [33X[0;0YPrintTo and AppendTo (for streams)[133X[101X
  
  [33X[1;0Y[29X[2XPrintTo[102X( [3Xoutput-stream[103X, [3Xarg1[103X, [3X...[103X ) [32X function[133X
  [33X[1;0Y[29X[2XAppendTo[102X( [3Xoutput-stream[103X, [3Xarg1[103X, [3X...[103X ) [32X function[133X
  
  [33X[0;0YThese  functions work like [2XPrint[102X ([14X6.3-4[114X), except that the output is appended
  to the output stream [3Xoutput-stream[103X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xstr := "";; a := OutputTextString(str,true);;[127X[104X
    [4X[25Xgap>[125X [27XAppendTo( a, (1,2,3), ":", Z(3) );[127X[104X
    [4X[25Xgap>[125X [27XCloseStream(a);[127X[104X
    [4X[25Xgap>[125X [27XPrint( str, "\n" );[127X[104X
    [4X[28X(1,2,3):Z(3)[128X[104X
  [4X[32X[104X
  
  [1X10.4-5 LogTo[101X
  
  [33X[1;0Y[29X[2XLogTo[102X( [3Xstream[103X ) [32X operation[133X
  
  [33X[0;0Ycauses  the subsequent interaction to be logged to the output stream [3Xstream[103X.
  It works in precisely the same way as it does for files (see [2XLogTo[102X ([14X9.8-4[114X)).[133X
  
  [1X10.4-6 InputLogTo[101X
  
  [33X[1;0Y[29X[2XInputLogTo[102X( [3Xstream[103X ) [32X operation[133X
  
  [33X[0;0Ycauses  the  subsequent  input  to be logged to the output stream [3Xstream[103X. It
  works just like it does for files (see [2XInputLogTo[102X ([14X9.8-5[114X)).[133X
  
  [1X10.4-7 OutputLogTo[101X
  
  [33X[1;0Y[29X[2XOutputLogTo[102X( [3Xstream[103X ) [32X operation[133X
  
  [33X[0;0Ycauses  the  subsequent  output to be logged to the output stream [3Xstream[103X. It
  works just like it does for files (see [2XOutputLogTo[102X ([14X9.8-6[114X)).[133X
  
  [1X10.4-8 SetPrintFormattingStatus[101X
  
  [33X[1;0Y[29X[2XSetPrintFormattingStatus[102X( [3Xstream[103X, [3Xnewstatus[103X ) [32X operation[133X
  [33X[1;0Y[29X[2XPrintFormattingStatus[102X( [3Xstream[103X ) [32X operation[133X
  
  [33X[0;0YWhen  text  is  being  sent  to  an  output text stream via [2XPrintTo[102X ([14X9.8-3[114X),
  [2XAppendTo[102X  ([14X9.8-3[114X),  [2XLogTo[102X ([14X10.4-5[114X), etc., it is by default formatted just as
  it  would  be  were  it being printed to the screen. Thus, it is broken into
  lines  of  reasonable  length  at  (where  possible)  sensible places, lines
  containing  elements of lists or records are indented, and so forth. This is
  appropriate  if  the  output  is  eventually  to  be  viewed by a human, and
  harmless if it to passed as input to [5XGAP[105X, but may be unhelpful if the output
  is to be passed as input to another program. It is possible to turn off this
  behaviour  for a stream using the [2XSetPrintFormattingStatus[102X operation, and to
  test whether it is on or off using [2XPrintFormattingStatus[102X.[133X
  
  [33X[0;0Y[2XSetPrintFormattingStatus[102X  sets  whether  output  sent  to  the output stream
  [3Xstream[103X  via  [2XPrintTo[102X  ([14X9.8-3[114X), [2XAppendTo[102X ([14X9.8-3[114X), etc. will be formatted with
  line  breaks  and indentation. If the second argument [3Xnewstatus[103X is [9Xtrue[109X then
  output will be so formatted, and if [9Xfalse[109X then it will not. If the stream is
  not a text stream, only [9Xfalse[109X is allowed.[133X
  
  [33X[0;0Y[2XPrintFormattingStatus[102X  returns [9Xtrue[109X if output sent to the output text stream
  [3Xstream[103X  via  [2XPrintTo[102X  ([14X9.8-3[114X), [2XAppendTo[102X ([14X9.8-3[114X), etc. will be formatted with
  line  breaks  and indentation, and [9Xfalse[109X otherwise. For non-text streams, it
  returns  [9Xfalse[109X.  If as argument [3Xstream[103X the string [10X"*stdout*"[110X is given, these
  functions  refer to the formatting status of the standard output (so usually
  the user's terminal screen).[133X
  
  [33X[0;0YSimilarly,  the  string  [10X"*errout*"[110X  refers  to the formatting status of the
  standard error output, which influences how error messages are printed.[133X
  
  [33X[0;0YThese  functions  do  not influence the behaviour of the low level functions
  [2XWriteByte[102X  ([14X10.4-1[114X),  [2XWriteLine[102X  ([14X10.4-2[114X)  or [2XWriteAll[102X ([14X10.4-3[114X) which always
  write without formatting.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xs := "";; str := OutputTextString(s,false);;[127X[104X
    [4X[25Xgap>[125X [27XPrintTo(str,Primes{[1..30]});[127X[104X
    [4X[25Xgap>[125X [27Xs;[127X[104X
    [4X[28X"[ 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61,\[128X[104X
    [4X[28X \n  67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113 ]"[128X[104X
    [4X[25Xgap>[125X [27XPrint(s,"\n");[127X[104X
    [4X[28X[ 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61,[128X[104X
    [4X[28X  67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113 ][128X[104X
    [4X[25Xgap>[125X [27XSetPrintFormattingStatus(str, false);[127X[104X
    [4X[25Xgap>[125X [27XPrintTo(str,Primes{[1..30]});[127X[104X
    [4X[25Xgap>[125X [27Xs;[127X[104X
    [4X[28X"[ 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61,\[128X[104X
    [4X[28X \n  67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113 ][ 2, 3, 5, 7\[128X[104X
    [4X[28X, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, \[128X[104X
    [4X[28X79, 83, 89, 97, 101, 103, 107, 109, 113 ]"[128X[104X
    [4X[25Xgap>[125X [27XPrint(s,"\n");[127X[104X
    [4X[28X[ 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61,[128X[104X
    [4X[28X  67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113 ][ 2, 3, 5, 7, 1\[128X[104X
    [4X[28X1, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79,\[128X[104X
    [4X[28X 83, 89, 97, 101, 103, 107, 109, 113 ][128X[104X
  [4X[32X[104X
  
  
  [1X10.5 [33X[0;0YFile Streams[133X[101X
  
  [33X[0;0YFile  streams  are streams associated with files. An input file stream reads
  the  characters  it  delivers  from a file, an output file stream prints the
  characters  it  receives  to  a file. The following functions can be used to
  create  such  streams.  They  return [9Xfail[109X if an error occurred, in this case
  [2XLastSystemError[102X ([14X9.1-1[114X) can be used to get information about the error.[133X
  
  [1X10.5-1 InputTextFile[101X
  
  [33X[1;0Y[29X[2XInputTextFile[102X( [3Xfilename[103X ) [32X operation[133X
  
  [33X[0;0Y[10XInputTextFile(   [3Xfilename[103X[10X   )[110X  returns  an  input  stream  in  the  category
  [2XIsInputTextStream[102X  ([14X10.1-4[114X)  that  delivers  the  characters  from  the file
  [3Xfilename[103X. If [3Xfilename[103X ends in [10X.gz[110X and the file is a valid gzipped file, then
  the file will be transparently uncompressed.[133X
  
  [33X[0;0Y[10XInputTextFile[110X  is designed for use with text files and automatically handles
  windows-style  line  endings.  This  means  it should [13Xnot[113X be used for binary
  data.  The  [2XIO_File[102X ([14XIO_File???[114X) function from the [5XIO[105X package should be used
  to access binary data.[133X
  
  [33X[0;0YNote: At most 256 files may be open for reading or writing at the same time.
  Use  [2XCloseStream[102X  ([14X10.2-1[114X)  to close the input stream once you have finished
  reading from it.[133X
  
  [1X10.5-2 OutputTextFile[101X
  
  [33X[1;0Y[29X[2XOutputTextFile[102X( [3Xfilename[103X, [3Xappend[103X ) [32X operation[133X
  [33X[1;0Y[29X[2XOutputGzipFile[102X( [3Xfilename[103X, [3Xappend[103X ) [32X operation[133X
  
  [33X[0;0Y[10XOutputTextFile(  [3Xfilename[103X[10X, [3Xappend[103X[10X )[110X returns an output stream in the category
  [10XIsOutputTextFile[110X  that  writes  received characters to the file [3Xfilename[103X. If
  [3Xappend[103X  is  [9Xfalse[109X,  then  the  file  is  emptied  first,  otherwise received
  characters are added at the end of the file. [10XOutputGzipFile[110X acts identically
  to [10XOutputTextFile[110X, except it compresses the output with gzip.[133X
  
  [33X[0;0YNote: At most 256 files may be open for reading or writing at the same time.
  Use  [2XCloseStream[102X  ([14X10.2-1[114X) to close the output stream once you have finished
  writing to it.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27X# use a temporary directory[127X[104X
    [4X[25Xgap>[125X [27Xname := Filename( DirectoryTemporary(), "test" );;[127X[104X
    [4X[25Xgap>[125X [27X# create an output stream, append output, and close again[127X[104X
    [4X[25Xgap>[125X [27Xoutput := OutputTextFile( name, true );;[127X[104X
    [4X[25Xgap>[125X [27XAppendTo( output, "Hallo\n", "You\n" );[127X[104X
    [4X[25Xgap>[125X [27XCloseStream(output);[127X[104X
    [4X[25Xgap>[125X [27X# create an input, print complete contents of file, and close[127X[104X
    [4X[25Xgap>[125X [27Xinput := InputTextFile(name);;[127X[104X
    [4X[25Xgap>[125X [27XPrint( ReadAll(input) );[127X[104X
    [4X[28XHallo[128X[104X
    [4X[28XYou[128X[104X
    [4X[25Xgap>[125X [27XCloseStream(input);[127X[104X
    [4X[25Xgap>[125X [27X# append a single line[127X[104X
    [4X[25Xgap>[125X [27Xoutput := OutputTextFile( name, true );;[127X[104X
    [4X[25Xgap>[125X [27XAppendTo( output, "AppendLine\n" );[127X[104X
    [4X[25Xgap>[125X [27X# close output stream to flush the output[127X[104X
    [4X[25Xgap>[125X [27XCloseStream(output);[127X[104X
    [4X[25Xgap>[125X [27X# create an input, print complete contents of file, and close[127X[104X
    [4X[25Xgap>[125X [27Xinput := InputTextFile(name);;[127X[104X
    [4X[25Xgap>[125X [27XPrint( ReadAll(input) );[127X[104X
    [4X[28XHallo[128X[104X
    [4X[28XYou[128X[104X
    [4X[28XAppendLine[128X[104X
    [4X[25Xgap>[125X [27XCloseStream(input);[127X[104X
  [4X[32X[104X
  
  
  [1X10.6 [33X[0;0YUser Streams[133X[101X
  
  [33X[0;0YThe   commands  described  in  this  section  create  streams  which  accept
  characters from, or deliver characters to, the user, via the keyboard or the
  [5XGAP[105X session display.[133X
  
  [1X10.6-1 InputTextUser[101X
  
  [33X[1;0Y[29X[2XInputTextUser[102X(  ) [32X function[133X
  
  [33X[0;0Yreturns an input text stream which delivers characters typed by the user (or
  from  the  standard  input  device  if  it  has  been redirected). In normal
  circumstances,  characters  are  delivered  one  by  one  as they are typed,
  without waiting until the end of a line. No prompts are printed.[133X
  
  [1X10.6-2 OutputTextUser[101X
  
  [33X[1;0Y[29X[2XOutputTextUser[102X(  ) [32X function[133X
  
  [33X[0;0Yreturns an output stream which delivers characters to the user's display (or
  the  standard  output  device  if it has been redirected). Each character is
  delivered  immediately  it  is  written,  without waiting for a full line of
  output.  Text  written  in  this  way is [13Xnot[113X written to the session log (see
  [2XLogTo[102X ([14X9.8-4[114X)).[133X
  
  [1X10.6-3 InputFromUser[101X
  
  [33X[1;0Y[29X[2XInputFromUser[102X( [3Xarg[103X ) [32X function[133X
  
  [33X[0;0Yprints the [3Xarg[103X as a prompt, then waits until a text is typed by the user (or
  from the standard input device if it has been redirected). This text must be
  a  [13Xsingle[113X  expression,  followed  by  one  [13Xenter[113X.  This  is  evaluated  (see
  [2XEvalString[102X ([14X27.9-5[114X)) and the result is returned.[133X
  
  
  [1X10.7 [33X[0;0YString Streams[133X[101X
  
  [33X[0;0YString  streams  are streams associated with strings. An input string stream
  reads  the  characters  it  delivers  from a string, an output string stream
  appends  the characters it receives to a string. The following functions can
  be used to create such streams.[133X
  
  [1X10.7-1 InputTextString[101X
  
  [33X[1;0Y[29X[2XInputTextString[102X( [3Xstring[103X ) [32X operation[133X
  
  [33X[0;0Y[10XInputTextString(  [3Xstring[103X[10X  )[110X  returns  an  input  stream  that  delivers  the
  characters  from  the  string [3Xstring[103X. The [3Xstring[103X is not changed when reading
  characters from it and changing the [3Xstring[103X after the call to [2XInputTextString[102X
  has no influence on the input stream.[133X
  
  [1X10.7-2 OutputTextString[101X
  
  [33X[1;0Y[29X[2XOutputTextString[102X( [3Xlist[103X, [3Xappend[103X ) [32X operation[133X
  
  [33X[0;0Yreturns  an  output  stream  that puts all received characters into the list
  [3Xlist[103X. If [3Xappend[103X is [9Xfalse[109X, then the list is emptied first, otherwise received
  characters are added at the end of the list.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27X# read input from a string[127X[104X
    [4X[25Xgap>[125X [27Xinput := InputTextString( "Hallo\nYou\n" );;[127X[104X
    [4X[25Xgap>[125X [27XReadLine(input);[127X[104X
    [4X[28X"Hallo\n"[128X[104X
    [4X[25Xgap>[125X [27XReadLine(input);[127X[104X
    [4X[28X"You\n"[128X[104X
    [4X[25Xgap>[125X [27X# print to a string[127X[104X
    [4X[25Xgap>[125X [27Xstr := "";;[127X[104X
    [4X[25Xgap>[125X [27Xout := OutputTextString( str, true );;[127X[104X
    [4X[25Xgap>[125X [27XPrintTo( out, 1, "\n", (1,2,3,4)(5,6), "\n" );[127X[104X
    [4X[25Xgap>[125X [27XCloseStream(out);[127X[104X
    [4X[25Xgap>[125X [27XPrint( str );[127X[104X
    [4X[28X1[128X[104X
    [4X[28X(1,2,3,4)(5,6)[128X[104X
  [4X[32X[104X
  
  
  [1X10.8 [33X[0;0YInput-Output Streams[133X[101X
  
  [33X[0;0YInput-output  streams  capture  bidirectional communications between [5XGAP[105X and
  another process, either locally or (@as yet unimplemented@) remotely.[133X
  
  [33X[0;0YSuch  streams support the basic operations of both input and output streams.
  They  should  provide  some buffering, allowing output data to be written to
  the  stream,  even  when input data is waiting to be read, but the amount of
  this  buffering is operating system dependent, and the user should take care
  not  to  get too far ahead in writing, or behind in reading, or deadlock may
  occur.[133X
  
  [33X[0;0YAt  present  the  only  type  of  Input-Output  streams that are implemented
  provide communication with a local child process, using a pseudo-tty.[133X
  
  [33X[0;0YLike  other  streams,  write  operations  are blocking, read operations will
  block to get the first character, but not thereafter.[133X
  
  [33X[0;0YAs far as possible, no translation is done on characters written to, or read
  from  the  stream,  and  no control characters have special effects, but the
  details of particular pseudo-tty implementations may effect this.[133X
  
  [1X10.8-1 IsInputOutputStream[101X
  
  [33X[1;0Y[29X[2XIsInputOutputStream[102X( [3Xobj[103X ) [32X Category[133X
  
  [33X[0;0Y[2XIsInputOutputStream[102X is the Category of Input-Output Streams; it returns [9Xtrue[109X
  if the [3Xobj[103X is an input-output stream and [9Xfalse[109X otherwise.[133X
  
  [1X10.8-2 InputOutputLocalProcess[101X
  
  [33X[1;0Y[29X[2XInputOutputLocalProcess[102X( [3Xdir[103X, [3Xexecutable[103X, [3Xargs[103X ) [32X function[133X
  
  [33X[0;0Ystarts up a child process, whose executable file is [3Xexecutable[103X, with [21Xcommand
  line[121X  arguments  [3Xargs[103X  in  the  directory [3Xdir[103X. (Suitable choices for [3Xdir[103X are
  [10XDirectoryCurrent()[110X     or     [10XDirectoryTemporary()[110X     (see    Section [14X9.4[114X);
  [10XDirectoryTemporary()[110X  may  be a good choice when [3Xexecutable[103X generates output
  files  that  it  doesn't  itself remove afterwards.) [2XInputOutputLocalProcess[102X
  returns  an  InputOutputStream  object.  Bytes  written  to  this stream are
  received  by  the child process as if typed at a terminal on standard input.
  Bytes  written  to standard output by the child process can be read from the
  stream.[133X
  
  [33X[0;0YWhen  the  stream  is  closed,  the signal SIGTERM is delivered to the child
  process, which is expected to exit.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xd := DirectoryCurrent();[127X[104X
    [4X[28Xdir("./")[128X[104X
    [4X[25Xgap>[125X [27Xf := PathSystemProgram("rev");[127X[104X
    [4X[28X"/usr/bin/rev"[128X[104X
    [4X[25Xgap>[125X [27Xs := InputOutputLocalProcess(d,f,[]);[127X[104X
    [4X[28X< input/output stream to rev >[128X[104X
    [4X[25Xgap>[125X [27XWriteLine(s,"The cat sat on the mat");[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27XPrint(ReadLine(s));[127X[104X
    [4X[28Xtam eht no tas tac ehT[128X[104X
    [4X[25Xgap>[125X [27Xx := ListWithIdenticalEntries(10000,'x');;[127X[104X
    [4X[25Xgap>[125X [27XConvertToStringRep(x);[127X[104X
    [4X[25Xgap>[125X [27XWriteLine(s,x);[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27XWriteByte(s,INT_CHAR('\n'));[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27Xy := ReadAll(s);;[127X[104X
    [4X[25Xgap>[125X [27XLength(y);[127X[104X
    [4X[28X10002[128X[104X
    [4X[25Xgap>[125X [27XCloseStream(s);[127X[104X
    [4X[25Xgap>[125X [27Xs;[127X[104X
    [4X[28X< closed input/output stream to rev >[128X[104X
  [4X[32X[104X
  
  [1X10.8-3 ReadAllLine[101X
  
  [33X[1;0Y[29X[2XReadAllLine[102X( [3Xiostream[103X[, [3Xnofail[103X][, [3XIsAllLine[103X] ) [32X operation[133X
  
  [33X[0;0YFor  an  input/output  stream  [3Xiostream[103X  [2XReadAllLine[102X  reads  until a newline
  character  if  any  input  is  found  or  returns [9Xfail[109X if no input is found,
  i.e. if any input is found [2XReadAllLine[102X is non-blocking.[133X
  
  [33X[0;0YIf  the  argument [3Xnofail[103X (which must be [9Xfalse[109X or [9Xtrue[109X) is provided and it is
  set  to  [9Xtrue[109X  then [2XReadAllLine[102X will wait, if necessary, for input and never
  return [9Xfail[109X.[133X
  
  [33X[0;0YIf  the  argument  [3XIsAllLine[103X  (which  must be a function that takes a string
  argument and returns either [9Xtrue[109X or [9Xfalse[109X) then it is used to determine what
  constitutes a whole line. The default behaviour is equivalent to passing the
  function[133X
  
  [4X[32X  Example  [32X[104X
    [4X[28Xline -> 0 < Length(line) and Last(line) = '\n'[128X[104X
  [4X[32X[104X
  
  [33X[0;0Yfor  the  [3XIsAllLine[103X  argument.  The  purpose of the [3XIsAllLine[103X argument is to
  cater  for  the  case where the input being read is from an external process
  that writes a [21Xprompt[121X for data that does not terminate with a newline.[133X
  
  [33X[0;0YIf the first argument is an input stream but not an input/output stream then
  [2XReadAllLine[102X  behaves  as if [2XReadLine[102X ([14X10.3-4[114X) was called with just the first
  argument and any additional arguments are ignored.[133X
  
  
  [1X10.9 [33X[0;0YDummy Streams[133X[101X
  
  [33X[0;0YThe  following  two  commands  create  dummy  streams which will consume all
  characters and never deliver one.[133X
  
  [1X10.9-1 InputTextNone[101X
  
  [33X[1;0Y[29X[2XInputTextNone[102X(  ) [32X function[133X
  
  [33X[0;0Yreturns a dummy input text stream, which delivers no characters, i.e., it is
  always  at end of stream. Its main use is for calls to [2XProcess[102X ([14X11.1-1[114X) when
  the started program does not read anything.[133X
  
  [1X10.9-2 OutputTextNone[101X
  
  [33X[1;0Y[29X[2XOutputTextNone[102X(  ) [32X function[133X
  
  [33X[0;0Yreturns  a  dummy output stream, which discards all received characters. Its
  main  use is for calls to [2XProcess[102X ([14X11.1-1[114X) when the started program does not
  write anything.[133X
  
  
  [1X10.10 [33X[0;0YHandling of Streams in the Background[133X[101X
  
  [33X[0;0YThis  section  describes  a  feature  of  the [5XGAP[105X kernel that can be used to
  handle  pending streams somehow [21Xin the background[121X. This is only available on
  operating systems that have [10Xselect[110X.[133X
  
  [33X[0;0YRight  before  [5XGAP[105X  reads  a  keypress  from  the keyboard it calls a little
  subroutine  that can handle streams that are ready to be read or ready to be
  written.  This  means that [5XGAP[105X can handle these streams during user input on
  the  command line. Note that this does not work when [5XGAP[105X is in the middle of
  some calculation.[133X
  
  [33X[0;0YThis feature is used in the following way. One can install handler functions
  for  reading  or  writing  streams  via  [2XInstallCharReadHookFunc[102X  ([14X10.10-1[114X).
  Handlers can be removed via [2XUnInstallCharReadHookFunc[102X ([14X10.10-2[114X)[133X
  
  [33X[0;0YNote  that  handler  functions  must not return anything and get one integer
  argument, which refers to an index in one of the following arrays (according
  to whether the function was installed for input, output or exceptions on the
  stream).  Handler  functions  usually  should  not  output  anything  on the
  standard  output  because  this  ruins  the command line during command line
  editing.[133X
  
  [1X10.10-1 InstallCharReadHookFunc[101X
  
  [33X[1;0Y[29X[2XInstallCharReadHookFunc[102X( [3Xstream[103X, [3Xmode[103X, [3Xfunc[103X ) [32X function[133X
  
  [33X[0;0Yinstalls  the function [3Xfunc[103X as a handler function for the stream [3Xstream[103X. The
  argument  [3Xmode[103X  decides,  for what operations on the stream this function is
  installed.  [3Xmode[103X  must  be a string, in which a letter [10Xr[110X means [21Xread[121X, [10Xw[110X means
  [21Xwrite[121X  and  [10Xx[110X  means [21Xexception[121X, according to the [10Xselect[110X function call in the
  UNIX  C-library  (see  [10Xman  select[110X  and  [2XUNIXSelect[102X ([14X10.2-3[114X)). More than one
  letter  is  allowed  in [3Xmode[103X. As described above the function is called in a
  situation  when  [5XGAP[105X  is  reading  a  character  from  the keyboard. Handler
  functions should not use much time to complete.[133X
  
  [33X[0;0YThis functionality only works if the operating system has a [10Xselect[110X function.[133X
  
  [1X10.10-2 UnInstallCharReadHookFunc[101X
  
  [33X[1;0Y[29X[2XUnInstallCharReadHookFunc[102X( [3Xstream[103X, [3Xfunc[103X ) [32X function[133X
  
  [33X[0;0Yuninstalls  the  function  [3Xfunc[103X as a handler function for the stream [3Xstream[103X.
  All  instances  are  deinstalled, regardless of the mode of operation (read,
  write, exception).[133X
  
  [33X[0;0YThis functionality only works if the operating system has a [10Xselect[110X function.[133X
  
  
  [1X10.11 [33X[0;0YComma separated files[133X[101X
  
  [33X[0;0YIn  some situations it can be desirable to process data given in the form of
  a  spreadsheet  (such  as  Excel).  [5XGAP[105X  can  do  this  using the CSV (comma
  separated  values) format, which spreadsheet programs can usually read in or
  write out.[133X
  
  [33X[0;0YThe  first  line  of the spreadsheet is used as labels of record components,
  each  subsequent  line  then  corresponds  to  a record. Entries enclosed in
  double  quotes  are  considered  as strings and are permitted to contain the
  separation character (usually a comma).[133X
  
  [1X10.11-1 ReadCSV[101X
  
  [33X[1;0Y[29X[2XReadCSV[102X( [3Xfilename[103X[, [3Xnohead[103X][, [3Xseparator[103X] ) [32X function[133X
  
  [33X[0;0YThis  function  reads in a spreadsheet, saved in CSV format ([13Xc[113Xomma [13Xs[113Xeparated
  [13Xv[113Xalues)  and  returns  its  entries as a list of records. The entries of the
  first  line  of  the  spreadsheet are used to denote the names of the record
  components.  Blanks  will  be  translated into underscore characters. If the
  parameter  [3Xnohead[103X  is  given  as [9Xtrue[109X, instead the record components will be
  called  [10Xfieldn[110X.  Each  subsequent  line  will  create  one record. If given,
  [3Xseparator[103X is the character used to separate fields. Otherwise it defaults to
  a comma.[133X
  
  [1X10.11-2 PrintCSV[101X
  
  [33X[1;0Y[29X[2XPrintCSV[102X( [3Xfilename[103X, [3Xlist[103X[, [3Xfields[103X] ) [32X function[133X
  
  [33X[0;0YThis function prints a list of records as a spreadsheet in CSV format (which
  can  be  read in for example into Excel). The names of the record components
  will  be  printed  as  entries  in the first line. If the argument [3Xfields[103X is
  given  only  the  record fields listed in this list will be printed and they
  will be printed in the same arrangement as given in this list. If the option
  noheader  is  set  to  true the line with the record field names will not be
  printed.[133X
  
  
  [1X10.12 [33X[0;0YOpening files in the Operating System[133X[101X
  
  [33X[0;0YIn  some  situations  it can be desirable to open a file outside of [5XGAP[105X, for
  example HTML files, PDFs, or pictures.[133X
  
  [1X10.12-1 OpenExternal[101X
  
  [33X[1;0Y[29X[2XOpenExternal[102X( [3Xfilename[103X ) [32X function[133X
  
  [33X[0;0YOpen  the  file  [3Xfilename[103X using the default application for this file in the
  operating  system. This can be used to open files like HTML and PDF files in
  the GUI.[133X
  
