ftp.cs.arizona.edu/ftp.cs.arizona.edu.tar/ftp.cs.arizona.edu/icon/historic/v941.tgz/icon.v941src.tar/icon.v941src/ipl/progs/ipp.icn

Text File | 2002-03-26 | 36KB | 1,179 lines

############################################################################
#
#    File:     ipp.icn
#
#    Subject:  Program to preprocess Icon programs
#
#    Author:   Robert C. Wieland, revised by Frank J. Lhota
#
#    Date:     March 26, 2002
#
############################################################################
#
#   This file is in the public domain.
#
############################################################################
#
#     Ipp is a preprocessor for the Icon language.  Ipp has many operations and
#  features that are unique to the Icon environment and should not be used as
#  a generic preprocessor (such as m4).  Ipp produces output which when written
#  to a file is designed to be the source for icont, the command processor for
#  Icon programs.
#
############################################################################
#
#  Ipp may be invoked from the command line as:
#
#    ipp [option  ...] [ifile [ofile]]
#
#     Two file names may be specified as arguments.  'ifile' and 'ofile' are
#  respectively the input and output files for the preprocessor.  By default
#  these are standard input and standard output.  If the output file is to be
#  specified while the input file should remain standard input a dash ('-')
#  should be given as 'ifile'.  For example, 'ipp - test' makes test the output
#  file while retaining standard input as the input file.
#
#     The following special names are predefined by ipp and may not be
#  redefined #  or undefined.  The name _LINE_ is defined as the line number
#  (as an integer) of the line of the source file currently processed.  The
#  name _FILE_ is defined as the name of the current source file
#  (as a string).  If the source is standard input then it has the value
#  'stdin'.
#
#     Ipp will also set _LINE_ and _FILE_ from the "#line" directives it
#  encounters, and will insert line directives to indicate source origins.
#
#     Also predefined are names corresponding to the features supported by the
#  implementation of Icon at the location the preprocessor is run.  This allows
#  conditional translations using the 'if' commands, depending on what features
#  are available.  Given below is a list of the features on a 4.nbsd UNIX
#  implementation and the corresponding predefined names:
#
#      Feature                Name
#      -----------------------------------------------------
#      UNIX                UNIX
#      co-expressions            co_expressions
#      overflow checking        overflow_checking
#      direct execution        direct_execution
#      environment variables        environment_variables
#      error traceback            error_traceback
#      executable images        executable_images
#      string invocation        string_invocation
#      expandable regions        expandable_regions
#
#
#  Command-Line Options:
#  ---------------------
#
#    The following options to ipp are recognized:
#
#   -C        By default ipp strips Icon-style comments.  If this option
#         is specified all comments are passed along except those
#         found on ipp command lines (lines starting with  a '$'
#         command).
#
#   -D name
#   -D name=def    Allows the user to define a name on the command line instead
#         of using a $define command in a source file.  In the first
#         form the name is defined as '1'.  In the second form name is
#         defined as the text following the equal sign.  This is less
#         powerful than the $define command line since def can not
#         contain any white space (spaces or tabs).
#
#   -d depth    By default ipp allows include files to be nested to a depth
#         of ten.  This allows the preprocessor to detect infinitely
#         recursive include sequences.  If a different limit for the
#         nesting depth is needed it may changed by using this option
#         with an integer argument greater than zero. Also, if a file
#         is found to already be in a nested include sequence an
#         error message is written regardless of the limit.
#
#   -I dir    The following algorithm is normally used in searching for
#         $include files.  On a UNIX system names enclosed in "" are
#         searched for by trying in order the directories specified by the
#         PATH environment variable, and names enclosed in <> are always
#         expected to be in the /usr/icon/src directory.  On other systems
#         names enclosed in <> are searched for by trying in order the
#         directories specified by the IPATH environment variable; names
#         in "" are searched for in a similar fashion, except that the
#         current directory is tried first.  If the -I option is given the
#         directory specified is searched before the 'standard'
#          directories.  If this option is specified more than once the
#          directories specified are tried in the order that they appear on
#          the command line, then followed by the 'standard' directories.
#
#  Preprocessor commands:
#  ----------------------
#
#     All ipp commands start with a line that has '$' as its first non-space
#  character.  The name of the command must follow the '$'.  White space
#  (any number of spaces or tabs) may be used to separate the '$' and the
#  command name.  Any line beginning with a '$' and not followed by a valid
#  name will cause an error message to be sent to standard error and
#  termination of the preprocessor.  If the command requires an argument then
#  it must be separated from the command name by white space otherwise the
#  argument will be considered part of the name and the result will likely
#  produce an error.  In processing the $ commands ipp responds to exceptional
#  conditions in one of two ways.  It may produce a warning and continue
#  processing or produce an error message and terminate.  In both cases the
#  message is sent to standard error.  With the exception of error conditions
#  encountered during the processing of the command line, the messages normally
#  include the name and line number of the source file at the point the
#  condition was encountered.  Ipp was designed so that most exception
#  conditions encountered will produce errors and terminate.  This protects the
#  user since warnings could simply be overlooked or misinterpreted.
#
#     Many ipp command require names as arguments.  Names must begin with a
#  letter or an underscore, which may be followed by any number of letters,
#  underscores, and digits.  Icon-style comments may appear on ipp command
#  lines, however they must be separated from the normal end of the command by
#  white_space.  If any extraneous characters appear on a command line a
#  warning is issued.  This occurs when characters other than white-space or a
#  comment follow the normal end of a command.
#
#     The following commands are implemented:
#
#    $define:  This command may be used in one of two forms.  The first form
#           only allows simple textual substitution.  It would be invoked as
#          '$define name text'.  Subsequent occurrences of name are replaced
#          with text.  Name and text must be separated by one white space
#          character which is not considered to be part of the replacement
#          text.  Normally the replacement text ends at the end of the line.
#          The text however may be continued on the next line if the backslash
#          character '\' is the last character on the line.  If name occurs
#          in the replacement text an error message (recursive textual substi-
#          tution) is written.
#
#          The second form is '$define name(arg,...,arg) text' which defines
#          a macro with arguments.  There may be no white space between the
#          name and the '('.  Each occurrence of arg in the replacement text
#          is replaced by the formal arg specified when the macro is
#          encountered.   When a macro with arguments is expanded the arguments
#          are placed into the expanded replacement text unchanged.  After the
#          entire replacement text is expanded, ipp restarts its scan for names
#          to expand at the beginning of the newly formed replacement text.
#          As with the first form above, the replacement text may be continued
#          on following lines.  The replacement text starts immediately after
#          the ')'.
#          The names of arguments must comply with the convention for regular
#          names.  See the section below on Macro processing for more
#          information on the replacement process.
#
#    $undef:   Invoked as '$undef name'.   Removes the definition of name.  If
#          name is not a valid name or if name is one of the reserved names
#          _FILE_ or _LINE_ a message is issued.
#
#    $include: Invoked as '$include <filename>' or '$include "filename"'.  This
#          causes the preprocessor to make filename the new source until
#          end of file is reached upon which input is again taken from the
#          original source.  See the -I option above for more detail.
#
#    $dump:    This command, which has no arguments, causes the preprocessor to
#          write to standard error all names which are currently defined.
#          See '$ifdef' below for a definition of 'defined'.
#
#    $warning:
#           This command issues a warning, with the text coming from the
#        argument field of the command.
#
#    $error:   This command issues a error, with the text coming from the
#        argument field of the command.  As with all errors, processing
#        is terminated.
#
#    $ifdef:   Invoked as 'ifdef name'.  The lines following this command appear
#          in the output only if the name given is defined.  'Defined' means
#            1.  The name is a predefined name and was not undefined using
#            $undef, or
#            2.  The name was defined using $define and has not been undefined
#            by an intervening $undef.
#
#    $ifndef:  Invoked as 'ifndef name'.  The lines following this command do
#           not appear in the output if the name is not defined.
#
#    $if:      Invoked as 'if constant-expression'.  Lines following this
#           command are processed only if the constant-expression produces a
#           result. The following arithmetic operators may be applied to
#           integer arguments: + - * / % ^
#
#          If an argument to one of the above operators is not an integer an
#          error is produced.
#
#             The following functions are provided: def(name), ndef(name)
#          This allows the utility of $ifdef and $ifndef in a $if command.
#          def produces a result if name is defined and ndef produces a
#          result if name is not defined.
#
#             The following comparison operators may be used on integer
#           operands:
#
#          > >= = < <= ~=
#
#              Also provided are alternation (|), conjunction (&), and
#           negation (not).  The following table lists all operators with
#           regard to decreasing precedence:
#
#        not + - (unary)
#          ^ (associates right to left)
#          * / %
#          + - (binary)
#               > >= = < <= ~=
#          |
#          &
#
#          The precedence of '|' and '&' are the same as the corresponding
#          Icon counterparts.  Parentheses may be used for grouping.
#          Backtracking is performed, so that the expression
#
#          FOO = (1|2)
#
#          will produce a result precisely when FOO is either 1 or 2.
#
#    $elif:    Invoked as 'elif constant-expression'.  If the lines preceding
#          this command were processed, this command and the lines following
#           it up to the matching $endif command are ignored.  Otherwise,
#           the constant-expression is evaluated, and the lines following this
#          command are processed only if it produces a result.
#
#    $else:    This command has no arguments and reverses the notion of the
#           test command which matches this directive.  If the lines preceding
#           this command where ignored the lines following are processed, and
#           vice versa.
#
#    $endif:   This command has no arguments and ends the section of lines
#           begun by a test command ($ifdef, $ifndef, or $if).  Each test
#           command must have a matching $endif.
#
#  Macro Processing and Textual Substitution
#  -----------------------------------------
#     No substitution is performed on text inside single quotes (cset literals)
#  and double quotes (strings) when a line is processed.   The preprocessor
#  will #  detect unclosed cset literals or strings on a line and issue an
#  error message unless the underscore character is the last character on the
#  line.  The output from
#
#      $define foo bar
#      write("foo")
#
#  is
#
#       write("foo")
#
#     Unless the -C option is specified comments are stripped from the source.
#  Even if the option is given the text after the '#' is never expanded.
#
#     Macro formal parameters are recognized in $define bodies even inside cset
#  constants and strings.  The output from
#
#      $define test(a)        "a"
#      test(processed)
#
#  is the following sequence of characters: "processed".
#
#     Macros are not expanded while processing a $define or $undef.  Thus:
#
#      $define off invalid
#      $define bar off
#      $undef off
#      bar
#
#  produces off.  The name argument to $ifdef or $ifndef is also not expanded.
#
#     Mismatches between the number of formal and actual parameters in a macro
#  call are caught by ipp.  If the number of actual parameters is greater than
#  the number of formal parameters is error is produced.  If the number of
#  actual parameters is less than the number of formal parameters a warning is
#  issued and the missing actual parameters are turned into null strings.
#
############################################################################
#
#    The records and global variables used by ipp are described below:
#
#  Src_desc:        Record which holds the 'file descriptor' and name
#            of the corresponding file.  Used in a stack to keep
#                track of the source files when $includes are used.
#  Opt_rec         Record returned by the get_args() routine which returns
#            the options and arguments on the command line.  options
#            is a cset containing options that have no arguments.
#            pairs is a list of [option,  argument] pairs. ifile and
#            ofile are set if the input or output files have been
#            specified.
#  Defs_rec        Record stored in a table keyed by names.  Holds the
#            names of formal arguments, if any, and the replacement
#            text for that name.
#  Expr_node        Node of a parse tree for $if / $elif expressions.
#            Holds the operator, or a string representing the
#            control structure.  Also, holds a list of the args for
#            the operation / control structure, which are either
#            scalars or other Expr_node records.
#  Chars        Cset of all characters that may appear in the input.
#  Defs            The table holding the definition data for each name.
#  Depth        The maximum depth of the input source stack.
#  Ifile        Descriptor for the input file.
#  Ifile_name        Name of the input file.
#  Init_name_char     Cset of valid initial characters for names.
#  Line_no        The current line number.
#  Name_char        Cset of valid characters for names.
#  Non_name_char    The complement of the above cset.
#  Ofile        The descriptor of the output file.
#  Options        Cset of no-argument options specified on the command
#            line.
#  Path_list        List of directories to search in for "" include files.
#  Src_stack        The stack of input source records.
#  Std_include_paths    List of directories to search in for <> include files.
#  White_space        Cset for white-space characters.
#  TRUE            Defined as 1.
#
############################################################################

record Src_desc(fd, fname, line)
record Opt_rec(options, pairs, ifile, ofile)
record Defs_rec(arg_list, text)
record Expr_node(op, arg)

global Chars, Defs, Depth, Ifile, Ifile_name, Init_name_char,
  Line_no, Name_char, Non_name_char, Ofile, Options, Path_list,
  Src_stack, Std_include_paths, White_space, TRUE, DIR_SEP

procedure main(arg_list)
  local line, source

  init(arg_list)

  repeat {
    while line := get_line(Ifile) do
      line ? process_cmd(get_cmd())

    # Get new source
    close(Ifile)
    if source := pop(Src_stack) then {
      Ifile := source.fd
      Ifile_name := source.fname
      Line_no := source.line
      }
    else  break
  }
end

procedure conditional(expr)

  return if eval(expr) then
      true_cond()
    else
      false_cond()
end

#
# In order to simplify the parsing the four operators that are longer
# than one character (<= ~= >= not) are replaced by one character
# 'aliases'.  Also, all white space is removed.
#

procedure const_expr(expr)
  local new

  static White_space_plus

  initial White_space_plus := White_space ++ '<>~n'

  new := ""
  expr ? {
    while new ||:= tab(upto(White_space_plus)) ||
      if any(White_space) then {
        tab(many(White_space))
     ""
    }
      else if =">=" then "\x01"
      else if ="<=" then "\x02"
      else if ="~=" then "\x03"
      else if not any(Name_char, ,&pos - 1) &
              ="not" &
              not any(Name_char) then "\x04"
      else move (1)
    new ||:= tab(0)
    }
  #
  # Now recursively parse the transformed string.
  #
  return parse(new)

end

procedure decoded(op)
  return case op of {
    "\x01":     ">="
    "\x02":     "<="
    "\x03":     "~="
    "\x04":     "not"
    default:     op
    }
end

procedure def_opt(s)
  local name, text

  s ? {
    name := tab(find("=")) | tab(0)
    text := (move(1) & tab(0)) | "1"
    }
  if name == ("_LINE_" | "_FILE_") then
    error(name, " is a reserved name and can not be redefined by the -D option")
  if not name ? (get_name() & pos(0)) then
    error(name, " :  Illegal name argument to -D option")
  if member(Defs, name) then
    warning(name, " : redefined by -D option")
  insert(Defs, name, Defs_rec(, text))
end

procedure define()
  local args, name, text

  get_opt_ws()
  if name := get_name() & (any(White_space ++ '(') | pos(0)) then {
    if name == ("_LINE_" | "_FILE_") then
      error(name, " is a reserved name and can not be redefined")

    if match("(") then             # A macro
      args := get_formals()
    text := get_text(args)

    if member(Defs,name) then
      warning(name, " redefined")
    insert(Defs, name, Defs_rec(args, text))
    }
  else
    error("Illegal or missing name in define")
end

procedure dump()
  if not pos(0) then
    warning("Extraneous characters after dump command")
  every write(&errout, (!sort(Defs))[1])
end

procedure error(s1, s2)
  s1 ||:= \s2
  stop(Ifile_name, ":  ", Line_no, ":  ", "Error  ", s1)
end

procedure eval(node)
  suspend case type(node) of {
    "Expr_node": {
      case node.op of {
    "|"     : eval(node.arg[1]) | eval(node.arg[2])
    "&"     : eval(node.arg[1]) & eval(node.arg[2])
    "not"   : not eval(node.arg[1])
    "def"   : member(Defs, node.arg[1])
    "ndef"  : not member(Defs, node.arg[1])
    default :
      case *node.arg of {
        1 : node.op(eval(node.arg[1]))
        2 : node.op(eval(node.arg[1]), eval(node.arg[2]))
        }
     }
       }
    default: node
    }
end

procedure false_cond()
  local cmd, line

  # Skip to next $else / $elif branch, or $endif
  cmd := skip_to("elif", "else", "endif")
  case cmd of {
    "elif" : return if_cond(cmd)
    "else" : {
       while line := get_line(Ifile) do
      line ? {
        cmd := get_cmd()
        case cmd of {
          "elif"  :
        error("'elif' encountered after 'else'")
          "else"  :
        error("multiple 'else' sections")
          "endif" : return
          default : process_cmd(cmd)
          }
        }
       error("'endif' not encountered before end of file")
       }
    "endif": return
    }
end

procedure find_file(fname, path_list)
  local ifile, ifname, path

  every path := !path_list do {
    ifname :=
      if path == ("" | ".") then
    fname
      else
    path || DIR_SEP || fname


    if ifile := open(ifname) then {
      if *Src_stack >= Depth then {
    close(ifile)
    error("Possibly infinitely recursive file inclusion")
    }
      if ifname == (Ifile_name | (!Src_stack).fname) then
    error("Infinitely recursive file inclusion")
      push(Src_stack, Src_desc(Ifile, Ifile_name, Line_no))
      Ifile := ifile
      Ifile_name := ifname
      Line_no := 0
      return
      }
    }
  error("Can not open include file ", fname)
end

procedure func(expr)
  local op, arg

  expr ? {
    if op  := tab(find("(")) & move(1) &
       arg := get_name() & =")" & pos(0) then {
      if op == ("def" | "ndef") then
    return Expr_node(op, [arg])
      else
        error("Invalid function name")
      }
    }
end

procedure get_args(arg_list, simple_opts, arg_opts)
  local arg, ch, get_ofile, i, opts, queue
  opts := Opt_rec('', [])
  queue := []

  every arg := arg_list[i := 1 to *arg_list] do
    if arg == "-" then         # Next argument should be output file
      get_ofile := (i = *arg_list - 1) |
    stop("Invalid position of '-' argument")
    else if arg[1] == "-" then     # Get options
      every ch := !arg[2: 0] do
    if any(simple_opts, ch) then
      opts.options ++:= ch
    else if any(arg_opts, ch) then
      put(queue, ch)
    else
      stop("Invalid option - ", ch)
    else if ch := pop(queue) then     # Get argument for option
      push(opts.pairs, [ch, arg])
    else if \get_ofile then {     # Get output file
      opts.ofile := arg
      get_ofile := &null
      }
    else {            # Get input file
      opts.ifile := arg
      get_ofile := (i < *arg_list)
      }

  if \get_ofile | *queue ~= 0 then
    stop("Invalid number of arguments")

  return opts
end

procedure get_cmd()
  local cmd
  static  no_arg_cmds
  initial no_arg_cmds := set(["dump", "else", "endif"])

  if ="#" & cmd := ="line" then
    get_opt_ws()
  else if (get_opt_ws()) & ="$" then {
    get_opt_ws()
    (cmd := tab(many(Chars))) | error("Missing command")
    get_opt_ws()
    if not pos(0) & member(no_arg_cmds, cmd) then
      warning("Extraneous characters after argument to '" || cmd || "'")
    }
  else
    tab (1)
  return cmd
end

procedure get_formals()
  local formal, arglist, ch

  arglist := []
  ="("
  get_opt_ws()
  if not =")" then
    repeat {
      if (formal := get_name()) & get_opt_ws() & any(',)') then
    put(arglist, formal)
      else
    error("Invalid formal argument in macro definition")
      if =")" then break
      =","
      get_opt_ws()
      }
  get_opt_ws()
  return arglist
end

procedure get_line(Ifile)
  return 1(read(Ifile), Line_no +:= 1)
end

procedure get_name()
  return tab(any(Init_name_char)) || (tab(many(Name_char)) | "")
end

procedure get_opt_ws()
  return (tab(many(White_space)) | "") || (="#" || tab(0) | "")
end

procedure get_text(is_macro)
  local text

  if \is_macro then
    text := tab(0)
  else
    text := (tab(any(White_space)) & tab(0)) | ""
  while (text[-1] == "\\") do
    (text := text[1:-1] || get_line(Ifile)) |
      error("Continuation line not found before end of file")
  return text
end

# if_cond is the procedure for $if or $elif.
#
# Procedure true_cond is invoked if the evaluation of a previous $if, $ifdef, or
# $ifndef causes subsequent lines to be processed.  Lines will be processed
# upto an $elif, $else, or $endif.  If $elif or $else is encountered, lines
# are skipped until the matching $endif is encountered.
#
# Procedure false_cond is invoked if the evaluation of a previous $if, $ifdef,
# or $ifndef causes subsequent lines to be skipped.  Lines will be skipped
# upto an $elif, $else, or, $endif.  If $else is encountered, lines are
# processed until the $endif matching the $else is encountered.

procedure if_cond(cmd)
  if pos(0) then
    error("Constant expression argument to '" || cmd || "' missing")
  else
    return conditional(const_expr(tab(0)))
end

procedure ifdef()
  local name

  if name := get_name() then
    {
    get_opt_ws()
    if not pos(0) then
      warning("Extraneous characters after argument to 'ifdef'")
    return conditional(Expr_node("def", [name]))
    }
  else
    error("Argument to 'ifdef' is not a valid name")
end

procedure ifndef()
  local name

  if name := get_name() then {
    get_opt_ws()
    if not pos(0) then
      warning("Extraneous characters after argument to 'ifndef'")
    return conditional(Expr_node("ndef", [name]))
    }
  else
    error("Argument to 'ifndef' is not a valid name")
end

procedure in_text(name, text)
  return text ?
    tab(find(name)) &
    (if move(-1) then tab(any(Non_name_char)) else "") &
    move(*name) &
    (tab(any(Non_name_char)) | pos(0))
end

procedure include()
  local ch, fname
  static fname_chars, stopper

  initial {
    fname_chars := Chars -- '<>"'
    stopper := table()
    insert(stopper, "\"", "\"")
    insert(stopper, "<",  ">")
    }

  if (ch    := tab(any('"<'))) &
     (fname := tab(many(fname_chars))) &
      =stopper[ch] then {
    get_opt_ws()
    if not pos(0) then
      warning("Extraneous characters after include file name")
    find_file(fname,
      case ch of {
    "\"" : Path_list
    "<"  : Std_include_paths
    }
      )
    }
  else
    error("Missing or invalid include file name")
end

procedure init(arg_list)
  local s

  TRUE := 1
  Defs := table()
  Init_name_char := &letters ++ '_'
  Name_char := Init_name_char ++ &digits
  Non_name_char := ~Name_char
  White_space := ' \t\b'
  Chars := &ascii -- White_space
  Line_no := 0
  Depth := 10

  # Predefine features
  every s := &features do {
    s := map(s, " -/", "___")
    insert(Defs, s, Defs_rec(, "1"))
    }

  # Set path list for $include files given in "", <>
  if member(Defs, "UNIX") then {
    Path_list := []
    getenv("PATH") ? while put(Path_list, 1(tab(find(":")), move(1)))
    Std_include_paths := ["/usr/icon/src"]
    }
  else {
    Std_include_paths := []
    (getenv("IPATH") || " ") ?
       while put(Std_include_paths, tab(find(" "))) do move(1)
    Path_list := [""] ||| Std_include_paths
    }

  process_options(arg_list)
end

procedure lassoc(expr, op)
  local j, arg1, arg2

  expr ? {
    every j := bal(op)
    # Succeeds if op found.
    if arg1 := tab(\j) & op := decoded(move(1)) & arg2 := tab(0) then {
      op := proc(op, 2)        # Fails for control structures
      return Expr_node(op, [parse(arg1), parse(arg2)])
      }
    }
end

#
# Programmer's note: Ifile_name and Line_no should not be assigned new
# values until the very end, so that if there is an error, the error
# message will include the file/line no of the current line directive,
# instead of the file/line of the text that follows the directive.
#
procedure line()
  local new_line, new_file

  new_line := tab(many(&digits)) | error("No line number in line directive")
  get_opt_ws()
  if ="\"" then {
    new_file := ""
    #
    # Get escaped chars.  We assume that the only escaped chars
    # appearing in a file name would be \\ or \", where the actual
    # character to be used is simply the character following the slash.
    # In the unlikely event that other escape sequences are encountered,
    # this section would have to revised.
    #
    while new_file ||:= tab(find("\\")) || (move(1) & move(1))
    new_file ||:= tab(find("\"")) |
      error("Invalid file name in line directive")
    }

  Line_no    := integer(new_line)
  Ifile_name := \new_file
  return
end

procedure macro_call(entry, args)
  local i, value, result, token

  value := table()
  every i := 1 to *entry.arg_list do
    insert(value, entry.arg_list[i], args[i] | "")

  entry.text ? {
    result := tab(upto(Name_char) | 0)
    while token := tab(many(Name_char)) do {
      result ||:= \value[token] | token
      result ||:= tab(many(Non_name_char))
      }
    }
  return result
end

procedure no_endif_error()
  error("'endif' not encountered before end of file")
end

procedure parse(expr)
  # strip surrounding parens.
  while expr ?:= 2(="(", tab(bal (')')), pos(-1))

  return lassoc(expr, '&' | '|') |
    lassoc(expr, '<=>\x01\x02\x03' | '+-' | '*/%') |
    rassoc(expr, '^') |
    unary(expr, '+-\x04') |
    func(expr) |
    integer(process_text(expr)) |
    error(expr, " :  Integer expected")
end

procedure process_cmd(cmd)
  static last_cmd
  initial last_cmd := ""

  case cmd of {
    "dump"    : dump()
    "define"  : define()
    "undef"   : undefine()
    "include" :    include()
    "line"    : line()
    "error"   :    error(tab(0))
    "warning" :    warning(tab(0))
    "if"      : if_cond( last_cmd := cmd )
    "ifdef"   : ifdef(   last_cmd := cmd )
    "ifndef"  : ifndef(  last_cmd := cmd )
    "elif"   |
    "else"   |
    "endif"   :    error("No previous 'if' expression")
    &null     : {
      if \last_cmd then
    put_linedir(Ofile, Line_no, Ifile_name)
      write(Ofile, process_text(tab(0)))
      }
    default   :    error("Undefined command")
    }
  last_cmd := cmd
  return
end

procedure process_macro(name, entry, s)
  local arg, args, new_entry, news, token

  s ? {
    args := []
    if ="(" then {
      #
      # Get args if list is not empty.
      #
      get_opt_ws ()
      if not =")" then
    repeat {
      arg := get_opt_ws()
      if token := tab(many(Chars -- '(,)')) then {
        if /(new_entry := Defs[token]) then
          arg ||:= token
        else if /new_entry.arg_list then
          arg ||:= new_entry.text
        else {  # Macro with arguments
          if news := tab(bal(White_space ++ ',)')) then
        arg ||:= process_macro(token, new_entry, news)
          else
        error(token, ":  Error in arguments to macro call")
          } # if
        } # if
      else if not any(',)') then
        error(name, ":  Incomplete macro call")
      arg ||:= tab(many(White_space))
      put(args, arg)
      if match(")") then
        break
      move(1)
    } # repeat
      if *args > *entry.arg_list then
    error(name, ":  Too many arguments in macro call")
      else if *args < *entry.arg_list then
    warning(name, ":  Missing arguments in macro call")
      return macro_call(entry, args)
      } # if
    }
end

procedure process_options(arg_list)
  local args, arg_opts, pair, simple_opts, tmp_list, value

  simple_opts := 'C'
  arg_opts := 'dDI'
  Src_stack := []

  args := get_args(arg_list, simple_opts, arg_opts)
  if \args.ifile then {
    (Ifile := open(args.ifile)) | stop("Can not open input file ", args.ifile)
    Ifile_name := args.ifile
    }
  else {
    Ifile := &input
    Ifile_name := "stdin"
    }
  if \args.ofile then
    (Ofile := open(args.ofile, "w")) | stop("Can not open output file",
      args.ofile)
  else
    Ofile := &output

  Options := args.options
  tmp_list := []
  every pair := !args.pairs do
    case pair[1] of {
      "D":    def_opt(pair[2])
      "d":    if (value := integer(pair[2])) > 0 then
          Depth := value
        else
          stop("Invalid argument for depth")
      "I":    push(tmp_list, pair[2])
    }
  Path_list := tmp_list ||| Path_list
end

procedure process_text(line)
  local add, entry, new, position, s, token
  static in_string, in_cset

  new :=  ""
  while *line > 0 do {
    add := ""
    line ?:= {
      if \in_string then {
    # Ignore escaped chars
    while new ||:= tab(find("\\")) || move(2)
    if new ||:= tab(find("\"")) || move(1) then
      in_string := &null
    else {
      new ||:= tab(0)
      if line[-1] ~== "_" then {
        in_string := &null
        warning("Unclosed double quote")
        }
      }
    }
      else if \in_cset then {
    # Ignore escaped chars.
    while new ||:= tab(find("\\")) || move(2)
    if new ||:= (tab(find("'")) || move(1)) then
      in_cset := &null
    else {
      new ||:= tab(0)
      if line[-1] ~== "_" then {
        in_cset := &null
        warning("Unclosed single quote")
        }
      }
    }

      new ||:= tab(many(White_space))
      case token := tab(many(Name_char) | any(Non_name_char)) of {
    "\"": {
      new ||:= "\""
      if \in_string then
        in_string := &null
      else if not pos(0) then {
        in_string := TRUE
        }
      else {
        warning("Unclosed double quote")
        }
      add ||:= tab(0)
        }
    "'": {
      new ||:= "'"
      if \in_cset then
        in_cset := &null
      else if not pos(0) then {
        in_cset := TRUE
        }
      else {
        warning("Unclosed double quote")
        }
      add ||:= tab(0)
        }
    "#": {
        new ||:= if any(Options, 'C') then token || tab(0)
        else tab(0) & token ? tab(find("#"))
        }
    "__LINE__":
      new ||:= Line_no
    "__FILE__":
      new ||:= Ifile_name
    default: {
      if /(entry := Defs[token]) then
        new ||:= token
      else if /entry.arg_list then
        if in_text(token, entry.text) then
        error("Recursive textual substitution")
        else
        add := entry.text
      else {  # Macro with arguments
        s := tab(bal(White_space) | 0)
        if not any('(', s) then
        error(token, ":  Incomplete macro call")
        add := process_macro(token, entry, s)
        }
      } # default
    } # case
      add || tab(0)
      } # ?:=
    } # while
  return new
end

procedure put_linedir(Ofile, Line_no, Ifile_name)
  static last_filename
  initial last_filename := ""

  writes(Ofile, "#line ", Line_no - 1)
  #
  # Output file name part only if the
  # filename differs from the last one used.
  #
  if last_filename ~==:= Ifile_name then
    writes(Ofile, " ", image(last_filename))
  write(Ofile)
  return
end

procedure rassoc(expr, op)
  local arg1, arg2


  # Succeeds if op found.
  expr ? if arg1 := tab(bal(op)) & op := move(1) & arg2 := tab(0) then {
      op := decoded(op)
      op := proc(op, 2)        # Fails for control structures
      return Expr_node(op, [parse(arg1), parse(arg2)])
      }
end

#
# skip_to is used to skip over parts of the an '$if' structure. targets
# are the $if - related commands to skip to, and should always include
# "endif".
#
# We do not, of course, wish to skip to a command in an $if structure
# that is embedded in the current one; also, we want to make sure that
# embedded $if structures, even in skipped lines, are well formed.  We
# therefore maintain a stack, if_sects, of the currently applicable $if
# structure commands encountered in the skipped lines.  For example, if
# we have skipped over the commands
#
#    $ifdef ...
#       $if ...
#       $elif ...
#           $if ...
#           $else
#
# if_sect would be ["else", "elif", "ifdef"].
#
procedure skip_to(targets[])
  local cmd, if_sects, line, argpos

  if_sects := []
  while line := get_line(Ifile) | no_endif_error () do
    line ? {
      cmd := get_cmd()
      if *if_sects = 0 & \cmd == !targets then {
    argpos := &pos
    break
    }

      case cmd of {
    "if"    |
    "ifdef" |
    "ifndef" : {
      if pos(0) then
        error("Argument to '" || cmd || "' missing")
      push(if_sects, cmd)
      }
    "elif"   : {
      if pos(0) then
        error("Argument to '" || cmd || "' missing")
      if if_sects[1] == "else" then
        error("'elif' encountered after 'else'")
      else
        if_sects[1] := cmd
      }
    "else"   : {
      if if_sects[1] == "else" then
        error("multiple 'else' sections")
      else
        if_sects[1] := cmd
      }
    "endif"  : pop(if_sects)
    }
      }

  #
  # Now reset the &subject to the current line, and &pos to the argument
  # field of the current line, so that if we skipped to a line which will
  # require further processing (such as $elif), the scanning functions can
  # be used.
  #
  &subject := line
  &pos     := argpos
  return cmd

end

procedure true_cond()
  local cmd, line

  while line := get_line(Ifile) | no_endif_error () do
    line ? {
      case cmd := get_cmd() of {
    "elif" |
    "else"  : return skip_to("endif")
    "endif" : return cmd
    default : process_cmd(cmd)
    }
      }

end

procedure unary(expr, op)
  local arg1


  # Succeeds if op found.
  expr ?
    if op := decoded(tab(any(op))) & arg1 := tab(0) then {
      op := proc(op, 1)        # fails for control structures
      return Expr_node(op, [parse(arg1)])
      }
end

procedure undefine()
  local name

  if name := get_name() then {
    get_opt_ws()
    if not pos(0) then
      warning("Extraneous characters after argument to undef")
    if name == ("_LINE_" | "_FILE_") then
      error(name, " is a reserved name that can not be undefined")
    delete(Defs, name)
    }
  else
    error("Name missing in undefine")
end

procedure warning(s1, s2)
  s1 ||:= \s2
  write(&errout, Ifile_name, ":  ", Line_no, ":  ", "Warning  " || s1)
end