textutil::expanderText and string utilities, macro procestextutil::expander(n)



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NAME
       textutil::expander - Procedures to process templates and expand text.

SYNOPSIS
       package require Tcl  8.2

       package require textutil::expander  ?1.3.1?

       ::textutil::expander expanderName

       expanderName cappend text

       expanderName cget varname

       expanderName cis cname

       expanderName cname

       expanderName cpop cname

       expanderName ctopandclear

       expanderName cpush cname

       expanderName cset varname value

       expanderName cvar varname

       expanderName errmode newErrmode

       expanderName evalcmd ?newEvalCmd?

       expanderName expand string ?brackets?

       expanderName lb ?newbracket?

       expanderName rb ?newbracket?

       expanderName reset

       expanderName setbrackets lbrack rbrack

       expanderName textcmd ?newTextCmd?

       expanderName where

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DESCRIPTION
       The Tcl subst command is often used to support a kind of template
       processing. Given a string with embedded variables or function calls,
       subst will interpolate the variable and function values, returning the
       new string:

                  % set greeting "Howdy"
                  Howdy
                  % proc place {} {return "World"}
                  % subst {$greeting, [place]!}
                  Howdy, World!
                  %


       By defining a suitable set of Tcl commands, subst can be used to
       implement a markup language similar to HTML.

       The subst command is efficient, but it has three drawbacks for this
       kind of template processing:

       o      There's no way to identify and process the plain text between
              two embedded Tcl commands; that makes it difficult to handle
              plain text in a context-sensitive way.

       o      Embedded commands are necessarily bracketed by [ and ]; it's
              convenient to be able to choose different brackets in special
              cases.  Someone producing web pages that include a large
              quantity of Tcl code examples might easily prefer to use << and
              >> as the embedded code delimiters instead.

       o      There's no easy way to handle incremental input, as one might
              wish to do when reading data from a socket.

       At present, expander solves the first two problems; eventually it will
       solve the third problem as well.

       The following section describes the command API to the expander; this
       is followed by the tutorial sections, see TUTORIAL.

EXPANDER API
       The textutil::expander package provides only one command, described
       below. The rest of the section is taken by a description of the methods
       for the expander objects created by this command.

       ::textutil::expander expanderName
              The command creates a new expander object with an associated Tcl
              command whose name is expanderName. This command may be used to
              invoke various operations on the graph. If the expanderName is
              not fully qualified it is interpreted as relative to the current
              namespace.  The command has the following general form:

              expanderName option ?arg arg ...?

       Option and the args determine the exact behavior of the command.

       The following commands are possible for expander objects:

       expanderName cappend text
              Appends a string to the output in the current context.  This
              command should rarely be used by macros or application code.

       expanderName cget varname
              Retrieves the value of variable varname, defined in the current
              context.

       expanderName cis cname
              Determines whether or not the name of the current context is
              cname.

       expanderName cname
              Returns the name of the current context.

       expanderName cpop cname
              Pops a context from the context stack, returning all accumulated
              output in that context.  The context must be named cname, or an
              error results.

       expanderName ctopandclear
              Returns the output currently captured in the topmost context and
              clears that buffer. This is similar to a combination of cpop
              followed by cpush, except that internal state (brackets) is
              preserved here.

       expanderName cpush cname
              Pushes a context named cname onto the context stack.  The
              context must be popped by cpop before expansion ends or an error
              results.

       expanderName cset varname value
              Sets variable varname to value in the current context.

       expanderName cvar varname
              Retrieves the internal variable name of context variable
              varname; this allows the variable to be passed to commands like
              lappend.

       expanderName errmode newErrmode
              Sets the macro expansion error mode to one of nothing, macro,
              error, or fail; the default value is fail.  The value determines
              what the expander does if an error is detected during expansion
              of a macro.

              fail   The error propagates normally and can be caught or
                     ignored by the application.

              error  The macro expands into a detailed error message, and
                     expansion continues.

              macro  The macro expands to itself; that is, it is passed along
                     to the output unchanged.

              nothing
                     The macro expands to the empty string, and is effectively
                     ignored.


       expanderName evalcmd ?newEvalCmd?
              Returns the current evaluation command, which defaults to
              uplevel #0.  If specified, newEvalCmd will be saved for future
              use and then returned; it must be a Tcl command expecting one
              additional argument: the macro to evaluate.

       expanderName expand string ?brackets?
              Expands the input string, replacing embedded macros with their
              expanded values, and returns the expanded string.

              If brackets is given, it must be a list of two strings; the
              items will be used as the left and right macro expansion bracket
              sequences for this expansion only.

       expanderName lb ?newbracket?
              Returns the current value of the left macro expansion bracket;
              this is for use as or within a macro, when the bracket needs to
              be included in the output text.  If newbracket is specified, it
              becomes the new bracket, and is returned.

       expanderName rb ?newbracket?
              Returns the current value of the right macro expansion bracket;
              this is for use as or within a macro, when the bracket needs to
              be included in the output text.  If newbracket is specified, it
              becomes the new bracket, and is returned.

       expanderName reset
              Resets all expander settings to their initial values.  Unusual
              results are likely if this command is called from within a call
              to expand.

       expanderName setbrackets lbrack rbrack
              Sets the left and right macro expansion brackets.  This command
              is for use as or within a macro, or to permanently change the
              bracket definitions.  By default, the brackets are [ and ], but
              any non-empty string can be used; for example, < and > or (* and
              *) or even Hello, and World!.

       expanderName textcmd ?newTextCmd?
              Returns the current command for processing plain text, which
              defaults to the empty string, meaning identity. If specified,
              newTextCmd will be saved for future use and then returned; it
              must be a Tcl command expecting one additional argument: the
              text to process. The expander object will this command for all
              plain text it encounters, giving the user of the object the
              ability to process all plain text in some standard way before
              writing it to the output. The object expects that the command
              returns the processed plain text.

              Note that the combination of "textcmd plaintext" is run through
              the evalcmd for the actual evaluation. In other words, the
              textcmd is treated as a special macro implicitly surrounding all
              plain text in the template.

       expanderName where
              Returns a three-element list containing the current character
              position, line, and column the expander is at in the processing
              of the current input string.

TUTORIAL
   BASICS
       To begin, create an expander object:

                  % package require expander
                  1.2
                  % ::expander::expander myexp
                  ::myexp
                  %


       The created ::myexp object can be used to expand text strings
       containing embedded Tcl commands.  By default, embedded commands are
       delimited by square brackets.  Note that expander doesn't attempt to
       interpolate variables, since variables can be referenced by embedded
       commands:

                  % set greeting "Howdy"
                  Howdy
                  % proc place {} {return "World"}
                  % ::myexp expand {[set greeting], [place]!}
                  Howdy, World!
                  %


   EMBEDDING MACROS
       An expander macro is simply a Tcl script embedded within a text string.
       Expander evaluates the script in the global context, and replaces it
       with its result string.  For example,

                  % set greetings {Howdy Hi "What's up"}
                  Howdy Hi "What's up"
                  % ::myexp expand {There are many ways to say "Hello, World!":
                  [set result {}
                  foreach greeting $greetings {
                append result "$greeting, World!\\n"
                  }
                  set result]
                  And that's just a small sample!}
                  There are many ways to say "Hello, World!":
                  Howdy, World!
                  Hi, World!
                  What's up, World!

                  And that's just a small sample!
                  %


   WRITING MACRO COMMANDS
       More typically, macro commands are used to create a markup language.  A
       macro command is just a Tcl command that returns an output string.  For
       example, expand can be used to implement a generic document markup
       language that can be retargeted to HTML or any other output format:

                  % proc bold {} {return "<b>"}
                  % proc /bold {} {return "</b>"}
                  % ::myexp expand {Some of this text is in [bold]boldface[/bold]}
                  Some of this text is in <b>boldface</b>
                  %


       The above definitions of bold and /bold returns HTML, but such commands
       can be as complicated as needed; they could, for example, decide what
       to return based on the desired output format.

   CHANGING THE EXPANSION BRACKETS
       By default, embedded macros are enclosed in square brackets, [ and ].
       If square brackets need to be included in the output, the input can
       contain the lb and rb commands.  Alternatively, or if square brackets
       are objectionable for some other reason, the macro expansion brackets
       can be changed to any pair of non-empty strings.

       The setbrackets command changes the brackets permanently.  For example,
       you can write pseudo-html by change them to < and >:

                  % ::myexp setbrackets < >
                  % ::myexp expand {<bold>This is boldface</bold>}
                  <b>This is boldface</b>


       Alternatively, you can change the expansion brackets temporarily by
       passing the desired brackets to the expand command:

                  % ::myexp setbrackets "\\[" "\\]"
                  % ::myexp expand {<bold>This is boldface</bold>} {< >}
                  <b>This is boldface</b>
                  %


   CUSTOMIZED MACRO EXPANSION
       By default, macros are evaluated using the Tcl uplevel #0 command, so
       that the embedded code executes in the global context.  The application
       can provide a different evaluation command using evalcmd; this allows
       the application to use a safe interpreter, for example, or even to
       evaluated something other than Tcl code.  There is one caveat: to be
       recognized as valid, a macro must return 1 when passed to Tcl's "info
       complete" command.

       For example, the following code "evaluates" each macro by returning the
       macro text itself.

                  proc identity {macro} {return $macro}
                  ::myexp evalcmd identity


   USING THE CONTEXT STACK
       Often it's desirable to define a pair of macros which operate in some
       way on the plain text between them.  Consider a set of macros for
       adding footnotes to a web page: one could have implement something like
       this:

                  Dr. Pangloss, however, thinks that this is the best of all
                  possible worlds.[footnote "See Candide, by Voltaire"]


       The footnote macro would, presumably, assign a number to this footnote
       and save the text to be formatted later on.  However, this solution is
       ugly if the footnote text is long or should contain additional markup.
       Consider the following instead:

                  Dr. Pangloss, however, thinks that this is the best of all
                  possible worlds.[footnote]See [bookTitle "Candide"], by
                  [authorsName "Voltaire"], for more information.[/footnote]


       Here the footnote text is contained between footnote and /footnote
       macros, continues onto a second line, and contains several macros of
       its own.  This is both clearer and more flexible; however, with the
       features presented so far there's no easy way to do it.  That's the
       purpose of the context stack.

       All macro expansion takes place in a particular context.  Here, the
       footnote macro pushes a new context onto the context stack.  Then, all
       expanded text gets placed in that new context.  /footnote retrieves it
       by popping the context.  Here's a skeleton implementation of these two
       macros:

                  proc footnote {} {
                      ::myexp cpush footnote
                  }

                  proc /footnote {} {
                      set footnoteText [::myexp cpop footnote]

                      # Save the footnote text, and return an appropriate footnote
                      # number and link.
                  }


       The cpush command pushes a new context onto the stack; the argument is
       the context's name.  It can be any string, but would typically be the
       name of the macro itself.  Then, cpop verifies that the current context
       has the expected name, pops it off of the stack, and returns the
       accumulated text.

       Expand provides several other tools related to the context stack.
       Suppose the first macro in a context pair takes arguments or computes
       values which the second macro in the pair needs.  After calling cpush,
       the first macro can define one or more context variables; the second
       macro can retrieve their values any time before calling cpop.  For
       example, suppose the document must specify the footnote number
       explicitly:

                  proc footnote {footnoteNumber} {
                      ::myexp cpush footnote
                      ::myexp csave num $footnoteNumber
                      # Return an appropriate link
                  }

                  proc /footnote {} {
                      set footnoteNumber [::myexp cget num]
                      set footnoteText [::myexp cpop footnote]

                      # Save the footnote text and its footnoteNumber for future
                      # output.
                  }


       At times, it might be desirable to define macros that are valid only
       within a particular context pair; such macros should verify that they
       are only called within the correct context using either cis or cname.

HISTORY
       expander was written by William H. Duquette; it is a repackaging of the
       central algorithm of the expand macro processing tool.

BUGS, IDEAS, FEEDBACK
       This document, and the package it describes, will undoubtedly contain
       bugs and other problems.  Please report such in the category textutil
       :: expander of the Tcllib SF Trackers
       [http://sourceforge.net/tracker/?group_id=12883].  Please also report
       any ideas for enhancements you may have for either package and/or
       documentation.

SEE ALSO
       http://www.wjduquette.com/expand, regexp, split, string

KEYWORDS
       string, template processing, text expansion

CATEGORY
       Documentation tools

COPYRIGHT
       Copyright (c) William H. Duquette, http://www.wjduquette.com/expand




textutil                             1.3.1               textutil::expander(n)
