textutil::expanderTextland string utilities, macro protextutil::expander(3tcl)
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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 pro-
cessing. 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 imple-
ment 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 quan-
tity 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 be-
low. 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 pre-
served here.
expanderName cpush cname
Pushes a context named cname onto the context stack. The con-
text 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 var-
name; 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 ig-
nored by the application.
error The macro expands into a detailed error message, and ex-
pansion 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 up-
level #0. If specified, newEvalCmd will be saved for future use
and then returned; it must be a Tcl command expecting one addi-
tional 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.
Note that this method pushes a new (empty) context on the stack
of contexts while it is running, and removes it on return.
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 de-
faults to the empty string, meaning identity. If specified, new-
TextCmd 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 pro-
cessed 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 textutil::expander
1.2
% ::textutil::expander myexp
::myexp
%
The created ::myexp object can be used to expand text strings contain-
ing 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 lan-
guage 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 accu-
mulated text.
Expand provides several other tools related to the context stack. Sup-
pose the first macro in a context pair takes arguments or computes val-
ues 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
of the Tcllib Trackers [http://core.tcl.tk/tcllib/reportlist]. Please
also report any ideas for enhancements you may have for either package
and/or documentation.
When proposing code changes, please provide unified diffs, i.e the out-
put of diff -u.
Note further that attachments are strongly preferred over inlined
patches. Attachments can be made by going to the Edit form of the
ticket immediately after its creation, and then using the left-most
button in the secondary navigation bar.
SEE ALSO
[uri, 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
tcllib 1.3.1 textutil::expander(3tcl)