textutil::expander -
Procedures to process templates and expand text.
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
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:
-
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.
-
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.
-
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.
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.
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!
%
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!
%
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.
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>
%
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
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.
expander was written by William H. Duquette; it is a repackaging
of the central algorithm of the expand macro processing tool.
This document, and the package it describes, will undoubtedly contain
bugs and other problems.
Please report such in the category
textutil :: expander of the
http://sourceforge.net/tracker/?group_id=12883.
Please also report any ideas for enhancements you may have for either
package and/or documentation.
regexp, split, string, http://www.wjduquette.com/expand
string, template processing, text expansion