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
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:
- •
- 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.
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.
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 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 textutil::expander
1.2
% ::textutil::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 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.
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