Writing HTML with m4

 in
Ease your creation and maintenance of web pages using this handy pre-processor called m4.
Sharing HTML Elements Across Several Pages

In many “nests” of HTML pages, each page shares elements such as a button bar containing links to other pages like this:

[Home]  [Next]  [Prev]  [Index]

This is fairly easy to create in each page. The trouble is that if you make a change in the “standard” button-bar then you have the tedious job of finding each occurrence of it in every file and manually making the changes. With m4 we can more easily do this job by putting the shared elements into an m4_include statement, just like C.

Let's also automate the naming of pages by putting the following lines into an include file called button_bar.m4:

m4_define(`_BUTTON_BAR',
 <a href="homepage.html">[Home]</a>
 <a href="$1">[Next]</a>
 <a href="$2">[Prev]</a>
 <a href="indexpage.html">[Index]</a>)

and then these lines in the document:

m4_include button_bar.m4
_BUTTON_BAR(`page_after_this.html',
 `page_before_this.html')
The $1 and $2 parameters in the macro definition are replaced by the strings in the macro call.

Managing HTML elements that often change

It is troublesome to have items change in multiple HTML pages. For example, if your e-mail address changes, you need to change all references to it to the new address. Instead, with m4 you can put a line like the following in your stdlib.m4 file:

m4_define(`_EMAIL_ADDRESS', `MyName@foo.bar.com')

and then just put _EMAIL_ADDRESS in your m4 files.

A more substantial example comes from building strings with multiple components, any of which may change as the page is developed. If, like me, you develop on one machine, test out the page and then upload to another machine with a totally different address, then you could use the m4_ifdef command in your stdlib.m4 file (just like the #ifdef command in cpp). For example:

m4_define(`_LOCAL')
...
m4_define(`_HOMEPAGE',
 m4_ifdef(`_LOCAL',
 `//127.0.0.1/~YourAccount',
 `http://ISP.com/~YourAccount'))
m4_define(`_PLUG', `<A HREF="http://www.ssc.com/linux/">
<IMG SRC="_HOMEPAGE/gif/powered.gif"
ALT=<"[Linux Information]"> </A>')

Note the careful use of quotes to prevent the variable _LOCAL from being expanded. _HOMEPAGE takes on different values according to whether the variable _LOCAL is defined or not. This definition can then ripple through the entire project as you build the pages.

In this example, _PLUG is a macro to advertise Linux. When you are testing your pages, use the local version of _HOMEPAGE. When you are ready to upload, remove or comment out the _LOCAL definition in this way:

m4_dnl m4_define(`_LOCAL')

... and then re-make.

Creating New Text Styles

Styles built into HTML include things like <EM> for emphasis and <CITE> for citations. With m4 you can define your own new styles like this:

m4_define(`_MYQUOTE',
 <BLOCKQUOTE><EM>$1</EM></BLOCKQUOTE>)

If, later, you decide you prefer <STRONG> instead of <EM>, it is a simple matter to change the definition. Then, every _MYQUOTE paragraph falls into line with a quick make.

The classic guides to good HTML writing say things like “It is strongly recommended that you employ the logical styles such as <EM>...</EM> rather than the physical styles such as <I>...</I> in your documents.” Curiously, the WYSIWYG editors for HTML generate purely physical styles. Using the m4 styles may be a good way to keep on using logical styles.

Typing and Mnemonic Aids

I don't depend on WYSIWYG editing (having been brought up on troff) but all the same I'm not averse to using help where it's available. There is a choice (and maybe it's a fine line) to be made between:

<BLOCKQUOTE><PRE><CODE>Some code you want to display.
</CODE></PRE></BLOCKQUOTE>

and:

_CODE(Some code you want to display.)
In this case, you would define _CODE like this:
m4_define(`_CODE',
<BLOCKQUOTE><PRE><CODE>$1</CODE></PRE></BLOCKQUOTE>)
Which version you prefer is a matter of taste and convenience although the m4 macro certainly saves some typing. Another example I like to use, since I can never remember the syntax for links, is:
m4_define(`_LINK', <a href="$1">$2</a>)
Then, instead of typing:
<a href="URL_TO_SOMEWHERE">Click here to get to SOMEWHERE
</a>
I type:
_LINK(`URL_TO_SOMEWHERE', `Click here to get to SOMEWHERE')

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