# Increase the power of deficient languages.

tl;dr: How to use m4 to increase the power of deficient languages. Two examples: improve xslt syntax and make fractal with svg.

xml was a very nice idea about structuring data. Some people where so enthusiastic about xml they saw it everywhere. The idea was: the future is xml. Then some believed it would be a good idea to invent many xml compatible format and even programming languages with xml syntax.

Happy! Happy! Joy! Joy!

Unfortunately, xml was made to transfert structured data. Not a format a human should see or edit directly. The sad reality is xml syntax is simply verbose and ugly. Most of the time it shouldn’t be a problem, as nobody should see it. In a perfect nice world, we should never deal directly with xml but only use software which deal with it for us. Guess what? Our world isn’t perfect. Too sad, a bunch of developer have to deal directly with this ugly xml.

Unfortunately xml isn’t the only case of misused format I know. You have many format for which it would be very nice to add variables, loops, functions…

If like me you hate with passion xslt or writing xml, I will show you how you could deal with this bad format or language.

## The xslt Example

Let’s start by the worst case of misused xml I know: xslt. Any developer who had to deal with xslt know how horrible it is.

In order to reduce the verbosity of such a bad languages, there is a way. m4. Yes, the preprocessor you use when you program in C and C++.

Here are some example:

• Variable, instead of writing the natural myvar = value, here is the xslt way of doing this:
<xsl:variable name="myvar" select="value"/>
• Printing something. Instead of print "Hello world!" here is the xslt equivalent:
<xsl:text
disable-output-escaping="yes"><![CDATA[Hello world!
]]></xsl:text>
• printing the value of a variable, instead of print myvar the xslt is:
<xslt:value-of select="myvar"/>
• Just try to imagine how verbose it is to declare a function with this language.

## The cure (m4 to the rescue)

<?xml version="1.0" standalone="yes"?> <!-- YES its <span class="sc">xml</span> -->
<!-- ← start a comment, then write some m4 directives:

define(ydef',<xsl:variable name="$1" select="$2"/>')
define(yprint',<xsl:text disable-output-escaping="yes"><![CDATA[$1]]></xsl:text>') define(yshow',<xsl:value-of select="$1"/>')

-->
<!-- Yes, <span class="sc">xml</span> sucks to be read -->
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<!-- And it sucks even more to edit -->
<xsl:template match="/">
ydef(myvar,value)
yprint(Hello world!)
yshow(myvar)
</xsl:template>

Now just compile this file:

m4 myfile.m4 > myfile.xslt

Profit! Now xslt is more readable and easier to edit!

## The cool part: Fractals!

svg is an xml format used to represent vector graphics, it even support animations. At its beginning some people believed it would be the new Flash. Apparently, it will be more canvas + js.

Let me show you the result:

Click to view directly the svg. It might slow down your computers if you have an old one.

The positionning of the “esod” text with regards to the reversed “λ” was done by changing position in firebug. I didn’t had to manually regenerate to test.

Making such a fractal is mostly:

1. take a root element
2. duplicate and transform it (scaling, translating, rotate)
3. the result is a sub new element.
4. repeat from 2 but by taking the sub new element as new root.
5. Stop when recursion is deep enough.

If I had to do this for each step, I had make a lot of copy/paste in my svg, because the transformation is always the same, but I cannot say, use transformation named “titi”. Then instead of manually copying some xml, I used m4

and here is the commented code:

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!--
M4 Macros
define(YTRANSFORMONE', scale(.43) translate(-120,-69) rotate(-10)')
define(YTRANSFORMTWO', scale(.43) translate(-9,-67.5) rotate(10)')
define(YTRANSFORMTHREE', scale(.43) translate(53,41) rotate(120)')
define(YGENTRANSFORM', translate(364,274) scale(3)')
define(YTRANSCOMPLETE',
<g id="level_$1"> <use style="opacity: .8" transform="YTRANSFORMONE" xlink:href="#level_$2" />
<use style="opacity: .8" transform="YTRANSFORMTWO" xlink:href="#level_$2" /> <use style="opacity: .8" transform="YTRANSFORMTHREE" xlink:href="#level_$2" />
</g>
')
-->
<svg
xmlns="http://www.w3.org/2000/svg"
x="64" y="64" width="512" height="512" viewBox="64 64 512 512"
id="svg2" version="1.1">
<g id="level_0"> <!-- some group, if I want to add other elements -->
<!-- the text "λ" -->
<text id="lambda"
fill="#333" style="font-family:Ubuntu; font-size: 100px"
transform="rotate(180)">λ</text>
</g>
<!-- the text "esod" -->
<text
fill="#333"
style="font-family:Ubuntu; font-size: 28px; letter-spacing: -0.10em"
x="-17.3"
y="69"
transform="YGENTRANSFORM">esod</text>
<!-- ROOT ELEMENT -->

YTRANSCOMPLETE(1,0) <!-- First recursion -->
YTRANSCOMPLETE(2,1) <!-- deeper -->
YTRANSCOMPLETE(3,2) <!-- deeper -->
YTRANSCOMPLETE(4,3) <!-- even deeper -->
YTRANSCOMPLETE(5,4) <!-- Five level seems enough -->
</svg>

and I compiled it to svg and then to png with:

m4 yesodlogo.m4 > yesodlogo.svg && convert yesodlogo.svg yesodlogo.png

The main λ is duplicated 3 times. Each transformation is named by: YTRANSFORMONE, YTRANSFORMTWO and YTRANSFORMTHREE.

Each transformation is just a similarity (translate + rotation + scale).

Once fixed, we should now simply copy and repeat for each new level.

Now it is time to talk about where the magic occurs: YTRANSCOMPLETE. This macro takes two arguments. The current depth and the preceding one. It duplicates using the three transformations the preceding level.

• At level 0 there is only one λ,
• at level 1 there is 3 λ,
• at level 2 there is 9 λ
• etc…

At the final 5th level there is 35=243 λ. All level combined have 36-1 / 2 = 364 λ.

I could preview the final result easily. Without the macro system, I would have to make 5 copy/paste + modifications for each try.

## Conclusion

It was fun to make a fractal in svg, but the interesting part is how to augment the power of a language using this preprocessor method. I used the xslt trick at work for example. I also used it to make include inside obscure format. If all you want is to generate a minimal static website withou using nanoc, jekyll or hakyll (ther are plenty other alternatives). You can consider using m4 to generate your html instead of copy/paste the menu and the footer, or using AJAX.

Another usage I thouhgt about is to use m4 to organize languages such as brainfuck.