Added first draft of theme documentation

docs/themes.md

@@ -0,0 +1,377 @@
+# Themes
+
+The theming support in coleslaw is very flexible, and relatively easy
+to use.  However it does require some knowledge of HTML, CSS and
+[Closure Templates][clt], and of course how coleslaw converts
+posts into a collection of HTML files.
+
+This document will focus mainly on the part of how coleslaw converts
+posts into a blog, and how you can influence the resulting HTML.
+
+## Overall Structure
+
+Conceptually the process of creating a blog with coleslaw is the following:
+
+1.  Coleslaw read a directory containt `.post` files and processes them
+	into HTML fragments.
+
+2.  The HTML fragments of the posts are processed with the templating engine,
+	to produce different HTML files.
+
+And for theming purposes, an important final step
+
+3.  A browser renders the generated HTML and styles it with CSS.
+
+The first step, the translation from markdown to HTML fragments is fixed, you cannot
+really influence that in a (theming) meaningful way.
+
+The theming is done in the last two steps, the templating and the CSS.
+
+Now both steps have a different role:
+
+1.  **Templating**, this determines the file structure of the
+	generated HTML.   This part inserts headers, footers, include
+    the CSS stylesheets and other resources.
+
+	This is also the place where you can add for example a table of contents
+    of the posts, or where the list of tags will be inserted etc.
+
+	Also, very importantly, by generating the right HTML `<div>` elements
+	it will make it easy to style the resulting HTML with CSS.
+	
+
+2.  **CSS**, this is the part which will determine the look of all the components.
+	For example the font and font size of the titles and sub titles.
+
+	But *CSS* is very well covered in the literature and we will not
+	cover how to use *CSS* in this document.  But it is good to remember that
+	if you are struggling to achieve a certain effect with CSS, it might
+    be easy to solve by modifying the template.  Either by changing the structure
+	of the document or by adding the right `id` or `class` attributes.
+
+
+**NOTE** It is not possible to change the generated file names or the generated
+file structure on disk.  The templating/theming support allows changing the resulting
+HTML but nothing more.
+
+
+## What Files are generated anyway?
+
+Before we dive into the templating itself, it is important to know the
+directory structure of the generated content, because when writing a
+template you need to be aware of how coleslaw lays out the blog on disk.
+
+The toplevel looks like this:
+
+
+    index.html
+    posts/
+    tags/
+    date/
+    static/
+    css/
+
+
+### index.html
+
+This file is the front page of the blog.  It contains a list of the most recent posts and
+has links to the different archives.
+
+### posts directory
+
+This directory contains an `.html` file per post.  The name of the file
+is the `slug` of the post.
+
+### tags directory
+
+This directory contains an `.html` file per tag.  Such a file contains
+all posts which contain the tag.  The name of a tag file is the `slug` of the tag.
+
+### date directory
+
+This directory contains files of the form `yyyy-mm.html` with `yyyy`
+the year as a 4 digit number and `mm` as a two digit month number.
+These files contain all the posts of the indicated month.
+
+### static directory
+
+This directory is a copy of the `static/` directory of the source folder of coleslaw.
+
+### css directory
+
+This directory is a copy of the `css/` folder of the theme.
+
+## Two type of HTML files
+
+Coleslaw generate two types of HTML files: `index` files and `post` files.
+Except for the files in the `posts/` directory all files are `index` files.
+
+The HTML files, as mentioned before, are created by filling in the [Closure Templates] [clt].
+And to generate all the HTML files there are three templates relevant:
+
+*  `base.tmpl` This template generates the outer shell of the HTML.  This is used
+   to generate a consist look and feel and structure for all pages in the blog.
+
+   The actual content (besides fixed headers and footers etc.) is generated by one of the
+   other two templates
+
+*  `index.tmpl` This template generates the content of the `index` files, remember,
+   these are all files containing more than one post, so including the front page.
+
+
+*  `post.tmpl` This generates the HTML files for the individual posts.
+   Remember that Coleslaw already converts the content of the individual
+   post to HTML by using markdown (or ReST).  So this template is NOT
+   used to format or convert an individual post.  This template is used
+   to create the HTML containing that post
+
+Visual it might be clearer this way:
+
+
+     INDEX HTML FILES                    INDIVIDUAL POST HTML FILES
+	 
+     |-------------------------|         |-------------------------|
+	 | base.tmpl               |         | base.tmpl               |
+	 |                         |         |                         |
+	 |  |-------------------|  |         | |------------------|    |
+     |  |  index.tmpl       |  |         | | post.tmpl        |    |
+	 |  |                   |  |         | |                  |    |
+	 |  |-------------------|  |         | |------------------|    |
+	 |                         |         |                         |
+	 |-------------------------|         |-------------------------|
+
+
+## Creating a Theme from Scratch (with code)
+
+### Step 1. Pick a name
+
+A theme name should satisfy two conditions:
+
+1.  It is a valid lisp symbol name (not containing `:`)
+2.  It is a valid directory name.
+
+So for this example lets pick `trivial`.
+
+### Step 2. Create the right directory
+
+Now we need to create a directory containing the theme files.
+In the coleslaw system directory should be a directory called `themes/`.
+The directory for the theme files is a sub directory of the `themes/` directory,
+named after the theme name we picked in step 1.
+
+So in our case, we have to create the directory `themes/trivial/`.
+
+### Step 3. Create the 3 template files
+
+As described above, we need the 3 template files `base.tmpl`, `post.tmpl` and `index.tmpl`.
+Before we customize them, create the with the following content:
+
+base.tmpl:
+	 
+       {namespace coleslaw.theme.trivial}
+       {template base}
+       {/template}
+	
+
+post.tmpl
+
+       {namespace coleslaw.theme.trivial}
+       {template post}
+       {/template}
+
+index.tmpl
+
+       {namespace coleslaw.theme.trivial}
+       {template index}
+       {/template}
+
+
+The first line in these files, declares the namespace of the template.  The namespace must be
+`coleslaw.theme.` followed by the theme name.
+
+The remaining two lines `{template ...}{/template ...}` create the empty templates.
+
+### Step 4. Test
+
+This is enough to make coleslaw happy.  You can now use the new `trivial` theme in coleslaw
+by changing the `:theme` in `.coleslawrc` file to `trivial`.
+
+If you do this and generate the blog with `(coleslaw:main "")`, coleslaw will not complain
+and generate all the post files, tag files and front page files as normal.  Except that all
+these files are empty.
+
+The HTML files are empty because the templates are empty.
+
+### Step 5. Generating valid HTML
+
+The `base.tmpl` generates all the HTML pages, so if we change the base template to:
+
+       {namespace coleslaw.theme.trivial}
+       {template base}
+          <html>
+     	   <head><title>Trivial Theme For Coleslaw</title></head>
+     	   <body>
+     	   <h1>All my pages have this title</h1>
+     	   </body>
+    	   </html>
+       {/template}
+
+We will generate valid HTML.  Of course every page is still the same, but at least
+it shows that the templating engine works.
+
+
+### Intermezzo I, The Templating Language
+
+The templating language is documented at [Google closure templates][clt].
+However as a short primer:
+
+*   Everyting is outputed literally, except template commands
+*   Template commands are enclosed in `{` and `}`
+*   Variables, which are provided by coleslaw are referenced
+    as `$variable.key` inside a template command.
+	So to insert a variable you have to use `{$variable.key}`.
+
+	Variables are either simple variables, which are referenced as `{$var}`
+	Or are indexed by a key and written as `{$var.key}`.
+	
+*   If statements are written as
+
+           {if ...} ... {else} ... {/if}
+
+	Typical examples are:
+
+           {if $injections.body} ... {/fi}
+
+    Or
+
+           {if not isLast($link)} ... {/fi}
+
+*   Loops are typically written as
+
+           {foreach $var in $index.posts} ... {/foreach}
+
+
+
+
+### Intermezzo II, Variables provided by coleslaw
+
+The variable that is available in all templates is:
+
+- **config** This contains the .coleslawrc content
+
+#### Base Template Variables
+
+- **raw**          the HTML generated by the sub templates, `index.tmpl` or `post.tmpl`
+- **content**      the data which was used to generate **raw** HTML.
+- **pubdate**      A string containing the publication date
+- **injections**   A list containg all injections.  Injections are for the plugins to
+                   communicate additional content to be included.
+
+
+#### Index Template Variables
+
+- **tags**         A list containing all the tags, A tag has values `.name` and `.slug`.
+- **months**       A list of all months for which there are posts.  This is a list
+                   of strings.  The strings are formatted as `yyyy-mm`.
+- **index**        This is the meat of the content.  This variable has as keys
+ - `id`, the name of the page that will be rendered
+ - `posts` a list of posts (see below)
+ - `title` The title under which this index is know.  This should be used
+	       to display a title for the user.
+- **prev**         If this index file is part of a chain, the `id`
+                   of the previous index html in the chain.
+				   If this is the first file, the value will be empty.
+- **next**         If this index file is part of a chain, the `id`
+                   of the next index html in the chain.
+				   If this is the last file, the value will be empty.
+
+
+#### Post Template Variable
+
+- **prev**          
+- **next**          
+- **post**         All these variables are of the same type, they represent a post.
+                   The **prev** and **next** are the post before this one, or the one
+				   after this one when put in chronological order.
+                   These variables have the following keys
+   - `tags` a list of tags (a tag has keys `name` and `slug`)
+   - `slug` the slug of the post
+   - `date` the date of posting
+   - `text` the HTML version of the posts body.
+   - `title` The title of the post
+                   
+
+### Step 6.  Including the content
+
+We improve the `base.tmpl` to include the content generated by the sub templates.
+This is done by adding the line `{$raw |noAutoescape}` to the template in the body section.
+
+The `|noAutoescape` is added because the `$raw` variable is already html and we do
+not want the templating engine to escape the html.
+
+So the `base.tmpl` now looks like this:
+
+       {namespace coleslaw.theme.trivial}
+       {template base}
+
+	   <html>
+	   <head><title>Trivial Theme For Coleslaw</title></head>
+	   <body>
+	   <h1>All my pages have this title</h1>
+     	   {$raw |noAutoescape}
+	   </body>
+	   </html>
+
+       {/template}
+
+If we run this through coleslaw we do not see any difference and this is because
+we have not modified the `index.tmpl` and the `post.tmpl`.
+
+A simple `index.tmpl` looks like this
+
+     {namespace coleslaw.theme.trivial}
+     {template index}
+		 {foreach $post in $index.posts}
+		    <h1>{$post.title}</h1>
+		    {$post.text |noAutoescape}
+		 {/foreach}
+     {/template}
+
+     
+And a simple `post.tmpl` is similarly
+
+    {namespace coleslaw.theme.trivial}
+    {template post}
+    <h1>{$post.title}</h1>
+    {$post.text |noAutoescape}
+    {/template}
+
+### Wrapup of Example
+
+Basically all the files are now populated with content.  There are a few huge gaps still, for example there is no
+linking between the pages.  So although the archives are fully populated, you cannot get there from the
+front page, but if you know the URL it is there.
+
+However linking is not very difficult see next section.
+
+
+## Note on adding links
+
+As mentioned in the beginning, most files have a file name which is a slug of some sort. So if you want to create a link
+to a tag file you should do something like this:
+
+      <a href="${config.domain}/tags/$tag.slug">$tag.name</a>
+
+
+## Note on adding style sheets
+
+Style sheets are nothing special.  In order to get them to work, you
+have to add a `css` folder in your theme folder.  The content of this folder
+will be copied to the `css` folder at the place where the `html` is generated.
+
+Now to actual use them, you have to include in the head section of the html
+(which is in the `base.tmpl`) a link to include the CSS.
+
+
+
+[clt]: https://developers.google.com/closure/templates/