Adding Go Template Tutorial

docs/content/layout/go-templates.md

@@ -1,21 +1,278 @@
 ---
-title: "Go Templates"
+title: "Using Go Templates"
 date: "2013-07-01"
 groups: ["layout"]
-groups_weight: 80
-draft: true
+groups_weight: 15
 ---
 
 Hugo uses the excellent [golang][] [html/template][gohtmltemplate] library for
-its template engine.
-It is an extremely lightweight engine that provides a very small amount of
-logic.
-In our experience that it is just the right amount of logic to be able to
-create a good static website.
+its template engine. It is an extremely lightweight engine that provides a very
+small amount of logic. In our experience that it is just the right amount of
+logic to be able to create a good static website. If you have used other
+template systems from different languages or frameworks you will find a lot of
+similarities in go templates.
 
-This is a brief primer on using go templates. The [golang docs][gohtmltemplate]
+This document is a brief primer on using go templates. The [golang docs][gohtmltemplate]
 provide more details.
 
+## Introduction to Go Templates
+
+Go templates provide an extremely simple template language. It adheres to the
+belief that only the most basic of logic belongs in the template or view layer.
+One consequence of this simplicity is that go templates parse very quickly.
+
+A unique characteristic of go templates is they are content aware. Variables and
+content will be sanitized depending on the context of where they are used. More
+details can be found in the [golang docs][gohtmltemplate].
+
+## Basic Syntax
+
+Go lang templates are html files with the addition of variables and
+functions. 
+
+**Go variables and functions are accessible within {{ }}**
+
+Accessing a predefined variable "foo":
+
+    {{ foo }}
+
+**Parameters are separated using spaces**
+
+Calling the add function with input of 1, 2:
+
+    {{ add 1 2 }}
+
+**Methods and fields are accessed via dot notation**
+
+Accessing the Page Parameter "bar"
+
+    {{ .Params.bar }}
+
+**Parentheses can be used to group items together**
+
+    {{ if or (isset .Params "alt") (isset .Params "caption") }} Caption {{ end }}
+
+
+## Variables
+
+Each go template has a struct (object) made available to it. In hugo each
+template is passed either a page or a node struct depending on which type of
+page you are rendering. More details are available on the
+[variables](/layout/variables) page.
+
+A variable is accessed by referencing the variable name.
+
+    <title>{{ .Title }}</title>
+
+Variables can also be defined and referenced.
+
+    {{ $address := "123 Main St."}}
+    {{ $address }}
+
+
+## Functions
+
+Go template ship with a few functions which provide basic functionality. The go
+template system also provides a mechanism for applications to extend the
+available functions with their own. [Hugo template
+functions](/layout/functions) provide some additional functionality we believe
+are useful for building websites. Functions are called by using their name
+followed by the required parameters separated by spaces. Template
+functions cannot be added without recompiling hugo.
+
+**Example:**
+
+    {{ add 1 2 }}
+
+## Includes
+
+When including another template you will pass to it the data it will be
+able to access. To pass along the current context please remember to
+include a trailing dot. The templates location will always be starting at
+the /layout/ directory within Hugo.
+
+**Example:**
+
+    {{ template "chrome/header.html" . }}
+
+
+## Logic
+
+Go templates provide the most basic iteration and conditional logic.
+
+### Iteration 
+
+Just like in go, the go templates make heavy use of range to iterate over
+a map, array or slice. The following are different examples of how to use
+range.
+
+**Example 1: Using Context**
+
+    {{ range array }} 
+        {{ . }}
+    {{ end }}
+
+**Example 2: Declaring value variable name**
+
+    {{range $element := array}} 
+        {{ $element }} 
+    {{ end }}
+
+**Example 2: Declaring key and value variable name**
+
+    {{range $index, $element := array}}
+        {{ $index }} 
+        {{ $element }} 
+    {{ end }}
+
+### Conditionals 
+
+If, else, with, or, & and provide the framework for handling conditional
+logic in Go Templates. Like range, each statement is closed with `end`.
+There is not an elseif function.
+
+Go Templates treat the following values as false: 
+
+* false
+* 0 
+* any array, slice, map, or string of length zero
+
+**Example 1: If**
+
+    {{ if isset .Params "title" }}<h4>{{ index .Params "title" }}</h4>{{ end }}
+
+**Example 2: If -> Else** 
+
+    {{ if isset .Params "alt" }} 
+        {{ index .Params "alt" }}
+    {{else}}
+        {{ index .Params "caption" }}
+    {{ end }}
+
+**Example 3: And & Or**
+
+    {{ if and (or (isset .Params "title") (isset .Params "caption")) (isset .Params "attr")}}
+
+**Example 4: With**
+
+An alternative way of writing "if" and then referencing the same value
+is to use "with" instead. With rebinds the context `.` within its scope,
+and skips the block if the variable is absent.
+
+The first example above could be simplified as:
+
+    {{ with .Params.title }}<h4>{{ . }}</h4>{{ end }}
+
+## Pipes
+
+One of the most powerful components of go templates is the ability to
+stack actions one after another. This is done by using pipes. Borrowed
+from unix pipes, the concept is simple, each pipeline's output becomes the
+input of the following pipe. 
+
+Because of the very simple syntax of go templates, the pipe is essential
+to being able to chain together function calls. One limitation of the
+pipes is that they only can work with a single value and that value
+becomes the last parameter of the next pipeline. 
+
+A few simple examples should help convey how to use the pipe.
+
+**Example 1 :**
+
+    {{ if eq 1 1 }} Same {{ end }}
+
+is the same as 
+
+    {{ eq 1 1 | if }} Same {{ end }}
+
+It does look odd to place the if at the end, but it does provide a good
+illustration of how to use the pipes.
+
+**Example 2 :**
+
+    {{ index .Params "disqus_url" | html }}
+
+Access the page parameter called "disqus_url" and escape the HTML.
+
+**Example 3 :**
+
+    {{ if or (or (isset .Params "title") (isset .Params "caption")) (isset .Params "attr")}}
+    Stuff Here
+    {{ end }}
+
+Could be rewritten as 
+
+    {{  isset .Params "caption" | or isset .Params "title" | or isset .Params "attr" | if }}
+    Stuff Here 
+    {{ end }}
+
+
+## Context (aka. the dot)
+
+The most easily overlooked concept to understand about golang templates is that {{ . }}
+always refers to the current context. In the top level of your template this
+will be the data set made available to it. Inside of a iteration it will have
+the value of the current item. When inside of a loop the context has changed. .
+will no longer refer to the data available to the entire page. If you need to
+access this from within the loop you will likely want to set it to a variable
+instead of depending on the context.
+
+**Example:**
+
+      {{ $title := .Site.Title }}
+      {{ range .Params.tags }}
+        <li> <a href="{{ $baseurl }}/tags/{{ . | urlize }}">{{ . }}</a> - {{ $title }} </li>
+      {{ end }}
+
+Notice how once we have entered the loop the value of {{ . }} has changed. We
+have defined a variable outside of the loop so we have access to it from within
+the loop.
+
+# Hugo Parameters 
+
+Hugo provides the option of passing values to the template language
+through the site configuration (for sitewide values), or through the meta
+data of each specific piece of content. You can define any values of any
+type (supported by your front matter/config format) and use them however
+you want to inside of your templates. 
+
+
+## Using Content (page) Parameters 
+
+In each piece of content you can provide variables to be used by the
+templates. This happens in the [front matter](/content/front-matter). 
+
+An example of this is used in this documentation site. Most of the pages
+benefit from having the table of contents provided. Sometimes the TOC just
+doesn't make a lot of sense. We've defined a variable in our front matter
+of some pages to turn off the TOC from being displayed. 
+
+Here is the example front matter:
+
+```
+---
+title: "Permalinks"
+date: "2013-11-18"
+aliases:
+  - "/doc/permalinks/"
+groups: ["extras"]
+groups_weight: 30
+notoc: true
+---
+```
+
+Here is the corresponding code inside of the template:
+
+      {{ if .Params.notoc }}
+      {{ else }}
+        <div id="toc" class="well col-md-4 col-sm-6">
+        {{ .TableOfContents }}
+        </div>
+      {{ end }}
+
+
+
+## Using Site (config) Parameters
 In your top-level configuration file (eg, `config.yaml`) you can define site
 parameters, which are values which will be available to you in chrome.
 
@@ -40,9 +297,9 @@ January 1st, instead of hunting through your templates.
 </footer>{{end}}
 ```
 
-An alternative way of writing the "if" and then referencing the same value is
-to use "with" instead, which rebinds the pointer `.` within its scope, and
-elides the scope if the variable is absent:
+An alternative way of writing the "if" and then referencing the same value
+is to use "with" instead. With rebinds the context `.` within its scope,
+and skips the block if the variable is absent:
 
 ```
 {{with .Site.Params.TwitterUser}}<span class="twitter">