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content/posts/goisforlovers.md
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content/posts/goisforlovers.md
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title = "Yazmak Güzeldir"
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tags = [
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"go",
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"golang",
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"templates",
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"themes",
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"development",
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]
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date = "2014-04-02"
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toc = true
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Hugo uses the excellent [Go][] [html/template][gohtmltemplate] library for
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its template engine. It is an extremely lightweight engine that provides a very
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small amount of logic. In our experience that it is just the right amount of
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logic to be able to create a good static website. If you have used other
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template systems from different languages or frameworks you will find a lot of
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similarities in Go templates.
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This document is a brief primer on using Go templates. The [Go docs][gohtmltemplate]
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provide more details.
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## Introduction to Go Templates
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Go templates provide an extremely simple template language. It adheres to the
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belief that only the most basic of logic belongs in the template or view layer.
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One consequence of this simplicity is that Go templates parse very quickly.
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A unique characteristic of Go templates is they are content aware. Variables and
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content will be sanitized depending on the context of where they are used. More
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details can be found in the [Go docs][gohtmltemplate].
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## Basic Syntax
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Golang templates are HTML files with the addition of variables and
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functions.
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**Go variables and functions are accessible within {{ }}**
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Accessing a predefined variable "foo":
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{{ foo }}
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**Parameters are separated using spaces**
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Calling the add function with input of 1, 2:
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{{ add 1 2 }}
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**Methods and fields are accessed via dot notation**
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Accessing the Page Parameter "bar"
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{{ .Params.bar }}
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**Parentheses can be used to group items together**
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{{ if or (isset .Params "alt") (isset .Params "caption") }} Caption {{ end }}
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## Variables
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Each Go template has a struct (object) made available to it. In hugo each
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template is passed either a page or a node struct depending on which type of
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page you are rendering. More details are available on the
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[variables](/layout/variables) page.
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A variable is accessed by referencing the variable name.
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<title>{{ .Title }}</title>
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Variables can also be defined and referenced.
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{{ $address := "123 Main St."}}
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{{ $address }}
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## Functions
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Go template ship with a few functions which provide basic functionality. The Go
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template system also provides a mechanism for applications to extend the
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available functions with their own. [Hugo template
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functions](/layout/functions) provide some additional functionality we believe
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are useful for building websites. Functions are called by using their name
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followed by the required parameters separated by spaces. Template
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functions cannot be added without recompiling hugo.
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**Example:**
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{{ add 1 2 }}
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## Includes
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When including another template you will pass to it the data it will be
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able to access. To pass along the current context please remember to
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include a trailing dot. The templates location will always be starting at
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the /layout/ directory within Hugo.
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**Example:**
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{{ template "chrome/header.html" . }}
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## Logic
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Go templates provide the most basic iteration and conditional logic.
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### Iteration
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Just like in Go, the Go templates make heavy use of range to iterate over
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a map, array or slice. The following are different examples of how to use
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range.
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**Example 1: Using Context**
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{{ range array }}
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{{ . }}
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{{ end }}
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**Example 2: Declaring value variable name**
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{{range $element := array}}
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{{ $element }}
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{{ end }}
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**Example 2: Declaring key and value variable name**
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{{range $index, $element := array}}
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{{ $index }}
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{{ $element }}
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{{ end }}
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### Conditionals
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If, else, with, or, & and provide the framework for handling conditional
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logic in Go Templates. Like range, each statement is closed with `end`.
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Go Templates treat the following values as false:
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* false
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* 0
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* any array, slice, map, or string of length zero
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**Example 1: If**
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{{ if isset .Params "title" }}<h4>{{ index .Params "title" }}</h4>{{ end }}
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**Example 2: If -> Else**
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{{ if isset .Params "alt" }}
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{{ index .Params "alt" }}
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{{else}}
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{{ index .Params "caption" }}
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{{ end }}
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**Example 3: And & Or**
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{{ if and (or (isset .Params "title") (isset .Params "caption")) (isset .Params "attr")}}
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**Example 4: With**
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An alternative way of writing "if" and then referencing the same value
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is to use "with" instead. With rebinds the context `.` within its scope,
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and skips the block if the variable is absent.
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The first example above could be simplified as:
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{{ with .Params.title }}<h4>{{ . }}</h4>{{ end }}
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**Example 5: If -> Else If**
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{{ if isset .Params "alt" }}
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{{ index .Params "alt" }}
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{{ else if isset .Params "caption" }}
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{{ index .Params "caption" }}
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{{ end }}
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## Pipes
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One of the most powerful components of Go templates is the ability to
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stack actions one after another. This is done by using pipes. Borrowed
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from unix pipes, the concept is simple, each pipeline's output becomes the
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input of the following pipe.
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Because of the very simple syntax of Go templates, the pipe is essential
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to being able to chain together function calls. One limitation of the
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pipes is that they only can work with a single value and that value
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becomes the last parameter of the next pipeline.
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A few simple examples should help convey how to use the pipe.
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**Example 1 :**
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{{ if eq 1 1 }} Same {{ end }}
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is the same as
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{{ eq 1 1 | if }} Same {{ end }}
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It does look odd to place the if at the end, but it does provide a good
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illustration of how to use the pipes.
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**Example 2 :**
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{{ index .Params "disqus_url" | html }}
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Access the page parameter called "disqus_url" and escape the HTML.
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**Example 3 :**
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{{ if or (or (isset .Params "title") (isset .Params "caption")) (isset .Params "attr")}}
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Stuff Here
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{{ end }}
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Could be rewritten as
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{{ isset .Params "caption" | or isset .Params "title" | or isset .Params "attr" | if }}
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Stuff Here
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{{ end }}
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## Context (aka. the dot)
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The most easily overlooked concept to understand about Go templates is that {{ . }}
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always refers to the current context. In the top level of your template this
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will be the data set made available to it. Inside of a iteration it will have
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the value of the current item. When inside of a loop the context has changed. .
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will no longer refer to the data available to the entire page. If you need to
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access this from within the loop you will likely want to set it to a variable
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instead of depending on the context.
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**Example:**
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{{ $title := .Site.Title }}
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{{ range .Params.tags }}
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<li> <a href="{{ $baseurl }}/tags/{{ . | urlize }}">{{ . }}</a> - {{ $title }} </li>
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{{ end }}
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Notice how once we have entered the loop the value of {{ . }} has changed. We
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have defined a variable outside of the loop so we have access to it from within
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the loop.
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# Hugo Parameters
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Hugo provides the option of passing values to the template language
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through the site configuration (for sitewide values), or through the meta
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data of each specific piece of content. You can define any values of any
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type (supported by your front matter/config format) and use them however
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you want to inside of your templates.
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## Using Content (page) Parameters
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In each piece of content you can provide variables to be used by the
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templates. This happens in the [front matter](/content/front-matter).
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An example of this is used in this documentation site. Most of the pages
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benefit from having the table of contents provided. Sometimes the TOC just
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doesn't make a lot of sense. We've defined a variable in our front matter
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of some pages to turn off the TOC from being displayed.
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Here is the example front matter:
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```
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---
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title: "Permalinks"
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date: "2013-11-18"
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aliases:
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- "/doc/permalinks/"
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groups: ["extras"]
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groups_weight: 30
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notoc: true
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---
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```
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Here is the corresponding code inside of the template:
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{{ if not .Params.notoc }}
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<div id="toc" class="well col-md-4 col-sm-6">
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{{ .TableOfContents }}
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</div>
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{{ end }}
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## Using Site (config) Parameters
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In your top-level configuration file (eg, `config.yaml`) you can define site
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parameters, which are values which will be available to you in chrome.
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For instance, you might declare:
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```yaml
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params:
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CopyrightHTML: "Copyright © 2013 John Doe. All Rights Reserved."
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TwitterUser: "spf13"
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SidebarRecentLimit: 5
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```
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Within a footer layout, you might then declare a `<footer>` which is only
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provided if the `CopyrightHTML` parameter is provided, and if it is given,
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you would declare it to be HTML-safe, so that the HTML entity is not escaped
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again. This would let you easily update just your top-level config file each
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January 1st, instead of hunting through your templates.
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```
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{{if .Site.Params.CopyrightHTML}}<footer>
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<div class="text-center">{{.Site.Params.CopyrightHTML | safeHtml}}</div>
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</footer>{{end}}
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```
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An alternative way of writing the "if" and then referencing the same value
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is to use "with" instead. With rebinds the context `.` within its scope,
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and skips the block if the variable is absent:
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```
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{{with .Site.Params.TwitterUser}}<span class="twitter">
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<a href="https://twitter.com/{{.}}" rel="author">
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<img src="/images/twitter.png" width="48" height="48" title="Twitter: {{.}}"
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alt="Twitter"></a>
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</span>{{end}}
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```
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Finally, if you want to pull "magic constants" out of your layouts, you can do
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so, such as in this example:
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```
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<nav class="recent">
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<h1>Recent Posts</h1>
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<ul>{{range first .Site.Params.SidebarRecentLimit .Site.Recent}}
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<li><a href="{{.RelPermalink}}">{{.Title}}</a></li>
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{{end}}</ul>
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</nav>
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```
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[go]: https://golang.org/
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[gohtmltemplate]: https://golang.org/pkg/html/template/
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