Templating Zoo

What is Templating?

Templating in the sense of this articlee is when there is a text with placeholders that get filled in according to some set of rules (the templating language). Using templating is often easier than hardcoding some string assembly logic, it also has the benefit that the template could be read in from a configuration file.

In case you've ended up here because you have some sample templating but no documentation: Keep reading, you'll probably find what you're searching for. (No gurantees though, there are many templating mechanisms out there)

Since most Templating languages borrow from each other it makes sense to group them into Families.

The printf likes

The printf() function makes it easy to generate semi-templated text in C and other languages without having to do manual string manipulaion. These templates are sometimes even applicable in reverse like with scanf or strptime.

How to recognize

The placeholders usually start with a percent sign (%) and end in a single letter that depends on the datatype. Extra information on how exactly to format is between the two.

Special characters may be escapable using backslash (/) escape sequences.

Examples

Template that contains a string: %s

Formatted number: %.2f

Literal percent sign: %%

Languages

printf likes are implemented in many places

The fully featured printf is likely to be found around C programs or in scripting languages like string.format() in lua or the printf shell command.

Documentation

printf for generic number and string formatting

strftime for formatting timestamps (quite popular across different libraries)

custom formats like for the find command also exist, those are usually documented in a manual somewhere

Compatibility Warning: printf implementations are not consistent and not all implementations support all features. For example encoding characters as octal (i.e. /033) is more widely supported than its hexadecimal cousin (i.e. \x1B).

Python printf-style

Python has "printf-style String Formatting" using the %-operator. The printf syntax has been extended to allow indexing dictionaries and is usually encoutered as %(key)s where key is the key used to index the dictionary and s is an example of the most common printf type, but all other types work too, if the data allows.

While this approach is dicouraged (in favor of pythons new format function) it will be used for quite some time in configuration files.

Go-Templates

Go-templates are named after ther implementation in the go programming language, the are part of the de-facto standard library. They are best known from the Hugo static site generator, though the hugo extension usually aren't reimplemented elsewhere. The template packages by themselves are pretty barebones.

How to recognize

All template syntax happens eclosed between {{ and }}

Placeholders usually start with a dot (.)

Examples

{{.Count}} items are made of {{.Material}}

{{23 -}} < {{- 45}} renders to 23<45

Hello, {{.}}!

{{ if and (eq .View "search") (ne .Data.Site "") }}…{{ end }}

Langugage

Go

Documentation

text/template

html/template

Jinja2 and friends

Jinja2 is a flexible and popular templating engine, that is used almost everywhere, where moere advanced templating is used. It also heavily inspired the tera crate for rust, which has almost identical syntax.

How to recognize

Variables are enclosed in {{ and }}, template logic uses {% and %}

Templates based on other templates have {% extends "…" %} as their first line.

Variables don't start with a dot (.) (that''s a go template thing)

Examples

This morning I need {{ size | default(value="a huge cup of" }} coffe.

{% if thing.is_working %}🥳{% else %}🔧{% endif %}

Languages

Python

Rust

Documentation

Jinja2

tera

Where to find

In web-related contexts, generating HTML or XML

Configuration templates, for example in Ansible

In other contexts too …

XSLT

XSLT (Extensible Stylesheet Language Transformations) is usually used to convert XML to different XML, but it can also be (ab)used to convert XML to text.

How to recognize

The template is XML and mentions the xsl: namespace (http://www.w3.org/1999/XSL/Transform) a lot.

Example

See the Wikipedia Article on XSLT

Languages

Java, but not exclusiely

Where to find

In enterprisey apllications from the 90s

In your web-browser