haml

Haml is a templating language for HTML that uses indentation to reflect the structure of the document. It stands for HTML Abstraction Markup Language and was created to simplify HTML markup for Ruby-based web development. Haml templates are compiled into HTML by a Ruby runtime and are commonly used with Ruby web frameworks, most notably Rails. The .haml file extension is typically used for such templates.

The syntax emphasizes minimal markup and nesting through indentation. Elements are declared with a percent sign, such as %div or %ul, and nesting is indicated by indentation. Class and id can be added using shorthand like %div#main.panel or %ul.list. Attributes can be supplied in a Ruby-like hash form, for example %div{ data: { id: 1 }, class: "panel" }. Ruby code and dynamic content are integrated directly into templates: lines starting with - introduce Ruby code blocks, while lines starting with = insert the result of Ruby expressions into the output, usually escaped by default; != can be used for unescaped HTML output.

Haml is designed to reduce boilerplate and improve readability compared with plain HTML. It integrates with Rails through gems such as haml-rails and is used in other Ruby frameworks as well. The language has inspired alternatives like Slim, which aim to further streamline templating, and there are ports and implementations in other ecosystems. Critics point to the learning curve, indentation sensitivity, and the need for a templating step, which can be less familiar to developers accustomed to HTML-centric workflows.