What Template Engines Solve

A template engine takes a template (text with placeholders) + data → final string.

Template:    Hello, {{ name }}! You have {{ count }} new messages.
Data:        {name: "Alice", count: 5}
Output:      Hello, Alice! You have 5 new messages.

Use cases:

• HTML generation (server-side rendered web pages).
• Email composition.
• Code generation (Jinja2 in Ansible templates).
• Config file rendering (Helm charts, k8s manifests).
• Documentation generation.

Real engines (target Jinja2-like for our build):

• Jinja2 (Python, used by Flask, Ansible, Salt).
• Mustache (logic-less, multi-language).
• Handlebars (JS, extends Mustache).
• EJS (JS, embedded JavaScript).
• ERB (Ruby on Rails).
• Go templates (text/template, html/template).
• Liquid (Shopify, Jekyll).

Two design philosophies:

1. Logic-less (Mustache): no conditionals/loops in templates; all logic in data.
2. Logic-full (Jinja2, ERB): full conditionals, loops, even arbitrary code.

Trade-off: less logic = simpler templates, but more code preparing data; more logic = templates harder to test, easier to write.

Modern web frameworks (React, Vue, Svelte) use component-based rendering, blurring the template/code boundary.

Key features we'll implement:

• {{ var }} substitution.
• {% if %}, {% for %}, {% else %}, {% endif %}.
• Filters: {{ name | upper }}.
• Auto-escaping for HTML safety.
• Template inheritance ({% extends %}, {% block %}).
• Includes ({% include %}).