simple_template - Mustache Template Engine for Eiffel

Overview

simple_template is a Mustache-style template engine for Eiffel. It provides logic-less templates with automatic HTML escaping to prevent XSS attacks, sections for conditionals and loops, and partial template support.

Part of the simple_* ecosystem of focused, single-purpose Eiffel libraries.

Quick Start

Installation

Add to your ECF file:

<library name="simple_template" location="$SIMPLE_TEMPLATE/simple_template.ecf"/>

Set environment variable:

set SIMPLE_TEMPLATE=D:\path\to\simple_template

Basic Variable Substitution

local
    tpl: SIMPLE_TEMPLATE
do
    create tpl.make_from_string ("Hello, {{name}}!")
    tpl.set_variable ("name", "World")
    print (tpl.render)  -- "Hello, World!"
end

HTML Escaping (Default)

-- Automatic XSS prevention
tpl.set_variable ("content", "<script>alert('xss')</script>")
print (tpl.render)  -- "&lt;script&gt;alert(&#39;xss&#39;)&lt;/script&gt;"

Raw/Unescaped Output

-- Triple braces for raw HTML
create tpl.make_from_string ("{{{html}}}")
tpl.set_variable ("html", "<b>Bold</b>")
print (tpl.render)  -- "<b>Bold</b>"

Conditional Sections

-- Show section when truthy
create tpl.make_from_string ("{{#logged_in}}Welcome back!{{/logged_in}}")
tpl.set_section ("logged_in", True)
print (tpl.render)  -- "Welcome back!"

-- Inverted section (show when falsy)
create tpl.make_from_string ("{{^has_items}}No items{{/has_items}}")
tpl.set_section ("has_items", False)
print (tpl.render)  -- "No items"

List Iteration

local
    items: ARRAYED_LIST [HASH_TABLE [STRING, STRING]]
    item: HASH_TABLE [STRING, STRING]
do
    create tpl.make_from_string ("{{#users}}{{name}} {{/users}}")

    create items.make (2)
    create item.make (1)
    item.put ("Alice", "name")
    items.extend (item)
    create item.make (1)
    item.put ("Bob", "name")
    items.extend (item)

    tpl.set_list ("users", items)
    print (tpl.render)  -- "Alice Bob "
end

Comments

-- Comments are not rendered
create tpl.make_from_string ("Hello{{! This is ignored }}World")
print (tpl.render)  -- "HelloWorld"

Partials

local
    tpl, header: SIMPLE_TEMPLATE
do
    create header.make_from_string ("<h1>{{title}}</h1>")
    create tpl.make_from_string ("{{>header}}<main>Content</main>")
    tpl.register_partial ("header", header)
    tpl.set_variable ("title", "My Page")
    print (tpl.render)
end

Template Syntax

Syntax	Description
`{{variable}}`	Output variable (HTML escaped)
`{{{variable}}}`	Output variable (raw, no escaping)
`{{#section}}...{{/section}}`	Conditional section (show if truthy) or list iteration
`{{^section}}...{{/section}}`	Inverted section (show if falsy)
`{{! comment }}`	Comment (not rendered)
`{{>partial}}`	Include partial template

Features

Mustache Syntax

Standard Mustache template syntax compatible with other implementations.

Auto HTML Escaping

XSS prevention by default. Use triple braces for raw output.

Sections

Conditional rendering with truthy/falsy evaluation.

Inverted Sections

Show content only when value is false or missing.

List Iteration

Repeat sections for each item in a list.

Partials

Include reusable sub-templates.

Comments

Template documentation that doesn't appear in output.

Design by Contract

Full preconditions, postconditions, and class invariants.

API Summary

Initialization

Feature	Description
`make`	Create empty template
`make_from_string (template)`	Create from string
`make_from_file (path)`	Load from file

Configuration

Feature	Description
`set_escape_html (enabled)`	Enable/disable HTML escaping
`set_missing_variable_policy (policy)`	Set behavior for missing variables
`register_partial (name, template)`	Register a partial template

Context Building

Feature	Description
`set_variable (name, value)`	Set a variable
`set_variables (table)`	Set multiple variables
`set_section (name, visible)`	Set section visibility
`set_list (name, items)`	Set list for iteration
`clear_variables`	Clear all context

Rendering

Feature	Description
`render: STRING`	Render template to string
`render_to_file (path)`	Render and write to file

Query

Feature	Description
`has_variable (name): BOOLEAN`	Is variable defined?
`required_variables: LIST`	Extract variable names from template
`is_valid: BOOLEAN`	Is template syntactically valid?
`last_error: STRING`	Last error message
`template_source: STRING`	The template string
`escape_html_enabled: BOOLEAN`	Is escaping enabled?

Missing Variable Policies

Constant	Description
`Policy_empty_string`	Missing vars become "" (default)
`Policy_keep_placeholder`	Keep {{name}} in output
`Policy_raise_exception`	Set error on missing var

Testing

The library includes a comprehensive test suite with 32 tests covering:

Initialization (empty, string, file)
Configuration (escaping, missing variable policy)
Variable handling (single, multiple, clear)
Basic rendering (plain text, variables, spaces)
HTML escaping (entities, raw output, disabled)
Sections (truthy, falsy, missing, inverted)
List iteration (items, empty list)
Comments (single line, multiline)
Partials
Nested sections

-- Run tests
ec.exe -batch -config simple_template.ecf -target simple_template_tests -c_compile
./EIFGENs/simple_template_tests/W_code/simple_template.exe