# Selmer
> Django-inspired HTML templating system for Clojure with filters, tags, and template inheritance
- Skill: `hugoduncan/selmer` (Agent Skill, multi-file: 3 files)
- Install (CLI): `npx skillmds@latest add hugoduncan/selmer`
- Raw SKILL.md: https://api.skillmd.com/api/skills/hugoduncan/selmer/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Web & Frontend
- Author: hugoduncan (https://skillmd.com/u/hugoduncan)
- Updated: 2026-09-10
- Page: https://skillmd.com/skills/hugoduncan/selmer
---
# Selmer
Django-inspired HTML templating system for Clojure providing a fast, productive templating experience with filters, tags, template inheritance, and extensive customization options.
## Overview
Selmer is a pure Clojure template engine inspired by Django's template syntax. It compiles templates at runtime, supports template inheritance and includes, provides extensive built-in filters and tags, and allows custom extensions. Designed for server-side rendering in web applications, email generation, reports, and any text-based output.
**Key Features:**
- Django-compatible template syntax
- Variable interpolation with nested data access
- 50+ built-in filters for data transformation
- Control flow tags (if, for, with)
- Template inheritance (extends, block, include)
- Custom filter and tag creation
- Template caching for performance
- Auto-escaping with override control
- Validation and error reporting
- Middleware support
**Installation:**
Leiningen:
```clojure
[selmer "1.12.65"]
```
deps.edn:
```clojure
{selmer/selmer {:mvn/version "1.12.65"}}
```
**Quick Start:**
```clojure
(require '[selmer.parser :refer [render render-file]])
;; Render string
(render "Hello {{name}}!" {:name "World"})
;=> "Hello World!"
;; Render file
(render-file "templates/home.html" {:user "Alice"})
```
## Core Concepts
### Variables
Variables use `{{variable}}` syntax and are replaced with values from the context map.
```clojure
(render "{{greeting}} {{name}}" {:greeting "Hello" :name "Bob"})
;=> "Hello Bob"
```
**Nested Access:**
```clojure
(render "{{person.name}}" {:person {:name "Alice"}})
;=> "Alice"
(render "{{items.0.title}}" {:items [{:title "First"}]})
;=> "First"
```
**Missing Values:**
```clojure
(render "{{missing}}" {})
;=> "" (empty string by default)
```
### Filters
Filters transform variable values using the pipe `|` operator.
```clojure
(render "{{name|upper}}" {:name "alice"})
;=> "ALICE"
;; Chain filters
(render "{{text|upper|take:5}}" {:text "hello world"})
;=> "HELLO"
```
### Tags
Tags use `{% tag %}` syntax for control flow and template structure.
```clojure
{% if user %}
Welcome {{user}}!
{% else %}
Please log in.
{% endif %}
```
### Template Inheritance
Parent template (`base.html`):
```html
{% block head %}Default Title{% endblock %}
{% block content %}{% endblock %}
```
Child template:
```html
{% extends "base.html" %}
{% block head %}Custom Title{% endblock %}
{% block content %}Hello!
{% endblock %}
```
## API Reference
### Rendering Functions
#### `render`
Render a template string with context.
```clojure
(render template-string context-map)
(render template-string context-map options)
;; Examples
(render "{{x}}" {:x 42})
;=> "42"
(render "[% x %]" {:x 42}
{:tag-open "[%" :tag-close "%]"})
;=> "42"
```
**Parameters:**
- `template-string` - Template as string
- `context-map` - Data map for template
- `options` - Optional map with `:tag-open`, `:tag-close`, `:filter-open`, `:filter-close`
#### `render-file`
Render a template file from classpath or resource path.
```clojure
(render-file filename context-map)
(render-file filename context-map options)
;; Examples
(render-file "templates/email.html" {:name "Alice"})
(render-file "custom.tpl" {:x 1}
{:tag-open "<%%" :tag-close "%%>"})
```
**File Resolution:**
1. Checks configured resource path
2. Falls back to classpath
3. Caches compiled template
### Caching
#### `cache-on!`
Enable template caching (default).
```clojure
(require '[selmer.parser :refer [cache-on!]])
(cache-on!)
```
Templates are compiled once and cached. Use in production.
#### `cache-off!`
Disable template caching for development.
```clojure
(require '[selmer.parser :refer [cache-off!]])
(cache-off!)
```
Templates recompile on each render. Use during development.
### Configuration
#### `set-resource-path!`
Configure base path for template files.
```clojure
(require '[selmer.parser :refer [set-resource-path!]])
(set-resource-path! "/var/html/templates/")
(set-resource-path! nil) ; Reset to classpath
```
#### `set-missing-value-formatter!`
Configure how missing values are rendered.
```clojure
(require '[selmer.parser :refer [set-missing-value-formatter!]])
(set-missing-value-formatter!
(fn [tag context-map]
(str "MISSING: " tag)))
(render "{{missing}}" {})
;=> "MISSING: missing"
```
### Introspection
#### `known-variables`
Extract all variables from a template.
```clojure
(require '[selmer.parser :refer [known-variables]])
(known-variables "{{x}} {{y.z}}")
;=> #{:x :y.z}
```
Useful for validation and documentation.
### Validation
#### `validate-on!` / `validate-off!`
Control template validation.
```clojure
(require '[selmer.validator :refer [validate-on! validate-off!]])
(validate-on!) ; Default - validates templates
(validate-off!) ; Skip validation for performance
```
Validation catches undefined filters, malformed tags, and syntax errors.
### Custom Filters
#### `add-filter!`
Register a custom filter.
```clojure
(require '[selmer.filters :refer [add-filter!]])
(add-filter! :shout
(fn [s] (str (clojure.string/upper-case s) "!!!")))
(render "{{msg|shout}}" {:msg "hello"})
;=> "HELLO!!!"
;; With arguments
(add-filter! :repeat
(fn [s n] (apply str (repeat (Integer/parseInt n) s))))
(render "{{x|repeat:3}}" {:x "ha"})
;=> "hahaha"
```
#### `remove-filter!`
Remove a filter.
```clojure
(require '[selmer.filters :refer [remove-filter!]])
(remove-filter! :shout)
```
### Custom Tags
#### `add-tag!`
Register a custom tag.
```clojure
(require '[selmer.parser :refer [add-tag!]])
(add-tag! :uppercase
(fn [args context-map]
(clojure.string/upper-case (first args))))
;; In template: {% uppercase "hello" %}
```
**Block Tags:**
```clojure
(add-tag! :bold
(fn [args context-map content]
(str "" (get-in content [:bold :content]) ""))
:bold :endbold)
;; In template:
;; {% bold %}text here{% endbold %}
```
#### `remove-tag!`
Remove a tag.
```clojure
(require '[selmer.parser :refer [remove-tag!]])
(remove-tag! :uppercase)
```
### Error Handling
#### `wrap-error-page`
Middleware to display template errors with context.
```clojure
(require '[selmer.middleware :refer [wrap-error-page]])
(def app
(wrap-error-page handler))
```
Shows error message, line number, and template snippet.
### Escaping Control
#### `without-escaping`
Render template without HTML escaping.
```clojure
(require '[selmer.util :refer [without-escaping]])
(render "{{html}}" {:html "Bold"})
;=> "<b>Bold</b>"
(without-escaping
(render "{{html}}" {:html "Bold"}))
;=> "Bold"
```
## Built-in Filters
### String Filters
**upper** - Convert to uppercase
```clojure
{{name|upper}} ; "alice" → "ALICE"
```
**lower** - Convert to lowercase
```clojure
{{NAME|lower}} ; "ALICE" → "alice"
```
**capitalize** - Capitalize first letter
```clojure
{{word|capitalize}} ; "hello" → "Hello"
```
**title** - Title case
```clojure
{{phrase|title}} ; "hello world" → "Hello World"
```
**addslashes** - Escape quotes
```clojure
{{text|addslashes}} ; "I'm" → "I'm"
```
**remove-tags** - Strip HTML tags
```clojure
{{html|remove-tags}} ; "text" → "text"
```
**safe** - Mark as safe (no escaping)
```clojure
{{html|safe}} ; Renders HTML without escaping
```
**replace** - Replace substring
```clojure
{{text|replace:"old":"new"}}
```
**subs** - Substring
```clojure
{{text|subs:0:5}} ; First 5 characters
```
**abbreviate** - Truncate with ellipsis
```clojure
{{text|abbreviate:10}} ; "Long text..." (max 10 chars)
```
### Formatting Filters
**date** - Format date
```clojure
{{timestamp|date:"yyyy-MM-dd"}}
{{timestamp|date:"MMM dd, yyyy"}}
```
**currency-format** - Format currency
```clojure
{{amount|currency-format}} ; 1234.5 → "$1,234.50"
```
**double-format** - Format decimal
```clojure
{{number|double-format:"%.2f"}} ; 3.14159 → "3.14"
```
**pluralize** - Pluralize noun
```clojure
{{count}} item{{count|pluralize}}
; 1 item, 2 items
{{count}} box{{count|pluralize:"es"}}
; 1 box, 2 boxes
```
### Collection Filters
**count** - Get collection size
```clojure
{{items|count}} ; [1 2 3] → "3"
```
**first** - First element
```clojure
{{items|first}} ; [1 2 3] → "1"
```
**last** - Last element
```clojure
{{items|last}} ; [1 2 3] → "3"
```
**join** - Join with separator
```clojure
{{items|join:", "}} ; [1 2 3] → "1, 2, 3"
```
**sort** - Sort collection
```clojure
{{items|sort}} ; [3 1 2] → [1 2 3]
```
**sort-by** - Sort by key
```clojure
{{people|sort-by:"age"}}
```
**reverse** - Reverse collection
```clojure
{{items|reverse}} ; [1 2 3] → [3 2 1]
```
**take** - Take first N
```clojure
{{items|take:2}} ; [1 2 3] → [1 2]
```
**drop** - Drop first N
```clojure
{{items|drop:1}} ; [1 2 3] → [2 3]
```
### Utility Filters
**default** - Default if falsy
```clojure
{{value|default:"N/A"}}
```
**default-if-empty** - Default if empty
```clojure
{{text|default-if-empty:"None"}}
```
**hash** - Compute hash
```clojure
{{text|hash:"md5"}}
{{text|hash:"sha256"}}
```
**json** - Convert to JSON
```clojure
{{data|json}} ; {:x 1} → "{\"x\":1}"
```
**length** - String/collection length
```clojure
{{text|length}} ; "hello" → "5"
```
## Built-in Tags
### Control Flow
#### `if` / `else` / `endif`
Conditional rendering.
```clojure
{% if user %}
Hello {{user}}!
{% else %}
Please log in.
{% endif %}
```
**With operators:**
```clojure
{% if count > 10 %}
Many items
{% elif count > 0 %}
Few items
{% else %}
No items
{% endif %}
```
**Operators:** `=`, `!=`, `<`, `>`, `<=`, `>=`, `and`, `or`, `not`
#### `ifequal` / `ifunequal`
Compare two values.
```clojure
{% ifequal user.role "admin" %}
Admin panel
{% endifequal %}
{% ifunequal status "active" %}
Inactive
{% endifunequal %}
```
#### `firstof`
Render first truthy value.
```clojure
{% firstof user.nickname user.name "Guest" %}
```
### Loops
#### `for`
Iterate over collections.
```clojure
{% for item in items %}
{{forloop.counter}}. {{item}}
{% endfor %}
```
**Loop Variables:**
- `forloop.counter` - 1-indexed position
- `forloop.counter0` - 0-indexed position
- `forloop.first` - True on first iteration
- `forloop.last` - True on last iteration
- `forloop.length` - Total items
**With empty:**
```clojure
{% for item in items %}
{{item}}
{% empty %}
No items found
{% endfor %}
```
**Destructuring:**
```clojure
{% for [k v] in pairs %}
{{k}}: {{v}}
{% endfor %}
```
#### `cycle`
Cycle through values in a loop.
```clojure
{% for item in items %}
{{item}}
{% endfor %}
```
### Template Structure
#### `extends`
Inherit from parent template.
```clojure
{% extends "base.html" %}
```
Must be first tag in template.
#### `block`
Define overridable section.
Parent template:
```html
{% block content %}Default content{% endblock %}
```
Child template:
```html
{% block content %}Custom content{% endblock %}
```
**Block super:**
```html
{% block content %}
{{block.super}} Additional content
{% endblock %}
```
#### `include`
Insert another template.
```clojure
{% include "header.html" %}
```
**With context:**
```clojure
{% include "item.html" with item=product %}
```
### Other Tags
#### `comment`
Template comments (not rendered).
```clojure
{% comment %}
This won't appear in output
{% endcomment %}
```
#### `now`
Render current timestamp.
```clojure
{% now "yyyy-MM-dd HH:mm" %}
```
#### `with`
Create local variables.
```clojure
{% with total=items|count %}
Total: {{total}}
{% endwith %}
```
#### `verbatim`
Render content without processing.
```clojure
{% verbatim %}
{{this}} won't be processed
{% endverbatim %}
```
Useful for client-side templates.
#### `script` / `style`
Include script/style blocks without escaping.
```clojure
{% script %}
var x = {{data|json}};
{% endscript %}
{% style %}
.class { color: {{color}}; }
{% endstyle %}
```
#### `debug`
Output context map for debugging.
```clojure
{% debug %}
```
## Common Patterns
### Email Templates
```clojure
(defn send-welcome-email [user]
(let [html (render-file "emails/welcome.html"
{:name (:name user)
:activation-link (generate-link user)})]
(send-email {:to (:email user)
:subject "Welcome!"
:body html})))
```
### Web Page Rendering
```clojure
(defn home-handler [request]
{:status 200
:headers {"Content-Type" "text/html"}
:body (render-file "pages/home.html"
{:user (:user request)
:posts (fetch-recent-posts)})})
```
### Template Fragments
```clojure
;; Reusable components
(render-file "components/button.html"
{:text "Click me"
:action "/submit"})
```
### Dynamic Form Generation
```clojure
(render-file "forms/user-form.html"
{:fields [{:name "username" :type "text"}
{:name "email" :type "email"}
{:name "password" :type "password"}]
:action "/register"})
```
### Report Generation
```clojure
(defn generate-report [data]
(render-file "reports/monthly.html"
{:period (format-period)
:totals (calculate-totals data)
:items data
:generated-at (java.time.LocalDateTime/now)}))
```
### Template Composition
```clojure
;; Base layout
{% extends "layouts/main.html" %}
;; Page-specific
{% block title %}Dashboard{% endblock %}
{% block content %}
{% include "components/stats.html" %}
{% include "components/chart.html" %}
{% endblock %}
```
### Custom Marker Syntax
```clojure
;; Compatible with client-side frameworks
(render-file "spa.html" data
{:tag-open "[%"
:tag-close "%]"
:filter-open "[["
:filter-close "]]"})
```
### Validation and Error Handling
```clojure
(require '[selmer.parser :refer [render-file known-variables]])
(require '[selmer.validator :refer [validate-on!]])
(validate-on!)
(defn safe-render [template-name data]
(try
(let [required (known-variables
(slurp (io/resource template-name)))]
(when-not (every? #(contains? data %) required)
(throw (ex-info "Missing template variables"
{:required required :provided (keys data)})))
(render-file template-name data))
(catch Exception e
(log/error e "Template rendering failed")
"Error rendering template")))
```
## Error Handling
### Common Errors
**Missing Template:**
```clojure
(render-file "nonexistent.html" {})
;=> Exception: resource nonexistent.html not found
```
**Solution:** Verify file exists in classpath or resource path.
**Undefined Filter:**
```clojure
(render "{{x|badfilter}}" {:x 1})
;=> Exception: filter badfilter not found
```
**Solution:** Check filter name or define custom filter.
**Malformed Tag:**
```clojure
(render "{% if %}" {})
;=> Exception: malformed if tag
```
**Solution:** Ensure tag syntax is correct.
### Error Middleware
```clojure
(require '[selmer.middleware :refer [wrap-error-page]])
(def app
(-> handler
wrap-error-page
wrap-other-middleware))
```
Displays detailed error page with:
- Error message
- Line number
- Template excerpt
- Context data
### Validation
```clojure
(require '[selmer.validator :refer [validate-on!]])
(validate-on!)
(render "{% unknown-tag %}" {})
;=> Validation error with details
```
Catches errors at compile time rather than runtime.
## Performance Considerations
### Template Caching
**Enable in production:**
```clojure
(cache-on!)
```
Templates compile once, cache compiled version. Significant performance improvement.
**Disable in development:**
```clojure
(cache-off!)
```
Recompiles on each render. See changes immediately.
### Resource Path Configuration
```clojure
(set-resource-path! "/var/templates/")
```
Reduces classpath scanning overhead.
### Filter Performance
**Expensive operations:**
```clojure
;; Avoid in loops
{% for item in items %}
{{item.data|json|hash:"sha256"}}
{% endfor %}
;; Better: preprocess in Clojure
(render-file "template.html"
{:items (map #(assoc % :hash (compute-hash %))
items)})
```
### Validation Overhead
```clojure
;; Development
(validate-on!)
;; Production (after testing)
(validate-off!)
```
Validation adds minimal overhead but can be disabled if templates are thoroughly tested.
### Template Inheritance
Shallow inheritance trees perform better than deep nesting.
**Good:**
```
base.html → page.html
```
**Slower:**
```
base.html → layout.html → section.html → page.html
```
## Best Practices
1. **Use template caching in production**
2. **Keep templates in dedicated directory** (`resources/templates/`)
3. **Validate templates in development**
4. **Preprocess complex data in Clojure** rather than in templates
5. **Use includes for reusable components**
6. **Leverage template inheritance** for consistent layouts
7. **Escape user content** (default behavior) unless explicitly safe
8. **Name templates descriptively** (`user-profile.html`, not `page1.html`)
9. **Document custom filters and tags**
10. **Test templates with various data** to catch edge cases
## Platform Notes
**Clojure:** Full support, production-ready.
**ClojureScript:** Not supported. Selmer is JVM-only due to template compilation requiring Java classes.
**Babashka:** Not supported. Selmer requires classes and compilation not available in Babashka.
**Alternatives for ClojureScript:**
- Reagent (Hiccup-style)
- Rum
- UIx
**Alternatives for Babashka:**
- Hiccup
- String templates with `format`