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selmer

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Django-inspired HTML templating system for Clojure with filters, tags, and template inheritance

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SKILL.md

name selmer
description Django-inspired HTML templating system for Clojure with filters, tags, and template inheritance

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:

[selmer "1.12.65"]

deps.edn:

{selmer/selmer {:mvn/version "1.12.65"}}

Quick Start:

(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.

(render "{{greeting}} {{name}}" {:greeting "Hello" :name "Bob"})
;=> "Hello Bob"

Nested Access:

(render "{{person.name}}" {:person {:name "Alice"}})
;=> "Alice"

(render "{{items.0.title}}" {:items [{:title "First"}]})
;=> "First"

Missing Values:

(render "{{missing}}" {})
;=> "" (empty string by default)

Filters

Filters transform variable values using the pipe | operator.

(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.

{% if user %}
  Welcome {{user}}!
{% else %}
  Please log in.
{% endif %}

Template Inheritance

Parent template (base.html):

<html>
  <head>{% block head %}Default Title{% endblock %}</head>
  <body>{% block content %}{% endblock %}</body>
</html>

Child template:

{% extends "base.html" %}
{% block head %}Custom Title{% endblock %}
{% block content %}<h1>Hello!</h1>{% endblock %}

API Reference

Rendering Functions

render

Render a template string with context.

(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.

(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).

(require '[selmer.parser :refer [cache-on!]])

(cache-on!)

Templates are compiled once and cached. Use in production.

cache-off!

Disable template caching for development.

(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.

(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.

(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.

(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.

(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.

(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.

(require '[selmer.filters :refer [remove-filter!]])

(remove-filter! :shout)

Custom Tags

add-tag!

Register a custom tag.

(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:

(add-tag! :bold
  (fn [args context-map content]
    (str "<b>" (get-in content [:bold :content]) "</b>"))
  :bold :endbold)

;; In template:
;; {% bold %}text here{% endbold %}

remove-tag!

Remove a tag.

(require '[selmer.parser :refer [remove-tag!]])

(remove-tag! :uppercase)

Error Handling

wrap-error-page

Middleware to display template errors with context.

(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.

(require '[selmer.util :refer [without-escaping]])

(render "{{html}}" {:html "<b>Bold</b>"})
;=> "&lt;b&gt;Bold&lt;/b&gt;"

(without-escaping
  (render "{{html}}" {:html "<b>Bold</b>"}))
;=> "<b>Bold</b>"

Built-in Filters

String Filters

upper - Convert to uppercase

{{name|upper}} ; "alice" → "ALICE"

lower - Convert to lowercase

{{NAME|lower}} ; "ALICE" → "alice"

capitalize - Capitalize first letter

{{word|capitalize}} ; "hello" → "Hello"

title - Title case

{{phrase|title}} ; "hello world" → "Hello World"

addslashes - Escape quotes

{{text|addslashes}} ; "I'm" → "I\'m"

remove-tags - Strip HTML tags

{{html|remove-tags}} ; "<b>text</b>" → "text"

safe - Mark as safe (no escaping)

{{html|safe}} ; Renders HTML without escaping

replace - Replace substring

{{text|replace:"old":"new"}}

subs - Substring

{{text|subs:0:5}} ; First 5 characters

abbreviate - Truncate with ellipsis

{{text|abbreviate:10}} ; "Long text..." (max 10 chars)

Formatting Filters

date - Format date

{{timestamp|date:"yyyy-MM-dd"}}
{{timestamp|date:"MMM dd, yyyy"}}

currency-format - Format currency

{{amount|currency-format}} ; 1234.5 → "$1,234.50"

double-format - Format decimal

{{number|double-format:"%.2f"}} ; 3.14159 → "3.14"

pluralize - Pluralize noun

{{count}} item{{count|pluralize}}
; 1 item, 2 items

{{count}} box{{count|pluralize:"es"}}
; 1 box, 2 boxes

Collection Filters

count - Get collection size

{{items|count}} ; [1 2 3] → "3"

first - First element

{{items|first}} ; [1 2 3] → "1"

last - Last element

{{items|last}} ; [1 2 3] → "3"

join - Join with separator

{{items|join:", "}} ; [1 2 3] → "1, 2, 3"

sort - Sort collection

{{items|sort}} ; [3 1 2] → [1 2 3]

sort-by - Sort by key

{{people|sort-by:"age"}}

reverse - Reverse collection

{{items|reverse}} ; [1 2 3] → [3 2 1]

take - Take first N

{{items|take:2}} ; [1 2 3] → [1 2]

drop - Drop first N

{{items|drop:1}} ; [1 2 3] → [2 3]

Utility Filters

default - Default if falsy

{{value|default:"N/A"}}

default-if-empty - Default if empty

{{text|default-if-empty:"None"}}

hash - Compute hash

{{text|hash:"md5"}}
{{text|hash:"sha256"}}

json - Convert to JSON

{{data|json}} ; {:x 1} → "{\"x\":1}"

length - String/collection length

{{text|length}} ; "hello" → "5"

Built-in Tags

Control Flow

if / else / endif

Conditional rendering.

{% if user %}
  Hello {{user}}!
{% else %}
  Please log in.
{% endif %}

With operators:

{% if count > 10 %}
  Many items
{% elif count > 0 %}
  Few items
{% else %}
  No items
{% endif %}

Operators: =, !=, <, >, <=, >=, and, or, not

ifequal / ifunequal

Compare two values.

{% ifequal user.role "admin" %}
  Admin panel
{% endifequal %}

{% ifunequal status "active" %}
  Inactive
{% endifunequal %}

firstof

Render first truthy value.

{% firstof user.nickname user.name "Guest" %}

Loops

for

Iterate over collections.

{% 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:

{% for item in items %}
  {{item}}
{% empty %}
  No items found
{% endfor %}

Destructuring:

{% for [k v] in pairs %}
  {{k}}: {{v}}
{% endfor %}

cycle

Cycle through values in a loop.

{% for item in items %}
  <tr class="{% cycle 'odd' 'even' %}">{{item}}</tr>
{% endfor %}

Template Structure

extends

Inherit from parent template.

{% extends "base.html" %}

Must be first tag in template.

block

Define overridable section.

Parent template:

{% block content %}Default content{% endblock %}

Child template:

{% block content %}Custom content{% endblock %}

Block super:

{% block content %}
  {{block.super}} Additional content
{% endblock %}

include

Insert another template.

{% include "header.html" %}

With context:

{% include "item.html" with item=product %}

Other Tags

comment

Template comments (not rendered).

{% comment %}
  This won't appear in output
{% endcomment %}

now

Render current timestamp.

{% now "yyyy-MM-dd HH:mm" %}

with

Create local variables.

{% with total=items|count %}
  Total: {{total}}
{% endwith %}

verbatim

Render content without processing.

{% verbatim %}
  {{this}} won't be processed
{% endverbatim %}

Useful for client-side templates.

script / style

Include script/style blocks without escaping.

{% script %}
  var x = {{data|json}};
{% endscript %}

{% style %}
  .class { color: {{color}}; }
{% endstyle %}

debug

Output context map for debugging.

{% debug %}

Common Patterns

Email Templates

(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

(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

;; Reusable components
(render-file "components/button.html"
             {:text "Click me"
              :action "/submit"})

Dynamic Form Generation

(render-file "forms/user-form.html"
             {:fields [{:name "username" :type "text"}
                       {:name "email" :type "email"}
                       {:name "password" :type "password"}]
              :action "/register"})

Report Generation

(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

;; 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

;; Compatible with client-side frameworks
(render-file "spa.html" data
             {:tag-open "[%"
              :tag-close "%]"
              :filter-open "[["
              :filter-close "]]"})

Validation and Error Handling

(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:

(render-file "nonexistent.html" {})
;=> Exception: resource nonexistent.html not found

Solution: Verify file exists in classpath or resource path.

Undefined Filter:

(render "{{x|badfilter}}" {:x 1})
;=> Exception: filter badfilter not found

Solution: Check filter name or define custom filter.

Malformed Tag:

(render "{% if %}" {})
;=> Exception: malformed if tag

Solution: Ensure tag syntax is correct.

Error Middleware

(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

(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:

(cache-on!)

Templates compile once, cache compiled version. Significant performance improvement.

Disable in development:

(cache-off!)

Recompiles on each render. See changes immediately.

Resource Path Configuration

(set-resource-path! "/var/templates/")

Reduces classpath scanning overhead.

Filter Performance

Expensive operations:

;; 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

;; 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