Templates are great way to learn how to assemble Clojure libraries into a working project and can save considerable time when creating new projects by providing common configuration, development tools, dependencies and code used by the engineering team.

seancorfield/deps-new provides a simple to understand approach to defining templates. Declarative rules express how file templates are copied to create a new project. Substitution values are passed to {{unique-key}} placeholders in template files creating a specific project each time. Programmatic transformation is also supported for significant customisation, e.g. adding options to templates.

practicalli/project-templates is a new project to provide comprehensive templates to support production level workflows, including Dockerfile configuration, GitHub continuous integration workflows, etc . Read on to see how the first template was built and learn tips to building your own templates.

deps-new provides a guide to writing templates

Existing templates

app, lib and scratch templates are built into deps-new tool. Additional template can be used from a local file space or Git repository, e.g. community templates.

deps-new also provides the template template to create a project for defining one or more custom project templates, highly recommended if sharing a template via Git repository.

Practicalli project templates provides the practicalli/service template to create a production grade HTTP service with Integrant component management system, mulog event logging, reitit routing for APIs, docker, tools.build and make tasks. More templates will be added to this project over time.

deps-new support templates published via a Git service or on the local file space. Templates published via Maven / Clojars are not supported

seancorfield/clj-new uses the Leiningen template format, providing wide range of project templates, although many of those templates do not include a Clojure CLI configuration. The clj-new project is not actively maintained.

Create template project

The deps-new built-in template called template will create a new project for creating a custom template (or multiple templates).

:project/create alias from Practicalli Clojure CLI Config provides the deps-new tool

Create a new project and specify the template, name of project (the template name) and optionally the directory to create the new project within.

clojure -T:project/create :template template :name practicalli/service :target-dir project-templates

clojure -Tnew template :name practicalli/service if deps-new was installed as a Clojure CLI tool

Version control the newly created template project and start customising.

Use new template

For an effective local development workflow, add the new template project as a dependency via :local/root, then template changes are available as they are saved.

:project/templates
{practicalli/templates {:local/root "home/practicalli/projects/practicalli/project-templates/"}}

Create a new project with this template to test what a new project looks like.

Use the :project/templates alias when calling deps-new, e.g. with the :project/create alias from Practicalli Clojure CLI Config.

Values for :template and :name keys are required for deps-new when calling deps-new

clojure -A:project/templates -T:project/create :template practicalli/service :name practicalli/gameboard

The :name form is the organisation or developer name / service name

New Template Overview

The files that make up the template reside under the resources directory, e.g. resources/practicalli/service/ for the Practicalli Service template

• build - source of the dependencies, a deps.edn template (deps.tmpl) and build script build.clj template (build.tmpl)
• root - files and directories to be added to the root of the new project
• src - files & directories providing default code examples
• test - files & directories providing default test code examples
• template.edn - values for substitution, declarative :transform rules for copying files and directories to a new project (values optionally used for programmatic transformation)

This is the default structure and can be modified as required. The template.edn should be modified to match any changes in directory structure and file names.

In the deps-new template design, Clojure files with {{}} substitution blocks have the tmpl extension to avoid errors from tooling that compiles or analyses code, should those files be added to the classpath.

Practicalli uses .clj.template and .edn.template file name extensions with Clojure files that contain substitutions to retain the original file name extension and clarity as to the files purpose

Copy files

deps-new creates a new project by copying all the files from the template using the :transform declarative rules.

Files in the root directory of the template are copied to the root of the new project by default.

If file names should be transformed or files moved to a different location than root, create a new directory in the template to contain those files and add an explicit rule to the :transform section. Remember to remove the files from the root directory, otherwise they will still be copied.

:transform key contains declarative rules that define where the contents of each directory in the template is copied to in a new project created from the template, e.g.:

• "build" "" copies files in the 'build' directory of the template to the root of a new project
• "src" "src/{{top/file}}" copies files to the src directory in a sub-directory provided by the template substitution, e.g src/practicalli/gameboard

In the Practicalli project templates the build files from the original deps-new template were renamed to deps.edn.template and build.clj.template to include their original filename extension and be very explicitly named.

[["api" "src/{{top/file}}/{{main/file}}/api"
   {"system_admin.clj.template" "system_admin.clj"
    "scoreboard.clj.template" "scoreboard.clj"}]
  ["build" ""
   {"build.clj.template" "build.clj"
    "deps.edn.template"  "deps.edn"}]
  ["resources" "resources"
   {"config.edn.template" "config.edn"}]
  ["src" "src/{{top/file}}/{{main/file}}"
   {"middleware.clj.template" "middleware.clj"
    "parse_system.clj.template" "parse_system.clj"
    "router.clj.template" "router.clj"
    "service.clj.template" "service.clj"}]
  ["test" "test/{{top/file}}/{{main/file}}"
   {"service_test.clj.template" "service_test.clj"}]]

The :only keyword added to the end of a transform mapping will only copy a specified file from the directory, ignoring all other files. :raw keyword will not use any substitution on that file, useful for binary files any files that use the {{}} substitution for purposes other than creating projects with deps-new.

HINT: practicalli/service template includes a resources/config.edn.template that will be copied to resources/config.edn in a new project. The existing resources directory in root was removed to ensure resources/.keep is not copied to the new project.

Substitution

{{key}} in any of the copied files will substitute the value associated with :keyword, if that key is found in:

• available options lists optional keys to override template defaults
• project names and variables lists derived, environmental and scm variables that can be used as substitutions
• template.edn file of the template in use

"{{description}}" substitutes the value associated with the :description key from the template.edn file or command-line argument. Typically used in the project readme.md file and/or the comment header of the main namespace

(ns {{top/ns}}.{{main/ns}}) substitutes the fully qualified namespace within an ns expression, creating the project specific namespace for source and test files.

{{raw-name}} uses value from :name passed as an argument

The following sub-sections show examples from the Practicalli Project templates for ideas on how to use these substitutions

Namespace and File names

Clojure projects need to use the correct case in file names (snake_case) and namespace ns definitions (kebab-case).

Appending any unqualified substitution keyword with /ns or /file will use the correct case

• {{unqualified-key/ns}} - hyphenated words use kebab-case, e.g. for ns and require expressions
• {{unqualified-key/file}} - hyphenated words use snake_case, e.g. for use as a filename or directory path

Use {{top/ns}} and {{main/ns}} substitutions to create a namespace form using the values derived from the :name value provided on the command line when creating a new project

(ns {{top/ns}}.{{main/ns}}.service
  "Gameboard service component lifecycle management"
  (:gen-class)
  (:require
   ;; Application dependencies
   [{{top/ns}}.{{main/ns}}.router :as router]
   ;; Component system
   [{{top/ns}}.{{main/ns}}.parse-system :as parse-system]
   ;; System dependencies
   [org.httpkit.server     :as http-server]
   [com.brunobonacci.mulog :as mulog]))

{{top/file}} and {{main/file}} are used in the template.edn to use the correct case for file names, derived from the :name value given when creating the project.

[["api" "src/{{top/file}}/{{main/file}}/api"
   {"system_admin.clj.template" "system_admin.clj"
    "scoreboard.clj.template" "scoreboard.clj"}]
  ["build" ""
   {"build.clj.template" "build.clj"
    "deps.edn.template"  "deps.edn"}]
  ["resources" "resources"
   {"config.edn.template" "config.edn"}]
  ["src" "src/{{top/file}}/{{main/file}}"
   {"middleware.clj.template" "middleware.clj"
    "parse_system.clj.template" "parse_system.clj"
    "router.clj.template" "router.clj"
    "service.clj.template" "service.clj"}]
  ["test" "test/{{top/file}}/{{main/file}}"
   {"service_test.clj.template" "service_test.clj"}]]

Build files directory

In the Practicalli Project templates, deps.edn.template is a Clojure CLI deps.edn configuration file which provides several aliases to support development.

• :run/service starts the service by running the -main function from the main namespace
• :run/greet calls the greet function, an example of using clojure.exec
• :test/env is a placeholder for adding libraries which would support the testing of the service.
• :test/run starts the kaocha test runner and runs all the unit tests within the test path (stops on first failing test) and can also be used with CI service
• :build runs the given build task (defined in build.clj), e.g. clojure -T:build uberjar

The :run/service uses a substitution to specify the fully qualified main namespace

  :run/service
  {:main-opts ["-m" "{{top/ns}}.{{main/ns}}"]}

Root files

README.md, CHANGELOG.md, .gitignore and pom.xml are provided by the deps-new template.

• README.md uses the {{raw-name}} as the title and {{description}} as the project description

The Practicall service template updates the root files and adds many others that support the Practicalli workflow

The following files were added to the resources/practicalli/service/root directory

• .cljstyle configuration for Cljstyle linter
• .dockerignore configuration to manage files copied to docker images
• .dir-local.el Clojure CLI aliases to use with the Emacs Cider jack-in command
• dev/user.clj for development tools
  • launches Portal data inspector, listening on nREPL and displays all evaluation results
  • Mulog event log with publisher to send event logs to portal
  • find-deps to search for Clojure libraries

src and test

The locations to copy the src and test files in a new project are defined in the :transform section of the template.edn file.

In each file within src and test, the namespace definition uses substitution to define the correct namespace

In the src/service.clj.template:

(ns {{top/ns}}.{{main/ns}}
  (:gen-class)
  (:require
    [com.brunobonacci.mulog :as mulog]))

In the test/service.clj.template the ns expression uses substitution before the -test postfix on the namespace.

The service namespace from source is also added via substitution

(ns {{top/ns}}.{{main/ns}}-test
  (:require [clojure.test :refer [deftest is testing]]
            [{{top/ns}}.{{main/ns}} :as {{main/ns}}]))

Custom substitution

Add key value pairs in template.edn for custom substitutions, e.g. defining the version of Clojure

{;; Values to pass into the template
 :description "TODO: Provide a meaningful description of the project"

 ;; Custom substitutions
 :clojure-version "1.11.1"

 ;; Programatic Transformation functions
 :data-fn practicalli.service/data-fn
 :template-fn practicalli.service/template-fn

 ;; Declarative Transformation rules
 :transform
 [[,,,]]}

The deps.edn.template can then use substitution to include the desired version of Clojure

 :deps
 {org.clojure/clojure {{clojure-version}}}

When using the template to create a project, the custom key can be given a different value as a command line option

clojure -T:project/create :template practicalli/application :name practicalli/playground :clojure-version "1.12.0-alpha"

Test templates

Each template should have a unit test that checks the template.edn file against the deps-new specification

Tests can be run with a Clojure test runner, e.g. kaocha test runner using :test/run alias from Practicalli Clojure CLI Config.

clojure -X:test/run

In Practicalli Project Templates, src/practicalli/service_test.clj defines a unit test with clojure.test and clojure.spec, which test the practicalli/template/service/template.edn configuration.

A unit test is provides when creating a new project via the deps-new template

Publish and Use template

Push the new project with custom template to a shared Git service (GitHub, GitLab, etc.)

Create a release with a Git tag version and a meaningful description, e.g. using the GitHub releases page https://github.com/practicalli/project-templates/releases

A alias could be used to add the new template project in the user deps.edn configuration

  :project/templates
  {extra-deps:
   {io.github.practicalli/project-templates {:git/tag "2023.04.19" :git/sha "975e771"}}}

However, it is simpler to include the new template project as a dependency within the alias to run deps-new, e.g. :project/create alias definition.

  :project/create
  {:replace-deps {io.github.seancorfield/deps-new
                  {:git/tag "v0.5.1" :git/sha "48bf01e"}
                  io.github.practicalli/project-templates
                  {:git/tag "2023.04.19" :git/sha "975e771"}}
   :exec-fn org.corfield.new/create
   :exec-args {:template practicalli.template/application
                  :name practicalli/playground}}

Default values for template.edn keys can be defined in the :exec-args {} of an alias for the project template. These values will be overridden by matching key values passed via the command line

The command to create a new project is greatly simplified

clojure -T:project/create

Providing a specific project name will override the default :name value

clojure -T:project/create :name practicalli.template/gameboard

And specifying a template will override the default :template value

clojure -T:project/create :template practicalli/application :name practicalli.template/gameboard

Practicalli Clojure CLI Config has been updated to include the practicalli/project-templates dependency, making available all the Practicalli templates.

Multiple template project

Multiple templates can be added to the same project, especially if they are logically connected (e.g. part of the same team or organisation).

A single dependency can be used if all templates are available in the same project.

Each template is referred to via its namespace, e.g. practicalli/service, practicalli/application, etc.

• resources/practicalli/new-template-name: add all the code and configuration for the new project, typically following the build, root, src and test directory name convention
• src/practicalli/: add template_name.clj
• test/practicalli/: add template_name_test.clj

Summary

seancorfield/deps-new template provides a fast way to start creating your own templates, which can quickly be assembled from code and configuration in existing project.

Practicalli project templates provides an example of how comprehensive a template can be with very simple declarative rules.

deps-new also provides Programmatic transformation for more advanced transformation.

A data-fn function is provided to modify the available substitution data and the transform-fn to modify the template.edn structure.

transform-fn could be used to update the declarative rules that control the file copying, using alternative source code and configuration files based on optional values passed into the template, e.g. :persistence :postgres command line option would use source code files that included next.jdbc, postgres library as a dependency and code to manage and query the database.

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