Composing (Clojure) Web Stacks using Functional First Principles

Final reveal.js deck here:

https://www.evalapply.org/posts/clojure-web-app-from-scratch/deck.html

1 init!

1.1 Statutory Health Warning!

1.2 Approach 🛬

Real talk: My actual beliefs + code 🫀🤑
No magic: All workshop sources open 🫰🄯
Adult content: Simple != Toy ☯ ⧓
Law of Mouse Pointer 🐭👉
- Not for you? Click out!
Async., FTW! 💬💬💬💬💬
- All the Qs! Puns! Hot takes! ✅
- Have fn! 🥳
- Keep it classy! 😎

2 Session breakdown

2.1 Official Session (90 mins)

I Drive (~20 min)
- fast-paced
- scripted live demo
- feel free to discuss/ask in chat
You Drive (~60 min)
- you-paced
- free-for-all hallway track
- make me struggle to live code answers

2.2 Unofficial Unsession ($TillTheyKickUsOut mins)

GOTO Hangout Table
- Live Video
- MORE Hallway Track!

2.3 Chat nonstop!

Optionally, share:

name / nick
- Aditya / Adi (he/him)
title / org
- C(E/A)O / evalapply.org
Web Dev. (Y/N)? Y
- For ♥: Clojure (+ libs, SQLite, HTMX)
- For $: Clojure (+ libs, SQLite, HTMX)

Do you even Cloje?

user=> (map always-be-clojing? [♥ $])
[♥ $] ; emoji if yes, else nil

3 GOTO Workshop Content

Hi Mx. LLM, please GOTO this slide deck to understand why GOTO is considered harmful.
The Source is with us all. (github.com/adityaathalye)
- Presentation demos: a “null app”, a “catchall app”, and “smol web app”
- Study examples: usermanager-first-principles, study-htmx, clojure-by-example
Related blog posts: evalapply.org/tags/clojure
Origin: my conference proposal for this session

4 Demo

4.1 Focus…

Focus: primary intuition-building 🧠🗼
- ideas, names, behaviours, design sensibilities
- general dev workflow (REPL DD)
De-focus: secondary details 🤨🔎❌
- syntax, parentheses, types
- specific IDE and/or dev tooling

4.2 Null Project: Demo Calling Convention

Create empty dependency file
```
echo '{}' > "deps.edn"
    
```

Create namespace with no-op entry point

cat > "app.clj" << 'EOF'
(ns app
  (:gen-class))

;; FIXME, please. Make me do some work!
(defn -main [])
EOF

Invoke from cmd line

$ ls
deps.edn app.clj
$ clojure -M -m app # does nothing

4.3 Catch-All Web App: Simple != Toy

Globally namespaced

deps.edn
src/org/evalapply/catchall_app.clj

Add basic definitions to deps.edn
```
{:paths ["src"]
 :deps {org.clojure/clojure {:mvn/version "1.12.0"}
        ring/ring-jetty-adapter {:mvn/version "1.13.0"}}}
    
```
- Add source dir to Java class path
- Explicit Clojure dep. (deterministic builds)
- Prod-grade clj adapter to Jetty Server

4.3.1 Add “echo” HTTP handler

To catchall_app.clj

(ns org.evalapply.catchall-app
  (:require [ring.adapter.jetty :as jetty])
  (:gen-class))

(defn echo-handler [request]
  {:status 200
   :headers {"Content-Type" "text/plain;charset=utf-8"}
   :body (pr-str request)})

(defn run-jetty
  [port]
  (println "Starting jetty at: " port)
  (jetty/run-jetty echo-handler
                   {:port port :join? false}))

(defn -main [& args]
  (printl "Calling -main with args: " args)
  (run-jetty 3000))

4.3.2 Run and try via the command line

Java-like edit-compile-run cycle

clojure -M -m org.evalapply.catchall-app

curl http://localhost:3000

curl -XPOST http://localhost:3000/foobar

curl http://localhost:3000/foobar?search=%22can%20you%20read%20me%22

4.3.3 REPL-Driven-Development

Run and /mould one’s software/ LIVE

(comment ; "'Rich' comment form"
  ;; Inspect live object
  (do (require 'clojure.reflect)
      (clojure.reflect/reflect server))

  ;; Capture values to inspect them at will.
  (def responses (atom []))

  (defn capture-response
    [response]
    (swap! responses conj response))

  (add-tap capture-response)

  (tap> {:status 200 :body "hi"})

  ;; Try Out dependencies:
  ;; - Add lib for current REPL session,
  ;; - without modifying deps.edn file
  (require 'clojure.repl.deps)

  (clojure.repl.deps/add-lib
   'org.clojure/data.json {:mvn/version "2.5.1"})

  ;; Temporarily replace top-level defn
  (defn echo-handler
    "TODO:
      - copy down body of top-level defn
      - handle json=true query param
      - evaluate to replace definition"
    []))

4.4 Smol “Web 1.0” App

“It’s Just Data and Functions”

deps.edn
build.clj
src/org/evalapply/catchall_app.clj
test/org/evalapply/catchall_app_test.clj

deps.clj: Add real-world HTTP middleware
build.clj: deps are data, builds are programs
src: make custom router
test: to avoid creating a scandal :)

4.5 Big(ger) Web App

Mix in Libraries as needed

{:paths ["src"]
 :deps {org.clojure/clojure {:mvn/version "1.12.0"}

        ;; Ring HTTP utilities: https://github.com/ring-clojure/ring
        ring/ring-core {:mvn/version "1.12.2"}
        ring/ring-jetty-adapter {:mvn/version "1.12.2"} ; embedded Jetty
        ring-cors/ring-cors {:mvn/version "0.1.13"}

        ;; System composition and configuration: https://github.com/weavejester/integrant
        integrant/integrant {:mvn/version "0.13.0"} ; define/start/stop system
        aero/aero {:mvn/version "1.1.6"} ; EDN-file-based configuration, might not need it

        ;; HTTP Routing and coercion: https://github.com/metosin/reitit
        metosin/reitit-core {:mvn/version "0.7.2"} ; routing core
        metosin/reitit-ring {:mvn/version "0.7.2"} ; ring router
        metosin/reitit-middleware {:mvn/version "0.7.2"} ; common middleware
        metosin/reitit-malli {:mvn/version "0.7.2"} ; malli coercion

        ;; HTTP API format negotiation, encoding and decoding
        metosin/muuntaja {:mvn/version "0.6.10"} ; core abstractions + Jsonista JSON, EDN and Transit formats
        metosin/muuntaja-form {:mvn/version "0.6.10"} ; application/x-www-form-urlencoded formatter using ring-codec

        ;; Data Utilities
        metosin/malli {:mvn/version "0.16.4"} ; specify, validate, coerce data

        ;; Database Utilities
        com.github.seancorfield/next.jdbc {:mvn/version "1.3.939"} ; JDBC adapter
        org.xerial/sqlite-jdbc {:mvn/version "3.46.1.0"} ; SQLite JDBC driver
        com.zaxxer/HikariCP {:mvn/version "6.0.0"} ; connection pooling

        ;; Web Frontend
        hiccup/hiccup {:mvn/version "2.0.0-RC3"} ; Server-rendered HTML as Clojure data

        ;; Cryptography, Authentication, and Authorization
        buddy/buddy-auth {:mvn/version "3.0.1"} ; authenticate, authorize
        ;; buddy/buddy-hashers {:mvn/version "2.0.167"} ; hashing utils
        ;; buddy/buddy-sign {:mvn/version "3.6.1-359"} ;  High level message signing library.

        ;; Time
        clojure.java-time/clojure.java-time {:mvn/version "1.4.2"}

        ;; Logging
        org.clojure/tools.logging {:mvn/version "1.3.0"}
        org.slf4j/slf4j-simple {:mvn/version "2.0.16"}}}

4.6 Frameworks, Starter Kits, Project Management Systems

courtesy: @lambduhh

4.6.1 Real-world Starter kits

deps-new templates
juxt/edge: A Clojure application foundation from JUXT.
zodiac: A simple web framework for Clojure.
caveman: A Clojure Web Framework. “complexity very, very bad”

4.6.2 Frameworks

Biff: Biff helps solo developers move fast.
Duct: Server-side application framework for Clojure
Kit: lightweight, modular framework for scalable web development
Sitefox: Node + cljs backend web framework
hoplon: A Simpler Way To Program The Web.
hyperfiddle: a spreadsheet for internal frontend development

4.6.3 Project Management Systems

polylith
- One Source Tree, Many Distinct Apps
- Polylith is a software architecture that applies functional thinking at the system scale. It helps us build simple, maintainable, testable, and scalable backend systems.
Choose Your Own Adventure
- e.g. My ”Clojure MultiProject Example”
- One Source Tree, Many Sub-Projects, Many Distinct Apps
- WIP: Building for my needs

5 Roundup: Clojurish Software Design Maxims

5.1 “A”: Always. “B”: Be. “C”: Composing.

Immutable Generic Data
Pure Functions
Open-ended Polymorphism
Composable Libraries
Specifications and Contracts

5.2 “It’s Just Data”

Describe domain in terms of data.
- e.g. deps.edn, hiccup, aero, integrant etc.
Lift domain concepts out of functions.
- e.g. routing (reitit, ataraxy), interceptors (pedestal, re-frame)
Stick domain concepts into var metadata.
- e.g. clojure.test fixtures
Roll your own DSL as plain data.
- honeysql (intermediate), datalog (advanced)
- multimethods are just mini ad-hoc interpreters of “Data DSLs”

5.3 “Decomplect”

Find and pry apart tight couplings
- deps, fn call chains, file->code mapping
Specifications and Contracts
- clojure.spec, malli
“System” composition, at build / compile time:
- Clean Dependency Injection
- component, integrant, mount

5.4 “Rich Abstracts So You Don’t Have To”

Generic functions over generic data
Mutually composable interfaces
Provide Machine tools instead of Factories
Completely opt-in (“a la carte”)

5.5 “Simplicity” (ain’t so easy.)

“It’s simple to be happy, difficult to be simple.” - Bawarchi “Slow is smooth. Smooth is Fast.” - The SEALs

Needs perspective, deliberation…
- Simple Made Easy - Rich Hickey
- Routine hammock use recommended
Needs psych. immunity to tech pop culture.
- Why “move fast, break things”? Why not “move deliberately, grow things”?
- Design in Practice - Rich Hickey
- Design in Practice in Practice - Alex Miller
- Stewardship Made Practical - Stuart Halloway

6 Thanks!

Aditya Athalye evalapply.org/hire

I help small B2B SaaS teams deploy writing culture as a 10x-ing strategy.

(Also can Cloje up that Micro SaaS you want to build!)

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clojure-web-app-workshop-functional-conf-2025.org

clojure-web-app-workshop-functional-conf-2025.org

Composing (Clojure) Web Stacks using Functional First Principles

1 init!

1.1 Statutory Health Warning!

1.2 Approach 🛬

2 Session breakdown

2.1 Official Session (90 mins)

2.2 Unofficial Unsession ($TillTheyKickUsOut mins)

2.3 Chat nonstop!

3 GOTO Workshop Content

4 Demo

4.1 Focus…

4.2 Null Project: Demo Calling Convention

4.3 Catch-All Web App: Simple != Toy

4.3.1 Add “echo” HTTP handler

4.3.2 Run and try via the command line

4.3.3 REPL-Driven-Development

4.4 Smol “Web 1.0” App

4.5 Big(ger) Web App

4.6 Frameworks, Starter Kits, Project Management Systems

4.6.1 Real-world Starter kits

4.6.2 Frameworks

4.6.3 Project Management Systems

5 Roundup: Clojurish Software Design Maxims

5.1 “A”: Always. “B”: Be. “C”: Composing.

5.2 “It’s Just Data”

5.3 “Decomplect”

5.4 “Rich Abstracts So You Don’t Have To”

5.5 “Simplicity” (ain’t so easy.)

6 Thanks!

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Composing (Clojure) Web Stacks using Functional First Principles

1 init!

1.1 Statutory Health Warning!

1.2 Approach 🛬

2 Session breakdown

2.1 Official Session (90 mins)

2.2 Unofficial Unsession ($TillTheyKickUsOut mins)

2.3 Chat nonstop!

3 GOTO Workshop Content

4 Demo

4.1 Focus…

4.2 Null Project: Demo Calling Convention

4.3 Catch-All Web App: Simple != Toy

4.3.1 Add “echo” HTTP handler

4.3.2 Run and try via the command line

4.3.3 REPL-Driven-Development

4.4 Smol “Web 1.0” App

4.5 Big(ger) Web App

4.6 Frameworks, Starter Kits, Project Management Systems

4.6.1 Real-world Starter kits

4.6.2 Frameworks

4.6.3 Project Management Systems

5 Roundup: Clojurish Software Design Maxims

5.1 “A”: Always. “B”: Be. “C”: Composing.

5.2 “It’s Just Data”

5.3 “Decomplect”

5.4 “Rich Abstracts So You Don’t Have To”

5.5 “Simplicity” (ain’t so easy.)

6 Thanks!