The most exciting feature of the upcoming Clojure 1.9 is clojure.spec, which delivers a game changer experience in a dynamically typed language such as clojure. With this new tool your code will be able to express constraints that are traditionally very hard to encode in traditional type systems, like describing a function that only accepts a sequence of strings of increasing length of which the third element starts with a capital letter. In this talk I'll start with the spec basics up to some of its advanced usages, from data validation to generative testing.

1. Above and beyond type systems with
clojure.spec
Carlo Sciolla
CODEMOTION MILAN - SPECIAL EDITION
10 – 11 NOVEMBER 2017

2. whoamicommunity edition

3. whoamienterprise edition

5. clojure.spec?

6. What spec is...
- a queryable registry of predicates
- a description of shapes of data
- a library to validate code interaction
- a gateway drug to generative testing

7. ...and what spec is not
- a type
- a type system
- a compiler
- a static interface
- a bug-free code guarantee

8. SHOW
ME
THE
CODE!!11oneone
WARN: parenthesis ahead!

9. A Clojure primer
(ns codemotion.demo)
(defn hello-user
[user]
(when (seq user)
(str "Hello, " user "!")))
(hello-user "world")

10. A Clojure primer
(ns codemotion.demo)
(defn hello-user
[user]
(when (seq user)
(str "Hello, " user "!")))
(hello-user "world")
(hello-user 42)
As clojure is a dynamically typed language, no compile time error is thrown when feeding functions with
unexpected kind of input.

11. Step 1:
string ok, number nok

12. We start with a predicate
user> (doc string?)
-------------------------
clojure.core/string?
([x])
Return true if x is a String
user> (string? 42)
false
user> (string? "I'm a string")
true
Predicates are functions that return boolean values. Any predicate can be used as a spec.

13. Attach the spec to a function
(ns codemotion.demo
(:require [clojure.spec.alpha :as s]))
(defn hello-user!
[user]
(when (seq user)
(str "Hello, " user "!")))
(s/fdef hello-user!
:args (s/cat :user string?))

14. demo timewish me luck

15. Function spec at its finest
(s/fdef hello-user!
:args (s/cat :user ::non-blank-string)
:ret ::non-blank-string
:fn (fn [{ret :ret
{user :user} :args}]
(= (count ret)
(+ 9 (count user)))))
You can spec the input arguments, the return value and the function invariants.

16. Step 2:
complex data types
union types and the likes

17. Greeting a JWT-authenticated user

18. demo time
wish me luck

19. Step 3:
generative testing

20. demo timebla bla bla.. ok, we got it

21. Conclusions

22. Philosophy and philology of

23. Philosophy and philology of
[Lisp] has assisted a number of our most gifted fellow humans in thinking previously impossible
thoughts
E. W. Dijkstra

24. Philosophy and philology of
We were not out to win over the Lisp programmers; we were after the C++ programmers. We managed
to drag a lot of them about halfway to Lisp
Guy Steele

25. Philosophy and philology of
[Clojure was] designed to be useful for the work I was doing, where you have customers that have
requirements that things run on one of these standard platforms
Rich Hickey

26. Philosophy and philology of
People change the world by using things. The focus must be on the "using", not the "thing"
Bret Victor

27. Thank you!
Carlo Sciolla
Sr Developer
https://codehopper.nl
https://synple.eu
@skuro