How to write correct Scala programs

Scala, because of its functional nature, already gives us some design tools to write correct programs: type polymorphism, higher-order functions, immutability, and others. There are also many test frameworks that allow us to detect some bugs. This is a good start, but we can do more: proving the absence of bugs. In this talk I will show how to use Isabelle, a proof assistant, to implement algorithms, prove them correct, and compile them as Scala programs. We’ll see how to formulate universal properties on programs and how to check them.

1. How to write correct Scala programs Lars Hupel (they/them) Scala Days 2025
2. Poll #1 Do you use property testing?
3. Image credits: CC-BY 2.0 / Greg, CC-BY-SA 3.0 / Dionisio Weiland
4. Property testing is great! • ScalaCheck came out ~2007 • Use is pervasive now • A few alternatives exist: scalaprops, hedgehog
5. What tools do we have? • Property testing • Polymorphism • Immutability • ...
6. Poll #2 Has property testing ever failed to find a bug?
7. Some things are hard with property testing • Anything with a large parameter space • Anything stateful • Anything with graphs
8. Corner cases ... val mid = (low + high) / 2
9. Corner cases ... val mid = (low + high) / 2
10. Poll #3 Will ScalaCheck find this bug?
11. scala> prop.check() ! Falsified after 0 passed tests. > ARG_0: 1 > ARG_1: 2147483647 > ARG_0_ORIGINAL: 39501542
12. What about this one? private void pushRun( int runBase, int runLen ) { this.runBase[stackSize] = runBase; this.runLen[stackSize] = runLen; StackSize++; }
13. What about this one? private void pushRun( int runBase, int runLen ) { this.runBase[stackSize] = runBase; this.runLen[stackSize] = runLen; StackSize++; }
15. What do we do? Use either: 1. Stainless 2.Isabelle Image credits: Viktor Kunčak
16. What do we do? Use either: 1. Stainless 2.Isabelle
17. Isabelle • Proof assistant for various logics • Typically: Higher-Order Logic (HOL) • Fully-fledged programming language and IDE
18. Isabelle/HOL HOL = Functional programming + logic
19. Demo time!
22. Why verification? Sometimes, you need code to be 100% correct ...
23. Why not verification? John Hughes: QuickCheck Testing for Fun and Profit (2007)
24. Q&A Lars Hupel + lars@hupel.info ✉️ 🐘