So cool! Any languages support STM first class besides Haskell?

This felt like a blast from the past. At a few times reading this article, I had to go back and check that, yes, it's actually a new article from the year 2025 on STM in Haskell and it's even using the old bank account example.

I remember 15 or 20 years (has it been that long?) when the Haskell people like dons were banging on about: 1) Moore's law being dead, 2) future CPUs will have tons of cores, and 3) good luck wrangling them in your stone age language! Check out the cool stuff we've got going on over in Haskell!

@OP perhaps there’s a comparison bug in withdraw(): if (a.balance <= amount)

When working with high throughput concurrent code like consumer producer pipelines, it's better to avoid shared mutable state entirely. Something actor like fits better and C# or Go channels works wonders.

This is a nice article, and I appreciate the examples. The next problem to solve is how to persist state on disk across two different accounts after a transfer has been done.

I found that a lot of the problems I had been having with mutexes, stem from the fact that traditionally the mutex and the data it protects are separate. Bolting them together, like Rust's Mutex<T> does, solves a lot these problems. It let's you write normal, synchronous code and leave the locking up to the caller, but without making it a nightmare. You can't even access the data without locking the mutex.

This isn't an attack on the (very well written) article though. Just wanted to add my two cents.

>Ugh, a correct transfer function should conceptually just be the composition of our well encapsulated withdraw and a deposit functions, but defining it correctly has forced us to remove the locking from both withdraw and deposit, making both of them less safe to use.

I know OOP isn't cool any more, but the above is what OOP solves.

TFA's transfer() and withdraw() functions aren't compliant with double-entry accounting anyways, so you'd mark them private and only expose Transfer to callers. Let the class handle its own details.