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Concurrency in Haskell: Fast, Simple, Correct

(bitbashing.io)
193 points ingve | 1 comments | 14 Apr 25 10:47 UTC | HN request time: 0.208s | source
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FuckButtons ◴[17 Apr 25 03:24 UTC] No.43712793[source]▶
I thought it was a bit odd that the author claims there’s no mutexes in sight, the TVar is effectively a mutex guard unless I’m misunderstanding this? (I’ve written exactly 0 lines of Haskel). Or is the claim that the lack of ceremony and accidental complexity around threading is the real win for concurrency here?
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1. ghusbands ◴[17 Apr 25 08:32 UTC] No.43714335[source]▶
As siblings note, TVar is a transactional variable. However, it's not just protective against concurrent writes but also against concurrent reads of altered variables, so it offers true atomicity across any accessed state in a transaction.

So if you have a thread altering `foo` and checking that `foo+bar` isn't greater than 5 and a thread altering `bar` and checking the same, then it's guaranteed that `foo+bar` does not exceed 5. Whereas if only write conflicts were detected (as is default with most databases) then `foo+bar` could end up greater than 5 through parallel changes.