[Haskell-cafe] MVar considered harmful

Ian Denhardt ian at zenhack.net
Wed Dec 19 22:14:47 UTC 2018

A few thoughts:

* IIRC, MVars (in Haskell anyway) also have some fairness guarantees,
  which I don't see otherwise mentioned in the discussion so far.
  Compare to STM, where large transactions are susceptible to
* If the goal is to have a simpler base primitive on which to build,
  everything has to boil down to compare-and-swap,
  load-linked/store-conditional, etc -- whatever the machine provides.
  When you start talking about even mutexes and semaphores, you're
  already dealing with higher-level abstractions, so the thinking about
  what's the right "primitive" seems silly to me at that point; you
  should be thinking about what the high-level (user) code should be
  able to do, and then figure out how to meet in the middle.
* If what you're after is *composable* concurrency, simpler locks won't
  get you that. STM is the best general solution I've seen for this, and
  if you're doing concurrency stuff in Haskell, I would say use STM
  unless you have a specific reason to do something else.
* Re: async exceptions, you're in the same situation with MVars as you
  are with all resource-acquiring IO actions in Haskell. `withMVar` and
  friends cover async-exception safety, and you can't get rid of mask and
  friends by swapping out MVars with another primitive anyway, because
  you still need them for acquiring other (non-concurrency related)
  resources, like files.


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