[Haskell-cafe] MVars and locks

[Ryan Yates]

When you are using an MVar as a lock, you will not want anyone to put
something into the MVar without already being the *one* taking from the
lock in the first place.  You can think of the value in the MVar as a token
indicating that you are in the critical section that no other thread can be
in unless they hold the token.

On Wed, May 6, 2015 at 3:19 PM, Tom Ellis <
tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk> wrote:

> In the following section of the book "Parallel and Concurrent Programming
> in
> Haskell"
>
>
> http://chimera.labs.oreilly.com/books/1230000000929/ch07.html#sec_conc-phonebook
>
> Simon Marlow explains that MVars can be used to implement something like
> locks on shared functional state: "To acquire the lock, we take the MVar,
> whereas, to update the variable and release the lock, we put the MVar."
>
> Am I right in thinking this only holds if we are careful to ensure both
> those operations happen in the given order?
>
> For example, if I take the MVar (lock) and I am about to put something back
> in (unlock) but someone else puts something in before I do, then the lock
> system becomes broken, doesn't it?  So if we want to be sure we have a
> robust lock system we have to wrap up MVars in another abstraction?
>
> Tom
>
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