Sorry I wasn't clear about that. The lock is not global, it is per cache. newCache creates a newMVar which serves as the thread safe lock for that cache. Concurrently fetching from different caches can indeed run their separate fallbacks concurrently. On Fri, Jun 13, 2014, 10:16 AM, Peter Simons wrote:

Hi Dan,

...

1. Create an empty cache with newCache 2. fetch repeatedly from the cache, each time providing a fallback in case it is empty.

Fetching from a Cache is thread safe: only one "fallback" will execute at a time, and after the first successful fallback, the cached value will be set and no other fallbacks will be called.

I'm not sure whether I get this correctly. If multiple threads fetch values from one cache, will only one of those threads run at a time? Or will those that fetch pairwise different values still run concurrently?

In other words, does the cache have a global lock or is there a lock per value?

Best regards, Peter

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