Using Cats Effect's MapRef

(all code samples can be run using scala-cli)

If you use Cats Effect (CE) it is likely you have found that you need a Map instance that has to be fiber-safe as several fibers can access it concurrently. MapRef is a type offered by CE std package that accommodates such need. This post shows how to instantiate and use it in a CE application.

Instantiate your MapRef, put and get data

Below you can find a simple example of how to instantiate and work with a MapRef[IO, Int, String], that is, a MapRef that 'works' in IO, and that contains a map with keys of type Int and values of type String :

Concurrency

Now, let's show off a bit the concurrent capabilities of MapRef, this time by first inserting 20 elements and then getting them back again concurrently:

Note also that get returns the value wrapped in a Ref instance, so you can modify it in a safe manner and changes will be visible to other fibers accessing the same value, see Ref documentation.

Full control over the wrapped Map

Sometimes you need a bit more control over the wrapped map that the one given by the MapRef API. To do so you can create an instance of Java's ConcurrentHashMap and then wrap it in a MapRef instance, below you can find an example of how to do so, see how this code sample uses the ConcurrentHashMap to retrieve the full list of keys: