Exploring F# through modeling #4


In three earlier posts we have explored modeling a Rock-paper-scissor game in F#. We have modeled the domain and infrastructure. In the last post we looked at refactoring and C# interop. In this post we’ll go back to basics when we explore using C# infrastructure in our solution. We’re going to look at record types, classes and interface implementation.


We want to use our Domain with Commands and Events together with some C# infrastructure. The infrastructure might have interfaces that our F# solution need to implement to use the infrastructure. We could do some adapter clue, but in this post we’re going to implement the interfaces in F#.

Command interface

The C# interface that the infrastructure uses looks like this;

So let’s take a look at one of our commmands – the CreateGameCommand.

We have used interfaces before in our solution – the Event marker interface.

To implement the C# interface we use the same syntax, specifying the the interface then each member of the interface as follow;

Note that all interface implementations in F# are Explicit

Here we changed the id to a Guid to align with the interface, pointing out the id property. The Correlation ID don’t have a corresponding property yet, as we create a new guid. The Correlation ID would return a new Guid each time – not ok. The correlation ID is just an identifier of each instance, nothing we need to set our self when creating a command. So what are our options regarding the Correlation ID ? If we want to stick to a record type we need to add a property to match Correlation ID.

If we would like to skip passing a correlation ID, we could create an factory, or we need to create a class instead. A simple factory;

In this case a record type fits better (Type inference, immutability and pattern matching – Record types vs Classes).

Just to have a look – lets go for the class approach;

Adding the interface to the class;

Event interface

The C# interface that the infrastructure uses for Event looks like this;

The SourceId would be the aggregate related to the change. And Correlation ID what command resulted in this event. This introduces a new challenge. The infrastructure needs to set something in our immutable world. Our events are record types – immutable. But to comply with this interface we could declare one of it’s properties as mutable;

We could skip the get();


Hope the infrastructure is worth it. Our record types is now bulkier. There might have been other way to use the infrastructure, wrapping it’s contracts. But we got a good back to basics look on record types, classes and interface implementation in F#. Due to that the infrastructure where in C#, we haven’t looked in creating interfaces in F#. Below is some resources on the topic we touched upon. Hope to get back to modeling in future posts.



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