haskell-es

An example of DDD + EventSourcing in Haskell.

The "main" modules of this example are:

• Aggregate - the Aggregate class
• Card - the model that represents a simple shopping card which has an Aggregate instance
• Repository - a simple event sourcing repository that works for any Aggregate and uses the EventStore.

To run the example

• Download and run the EventStore (https://geteventstore.com/)
  • Navigate to http://127.0.0.1:2113/ (default credentials are admin/changeit) to ensure you have ES running
• git clone https://github.com/AlexeyRaga/haskell-es-example.git
• cd haskell-es-example
• stack build
• ./haskell-es-exe

The following commands are accepted:

data Command Card = AddProduct ProductId
                  | RemoveProduct ProductId
                  | ClearCard
                  deriving (Show, Read)

The commands type derives Read (for example purposes only) so it can be used in the console as:

AddProduct (ProductId "prod-1")
RemoveProduct (ProductId "prod-1")
ClearCard

Enter exit to leave the session. Enter expose to show the current card state.

A session could look like:

Your current card is:
Card {cardId = CardId "test-card-1", products = []}

Enter your commands here:
AddProduct (ProductId "prod-1")
--> ProductAdded {addedProductId = ProductId "prod-1"}
AddProduct (ProductId "prod-2")
--> ProductAdded {addedProductId = ProductId "prod-2"}
AddProduct (ProductId "prod-1")
--> ProductAdded {addedProductId = ProductId "prod-1"}
AddProduct (ProductId "prod-1")
--! QuantityExceedsLimit (ProductId "prod-1") 2
RemoveProduct (ProductId "prod-1")
--> ProductRemoved {removedProductId = ProductId "prod-1"}
RemoveProduct (ProductId "prod-3")
--! NoProductInCard (ProductId "prod-3")
expose
Card {cardId = CardId "test-card-1", products = [CardItem {productId = ProductId "prod-1", quantity = 1},CardItem {productId = ProductId "prod-2", quantity = 1}]}
exit
Bye.

All the events are stored in the EventStore therefore the state of the aggregate is persisted between runs.

Go to the "Stream Browser" menu of the EventStore UI (http://127.0.0.1:2113/web/index.html#/streams) to see the streams. This example uses test-card-1 stream. Navigating to this stream will show all the history of the aggregate (all the events), which is one of the biggest points.