So, a little while ago, while I was studying Haskell, I decided to implement Greg Young’s SimpleCQRS example in Haskell.

During the last @DDDBE event, I was showing this to Mathias, and he decided to put it on the big screen.

The general response was:

What’s missing from Greg’s example?

And my reply was that nothing was missing. As people seemed to be flabbergasted by this code, I decided to convert it into a blog post.

This is the code:

A little background for the people who don’t know Haskell:

• The data statements define all the required commands, events and state; a pipe symbol for the data type means A or B
• handle :: [Event] -> Command -> [Event] implies that it is a function named handle that takes as parameters an array of events and a command, and it returns an array of Events.
• handle first creates an item by assigning apply events to it, then it passes it into a function named handle', which uses pattern matching to figure out which handle' to invoke
• item (i.e. the state) is either Nothing or Just Item.
• apply events starts out with Nothing as the state, and then calls apply' for every event in the events, resulting in the modified state upon applying every event.

I didn’t consider this such a big thing, except for the terseness and the very low signal to noise ratio in the code, but as people seemed to like it I decided to put this in a blog post for your enjoyment.

Have fun!