First Program

I came across1 the Memstate library (after reading about CQRS and eventsourcedb), which seemed very similar to acid-state on Haskell world, but based on CQRS and in the dotnet world. This would be a perfect candidate for a single-user Full-stack web development project, and as a replacement of LiteDB.

There were no examples for it in F#, however. So I bootstrapped a new dotnet console project and decided to get something working by referencing memstate’s tests.


// Learn more about F# at

open System
open Memstate
open Memstate.JsonNet

type Tweet =
    { Id: int
      Message: string

type TwitterModel() =
    let mutable tweets: Tweet list = []

    member this.Tweets = tweets

    member this.PostTweet(msg: string) = 
        let tweet = { Tweet.Id = 1; Message = msg }
        tweets <- tweets @ [tweet]

// Events 

type Tweeted(tweet: Tweet) =
    inherit Event()

// Commands

type PostTweet(msg: string) = 
    inherit Command<TwitterModel, int>()

    member this.Msg: string = msg
    override Command.Execute(model: TwitterModel): int =
        let tweet = model.PostTweet msg
        Command.RaiseEvent (new Tweeted(tweet))

// Query

type AllTweets() =
    inherit Query<TwitterModel, Tweet list>()

    override Query.Execute(model: TwitterModel) =

let main argv =
    // TODO: understand concepts used
    printfn "Begin"
    async {
        let! engine = Engine.Start<TwitterModel>() |> Async.AwaitTask
        let cmd = new PostTweet("Hello world")
        let! res = engine.Execute(cmd) |> Async.AwaitTask
        printfn "res: %d" res
        let query = new AllTweets()
        let! allTweets = engine.Execute(query) |> Async.AwaitTask
        printfn "%A" allTweets
        printfn "Fin."
    } |> Async.RunSynchronously
    0 // return an integer exit code

Keep in mind that this is the first F# code I wrote, and I did that without actually formally studying the language ahead. There is a TODO in there as a reminder for myself to study the concurrency topic (the async stuff). Microsoft has pretty good introductory docs on it.