Simplest Behaviour Tree

SimplestBehaviorTree.fsx

type Result = Success | Failure | Running

type Behavior =
    | Action of (unit -> Result)
    | Selector of Behavior list
    | Sequence of Behavior list
   
let rec behave = function
    | Action f -> f () 
    | Selector xs -> select xs      
    | Sequence xs -> sequence xs
and select xs =
    match xs with
    | [] -> Failure
    | x::xs' ->
        match behave x with
        | Failure -> select xs'
        | Success -> Success
        | Running -> Running
and sequence xs =
    match xs with
    | [] -> Failure
    | x::xs' ->
        match behave x with
        | Success -> sequence xs'
        | Failure -> Failure
        | Running -> sequence xs
        
let getToPlantingGrounds steps =
    let i = ref 0
    fun () ->
        incr i
        if !i < steps then
            printfn "Action - searching planting grounds"
            Running
        else
            Success

let plantSeeds () =
    printfn "Action - plant"
    Success

let weedsNotCoveringGround() =
    printfn "Conditional - is there space"
    Failure

let run () =
    let plant = Action(plantSeeds)
    let plantIfThereIsSpace =
        Selector [Action(weedsNotCoveringGround); plant]
    let sequence = Sequence [Action(getToPlantingGrounds 10); plant]
    let root = Selector [sequence; plantIfThereIsSpace]
    behave root
    
run ()