Counter

Let’s build a simple counter using Burr. When the counter terminates, print the count and the current time.

Imports

import datetime

from burr.core import action, State, ApplicationBuilder, expr

Define actions

We define the function increment() that has a State as input and outputs a new State. The decorator @action specifies it’s a Burr action and it will need to read and write from the counter attribute in State.

@action(reads=["counter"], writes=["counter"])
def increment(state: State) -> State:
    """Increment the counter by 1"""
    
    # get the value from the `state`
    current_count = state["counter"]
    current_count += 1

    print("Count: ", current_count)

    # use `.update()` to create a new `State`
    return state.update(counter=current_count)


@action(reads=["counter"], writes=[])
def exit_counter(state: State) -> State:
    """Print the current count and the current time"""
    current_count = state["counter"]

    print(f"Finished counting to {current_count} at {datetime.datetime.now():%H:%M:%S %Y-%m-%d}")

    return state

Build the Application

We pass the action to the ApplicationBuilder via .with_actions(). Then, we specify the valid transitions between actions: increment -> increment and increment -> exit_counter. The expression construct specifies to use the transition increment -> increment whenever the counter value in state is under ten. Then, default to increment -> exit_counter to terminate the application. We also use .with_entrypoint() to define where to start and .with_state() to set the starting state value.

app = (
    ApplicationBuilder()
    .with_actions(increment, exit_counter)
    .with_transitions(
        ("increment", "increment", expr("counter < 10")),
        ("increment", "exit_counter"),
    )
    .with_state(counter=0)
    .with_entrypoint("increment")
    .build()
)
app.visualize(include_conditions=True)

action_obj, result, state = app.run(halt_after=["exit_counter"])

Count:  1
Count:  2
Count:  3
Count:  4
Count:  5
Count:  6
Count:  7
Count:  8
Count:  9
Count:  10
Finished counting to 10 at 18:44:29 2024-08-26