Stackr provides a comprehensive way to interact with your state. The state transition function is the only way to change the state. It is a pure function that takes the current state and an action as arguments and returns the new state.
Do not perform any side effects or network calls in the state transition function. It is a pure function.

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Custom Action Types -> STF Input

These are inputs to the STF, the inputs from user are also similar though
Stackr calls transactions as Actions because it does not limit you to the type of transactions you can build. You are in-charge. You can define any custom action types you want. Fun-fact - “Actions” were called “UserIntents” in the initial versions of Stackr but then another definition of “Intents” came out.

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Defining Custom Action Types

export interface CounterActionInput {
  type: "increment" | "decrement";
}
This action expects objects of the type {type: "increment"} to be passed to the state transition function. 
export interface TransferActionInput {
  type: "transfer";
  to: string;
  amount: number;
}
This action expects a transaction object of the type {type: "transfer", to: "0x123", amount: 100} to be passed to the state transition function.
Do not worry about signatures yet

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Defining the State Transition Function

oof, the big one. This is where you define how your state changes when an action is performed. Stackr-JS comes with a STF interface that can be extended. 
import { STF } from "@stackr/stackr-js/execution";

export const veryEfficientSTF: STF<RollupState, ActionInput>;
You need to pass the state and action input types to the STF interface.

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Extending the STF Interface

The STF interface has a single method apply that takes the input and an identifier. There are THREE steps to build an STF.

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1. Get the current state

usually via transport.getState()

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2. modify the state based on the action input

update the state based on the action input, use “switch” or “if” statements based on input

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3. set the new state to the transport

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here’s an example

export const counterSTF: STF<CounterRollup, CounterActionInput> = {
  identifier: "counterSTF",

  apply(inputs: CounterActionInput, state: CounterRollup): void {
    // step  1
    let newState = state.getState();

    // step 2
    if (inputs.type === "increment") {
      newState += 1;
    } else if (inputs.type === "decrement") {
      throw new Error("Not implemented");
    }

    // step 3
    state.transport.currentCount = newState;
  },
};

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Future scope

Currently the SDK only supports a single STF and the various types are handled using a switch statement, However in the future, the Micro-rollups will be able to accept multiple different STFs