add ability to move from room to room.

Entity.ts

@@ -0,0 +1,102 @@
+/**
+ * Entity.ts contains the Entity class which is a base class that handles effects and
+ * containing items. It is the base class for Player and Scene.
+ */
+
+import type { Conditions, Item, EntityProperties } from "./types.ts";
+
+export default class Entity<T extends EntityProperties> {
+	protected _conditions: Conditions<T>;
+	protected _properties: T;
+	protected _activeEffects: string[];
+
+	constructor(
+		properties: T,
+		conditions: Conditions<T>,
+		activeEffects: string[] = []
+	) {
+		this._conditions = conditions; // Conditional properties
+		this._properties = properties; // Base properties
+		this._activeEffects = activeEffects;
+	}
+
+	get activeEffects(): string[] {
+		return this._activeEffects;
+	}
+
+	set conditions(conditions: Conditions<T>) {
+		this._conditions = conditions;
+	}
+
+	get properties(): T {
+		return this._properties;
+	}
+
+	set properties(properties: T) {
+		this._properties = properties;
+	}
+
+	addEffect(effect: string): void {
+		this.activeEffects.push(effect);
+	}
+
+	// Player effects should be applied first, then scene effects should be applied.
+	// This will mean that scene effects will take precedence over player effects.
+	// Returns base properties with conditional properties applied.
+	applyEffects(
+		activePlayerEffects: string[],
+		activeSceneEffects: string[]
+	): T {
+		const playerSet = new Set(activePlayerEffects);
+		const sceneSet = new Set(activeSceneEffects);
+
+		const effects = this._conditions.effects.filter(({ name, source }) => {
+			const set = source === "player" ? playerSet : sceneSet;
+			return set.has(name);
+		});
+
+		const appliedProperties = { ...this._properties };
+
+		// for each effect, apply changed properties to the
+		for (const effect of effects) {
+			Object.assign(appliedProperties, effect.properties);
+		}
+
+		return appliedProperties;
+	}
+
+	// Generate a description of the contained items
+	description(items: Item[]): string | null {
+		if (items.length === 0) return null;
+
+		const vowels = ["a", "e", "i", "o", "u"];
+		const description = items
+			.map(({ name }, i) => {
+				let anItem = `${vowels.includes(name[0]) ? "an" : "a"} ${name}`;
+
+				// if we have more than one item, and this is the last item...
+				if (i > 1 && i + 1 === items.length) {
+					anItem = `and ${anItem}`;
+				}
+
+				return anItem;
+			})
+			.join(", ");
+
+		return description;
+	}
+
+	// Checks if a given effect exists as an active effect on this entity
+	hasActiveEffect(effect: string): boolean {
+		return this._activeEffects.includes(effect);
+	}
+
+	// Removes an effect if it is active on this entity
+	removeEffect(effect: string): void {
+		const idx = this._activeEffects.findIndex((e) => e === effect);
+
+		if (idx >= 0) {
+			this._activeEffects.splice(idx, 1);
+		}
+	}
+}

Interpreter.ts

@@ -0,0 +1,136 @@
+/**
+ * Interpreter.ts contains the Interpreter class which takes the command that the user
+ * made as well as the player and scene state. By checking each word of the user command,
+ * the interpreter figures out what action the user wants to take and then execute that
+ * command.
+ */
+
+import type Player from "./Player.ts";
+import type Scene from "./Scene.ts";
+import { terms } from "./terms/terms.ts";
+import type {
+	ActionTerm,
+	ActionFn,
+	Category,
+	Constant,
+	Term,
+} from "./terms/Term.ts";
+
+const DEFAULT_ISSUE = "I don't understand that.";
+
+export default class Interpreter {
+	action: ActionFn | null; // The final command to run
+	expectedCategories: Category[]; // command categories that might follow
+	expectedConstants: Constant[]; // constants that might follow
+	isExpectingVariable: boolean; // a variable might follow
+	isValid: boolean; // signals whether the command is understood
+	issue: string | null; // a statement for a command that isn't clear
+	player: Player; // represents the player state
+	scene: Scene; // represents the scene state
+	tokens: string[]; // user command split by spaces (tokens)
+	vars: string[]; // variables taken from commandf
+
+	constructor(player: Player, scene: Scene, command: string) {
+		this.action = null;
+		this.expectedCategories = ["action"];
+		this.expectedConstants = [];
+		this.isExpectingVariable = false;
+		this.isValid = true;
+		this.issue = null;
+		this.player = player;
+		this.scene = scene;
+		this.tokens = command.split(" ");
+		this.vars = [];
+	}
+
+	// If the intepreter can figure out what the user meant, execute runs the action
+	// function it found with whatever variables it found. If the interpreter could not
+	// interpret the user command, then it will respond with whatever the issue was.
+	execute(): string {
+		let response = DEFAULT_ISSUE;
+
+		if (this.isValid && this.action) {
+			response = this.action(this.player, this.scene, ...this.vars);
+		} else if (this.issue) {
+			response = this.issue;
+		}
+
+		return response;
+	}
+
+	// interpret tries to figure out what the user wants. If it can, it sets as
+	// action function for this object and whatever variables it finds. If it
+	// cannot, then it sets the issue.
+	interpret() {
+		for (const token of this.tokens) {
+			if (!this.isValid) break;
+
+			this.interpretToken(token);
+		}
+	}
+
+	private interpretToken(token: string) {
+		const term = Interpreter.lookupTerm(token); // is the token a term?
+
+		if (!term) {
+			// if not...
+			if (this.isExpectingVariable) {
+				// ...are we expecting a variable?
+				this.processVariable(token); // add variable
+			} else {
+				// ...or...
+				this.stopParsing(); // this isn't a valid command.
+			}
+		} else if (
+			this.expectedConstants.find(({ constant }) => constant === token)
+		) {
+			// if this matches an expected constant string...
+			this.processTerm(term);
+		} else if (this.expectedCategories.includes(term.category)) {
+			// ...or category
+			this.processTerm(term);
+		} else {
+			this.stopParsing(); // ...this is not a valid command
+		}
+	}
+
+	private processTerm(term: ActionTerm | Term): void {
+		if ("action" in term) {
+			this.addAction(term);
+		}
+
+		this.expectedCategories = term.precedesCategories;
+		this.expectedConstants = term.precedesConstants;
+		this.isExpectingVariable = term.canPrecedeVariable;
+	}
+
+	private processVariable(token: string) {
+		if (this.vars.length >= 2) {
+			this.stopParsing();
+		} else {
+			this.vars.unshift(token.toLocaleLowerCase());
+		}
+	}
+
+	private addAction(term: ActionTerm) {
+		// make sure we expected an action and process it if it was
+		if (this.expectedCategories.includes("action")) {
+			this.action = term.action;
+		} else {
+			this.stopParsing();
+		}
+	}
+
+	private stopParsing(issue = DEFAULT_ISSUE) {
+		this.issue = issue;
+		this.isValid = false;
+	}
+
+	private static lookupTerm(token: string): Term | null {
+		if (token in terms) {
+			return terms[token];
+		} else {
+			return null;
+		}
+	}
+}

Player.ts

@@ -1,33 +1,37 @@
-import { Item } from "./data/data.ts";
-import User from "./User.ts";
+/**
+ * Player.ts contains the Player class. It represents the player's state as the game
+ * goes on.
+ */
 
-export default class Player {
-	#items: Item[];
-	#user: User;
+import { Effect, Item, EntityProperties } from "./types.ts";
+import Entity from "./Entity.ts";
 
-	constructor(items?: Item[]) {
-		this.#items = items || [];
-		this.#user = new User();
+// The Player is a type of "entity" which is a generic object that can have effects
+// and items.
+export default class Player extends Entity<EntityProperties> {
+	constructor(items: Item[] = [], _effects: Effect<EntityProperties>[] = []) {
+		super({ items }, { effects: [] });
 	}
 
-	drop(item: Item) {
-		this.#items.push(item);
+	// inventory is a getter that returns the items in the user's inventory
+	get inventory(): Item[] | null {
+		return this._properties.items || null;
 	}
 
-	async inventory() {
-		const vowels = ["a", "e", "i", "o", "u"];
-		const description = this.#items
-			.map(({ name }, i) => {
-				let anItem = `${vowels.includes(name[0]) ? "an" : "a"} ${name}`;
+	// look returns a string that describes the players inventory
+	look(): string {
+		const description = super.description(this._properties.items || []);
 
-				if (i + 1 === this.#items.length) {
-					anItem = `and ${anItem}`;
+		if (description) {
+			return `You have ${description}`;
+		} else {
+			return "You have nothing.";
+		}
 	}
 
-				return anItem;
-			})
-			.join(", ");
-
-		await this.#user.tell(`You have ${description}.`);
+	// put takes an item and puts it into the players inventory
+	put(item: Item) {
+		if (!this._properties.items) this._properties.items = [];
+		this._properties.items.push(item);
 	}
 }

Scene.ts

@@ -1,35 +1,109 @@
-import { GameData, Item } from "./data/data.ts";
-import User from "./User.ts";
+/**
+ * Scene.ts contains the Scene class. It represents the state of the current scene.
+ */
 
-export default class Scene {
-	#items: Item[];
-	#user: User;
+import type { Exit, StoryScene, Item, SceneProperties } from "./types.ts";
+import Entity from "./Entity.ts";
 
-	constructor(gameData: GameData) {
-		this.#items = gameData.items;
-		this.#user = new User();
+// The Scene is a type of "Entity" which is a generic object that can have effects
+// and items.
+export default class Scene extends Entity<SceneProperties> {
+	protected _map: StoryScene<SceneProperties>[];
+	protected _identifier: string;
+
+	constructor(identifier: string, map: StoryScene<SceneProperties>[]) {
+		const scene = Scene.getScene(identifier, map);
+
+		if (!scene) {
+			throw new Error("cannot find scene!");
 		}
 
-	async look() {
-		const description = this.#items
-			.map(({ description }) => description)
-			.join(" ");
+		const { properties, conditions } = scene;
 
-		await this.#user.tell(description);
+		super(properties, conditions);
+
+		this._identifier = identifier;
+		this._map = map;
 	}
 
-	async take(target: string): Promise<Item | null> {
-		const idx = this.#items.findIndex(({ name }) => name === target);
+	changeScene(identifier: string) {
+		const scene = Scene.getScene(identifier, this._map);
 
+		if (scene) {
+			const { conditions, properties } = scene;
+
+			this.properties = properties;
+			this.conditions = conditions;
+		}
+	}
+
+	// get removes an items from the scene and returns it
+	get(target: string): Item | null {
+		const { items = [] } = this._properties;
+		const idx = items.findIndex(({ name }) => name === target);
+
+		// if we found an index for the given item
 		if (idx >= 0) {
-			const item = this.#items[idx];
-			this.#items.splice(idx, 1);
-
-			await this.#user.tell("Taken.");
+			const item = items[idx];
+			items.splice(idx, 1);
 
 			return item;
 		}
 
+		// if an item wasn't found and returned, return null
 		return null;
 	}
+
+	// look returns a string that describes the scene and the items in it
+	look(activePlayerEffects: string[], activeSceneEffects: string[]): string {
+		// the properties of the scene after effects have been examined an applied
+		const properties = this.applyEffects(
+			activePlayerEffects,
+			activeSceneEffects
+		);
+
+		// description of the items in the scene
+		const itemsDescription = super.description(properties.items || []);
+		const { description, exits = [] } = properties; // description of the room
+
+		// will be a string that includes both the room and item descriptions
+		let fullDescription = description || "Nothing to see here...";
+
+		const exitsDescription = Scene.describeExits(exits);
+
+		fullDescription +=
+			exitsDescription && `\n\nThere is ${exitsDescription}.`;
+
+		// if there is a description of the items...
+		if (itemsDescription) {
+			fullDescription += `\n\nThere is ${itemsDescription}`;
+		}
+
+		return fullDescription;
+	}
+
+	// Turn exits into a string description with an "and" separating the last two items
+	private static describeExits(exits: Exit[]) {
+		const exitDescriptions = exits.map(({ description }) => description);
+		if (exits.length === 0) {
+			return "";
+		} else if (exits.length === 1) {
+			return exitDescriptions[0];
+		} else {
+			const lastExit = exitDescriptions[exitDescriptions.length - 1];
+			const restExits = exitDescriptions.slice(
+				0,
+				exitDescriptions.length - 1
+			);
+			return `${restExits.join(", ")} and ${lastExit}`;
+		}
+	}
+
+	// This needs to be static so that we can use it in the constructor
+	private static getScene(
+		identifier: string,
+		map: StoryScene<SceneProperties>[]
+	): StoryScene<SceneProperties> | null {
+		return map.find((scene) => scene.identifier === identifier) || null;
+	}
 }

User.ts

@@ -1,15 +1,14 @@
+/**
+ * User.ts export the User class that provides methods for communicating with the player
+ */
+
 const DEFAULT_PROMPT = ">";
 
 export default class User {
 	#prompt: string;
-	#out: (text: string) => Promise<void>;
 
-	constructor() {
-		this.#prompt = DEFAULT_PROMPT;
-		this.#out = async (text: string) => {
-			const data = new TextEncoder().encode(text);
-			await Deno.stdout.write(data);
-		};
+	constructor(prompt: string = DEFAULT_PROMPT) {
+		this.#prompt = prompt;
 	}
 
 	ask(question: string): string {
@@ -23,6 +22,11 @@ export default class User {
 	}
 
 	async tell(statement: string): Promise<void> {
-		await this.#out(`\n${statement}\n`);
+		await this.out(`\n${statement}\n`);
+	}
+
+	private async out(text: string): Promise<void> {
+		const data = new TextEncoder().encode(text);
+		await Deno.stdout.write(data);
 	}
 }

data/data.ts

@@ -1,10 +0,0 @@
-export * from "./rooms.ts";
-
-export interface Item {
-	name: string;
-	description: string;
-}
-
-export interface GameData {
-	items: Item[];
-}

data/rooms.ts

@@ -1,3 +1,127 @@
-export const hall = {
-	items: [{ name: "flashlight", description: "There is a flashlight." }],
+/**
+ * rooms.ts represents the game data that drives the story. It should contain all of the
+ * elements that are needed to tell the story. Although it is currently written in
+ * TypeScript, it should only contain objects which could just as easily be JSON. The
+ * reason to make this file TypeScript for now is that it lets the TypeScript compiler
+ * tell us if we have made errors. Later, we should add a file parser that can figure out
+ * if there are errors in a JSON file and load it if there are not.
+ */
+
+import { StoryScene, SceneProperties } from "../types.ts";
+
+const game: { map: StoryScene<SceneProperties>[] } = {
+	map: [
+		{
+			identifier: "hall",
+			// properties represent the base, default state for this room.
+			properties: {
+				// The room is called "Hall"
+				name: "Hall",
+
+				// This is what the user will see if the look while in the room.
+				description: "It's very dark",
+				// This array lists all the items that are in the room.
+				items: [
+					// flashlight is an item that can be used. When it is, it applies an effect
+					// to the room that it's being used in.
+					{
+						name: "flashlight",
+						actions: {
+							// Use is the name of the ActionTerm that will be used here. If a
+							// player types "use flashlight" then this describes what should
+							// happen.
+							use: {
+								// These are the pieces of information that the engine needs to
+								// exectue a use action of type "applyEffect."
+								args: {
+									// This is the name of the effect. It is defined in this file
+									// and should correspond to some condition on the player or
+									// scene.
+									effect: "flashlight-on",
+
+									// can be "player" or "scene"
+									applyTo: "player",
+
+									// When it's used, this is what should be reported back to
+									// the user.
+									result: "The flashlight is on.",
+
+									// If the effect is reversed—unapplied—this is what should be
+									// reported back to the user.
+									reverseResult: "The flashlight is off.",
+
+									// This indicates that the effect can be unapplied after it
+									// is applied. This should be done by calling use again after
+									// it has already been called and the effect has been applied.
+									reversible: true,
+								},
+								// applyEffect means that when this item is used, it will apply
+								// an effect to either the player or the scene.
+								type: "applyEffect",
+							},
+						},
+					},
+				],
+				// exit to the north leads to the Hall Closet
+				exits: [
+					{
+						description: "a closet to the north",
+						direction: "north",
+						scene: "hall closet",
+					},
+					{
+						description: "a dining room to the south",
+						direction: "south",
+						scene: "dining room",
+					},
+				],
+			},
+			// conditions are a set of conditions that, when met, will make alterations to the
+			// player or the scene.
+			conditions: {
+				// effects are conditions that revolve around effects that have been applied to
+				// either the player or the scene.
+				effects: [
+					{
+						// This is the name of the effect that, if present, will make alterations
+						// to the scene.
+						name: "flashlight-on",
+
+						// properties describes the properties on the scene that will be altered
+						// by the presents of this effect.
+						properties: {
+							// description means that, when this effect is applied, i.e. when
+							// the flashlight is on, the description will change to the one
+							// written here.
+							description:
+								"You are standing in a big hall. There's lots of nooks, " +
+								"crannies, and room for general testing. Aw yeah... sweet testing!",
+						},
+
+						// source indicates where the effect should be applied. In this case, the
+						// effect should be applied to the player.
+						source: "player",
+					},
+				],
+			},
+		},
+		{
+			identifier: "hall closet",
+			properties: {
+				name: "Hall Closet",
+				description:
+					"It's a closet. You feel a little silly just standing in it.",
+				items: [
+					{
+						name: "raincoat",
+					},
+				],
+			},
+			conditions: {
+				effects: [],
+			},
+		},
+	],
 };
+
+export default game;

main.ts

@@ -1,66 +1,64 @@
-import User from "./User.ts";
-import Scene from "./Scene.ts";
+import Interpreter from "./Interpreter.ts";
 import Player from "./Player.ts";
-import { hall } from "./data/data.ts";
-import parseCommand from "./parseCommand.ts";
+import Scene from "./Scene.ts";
+import User from "./User.ts";
+import game from "./data/rooms.ts";
 
 async function main() {
-	const user = new User();
-	const scene = new Scene(hall);
-	const player = new Player();
-	const question = "";
-	let running = true;
-	let statement = "";
+	const user = new User(); // for communication with the user
+	const scene = new Scene("hall", game.map); // the room that player is in.
+	const player = new Player(); // the players current state
+	let running = true; // running flag for the game loop. Stops on false.
+	let statement = ""; // holds a statement for the user.
 
 	while (running) {
-		const prompts = `${statement}${question}`;
+		const prompts = `${statement}\n`;
 		const answer = await user.ask(prompts);
 
-		const { action, target } = parseCommand(answer);
-
-		statement = "";
-
-		switch (action) {
+		// User commands that are not player moves, but are about running the
+		// game itself (e.g. loading, saving, quitting) may need to get handled
+		// here.
+		switch (answer) {
+			// kills the game loop.
 			case "quit":
 				running = quit(user);
-				break;
-
-			case "look":
-				await scene.look();
-				break;
-
-			case "take":
-				await pickUpItem(scene, player, target);
-				break;
-
-			case "inventory":
-				await player.inventory();
+				statement = "";
 				break;
 
 			default:
-				statement = "I didn't understand that.\n";
-
+				// Game moves require more complicated commands, this switch is
+				// not adequate to handle them. Here we will pass them off to
+				// other functions to parse.
+				statement = interpretUserCommand(player, scene, answer);
 				break;
 		}
 	}
 }
 
+// Takes player and scene state and a command. Tries to figure out what the
+// user is trying to do and, if it can, it executes the command.
+function interpretUserCommand(
+	player: Player,
+	scene: Scene,
+	command: string
+): string {
+	const interpreter = new Interpreter(player, scene, command);
+	interpreter.interpret();
+	return interpreter.execute();
+}
+
+// When it returns false, the loop should be stopped. When it returns true,
+// the loop should continue.
 function quit(user: User): boolean {
 	const confirmQuit = user.ask("Are you sure you want to quit?\n");
 
-	if (confirmQuit[0] === "y") {
+	// Anything that starts with a "y" or a "Y" is considered an affirmative
+	// response.
+	if (confirmQuit[0].toLocaleLowerCase() === "y") {
 		return false;
 	}
 
 	return true;
 }
 
-async function pickUpItem(scene: Scene, player: Player, target: string) {
-	const item = await scene.take(target);
-
-	if (item !== null) {
-		player.drop(item);
-	}
-}
-
 main();

parseCommand.ts

@@ -1,5 +0,0 @@
-export default function parseCommand(command: string) {
-	const [action, target] = command.toLocaleLowerCase().split(" ");
-
-	return { action, target };
-}

terms/Term.ts

@@ -0,0 +1,43 @@
+/**
+ * Term.ts contains the type information for parsing user sentences.
+ */
+
+import Player from "../Player.ts";
+import Scene from "../Scene.ts";
+
+// ActionFn is the signature for functions that contain the logic associated with verbs
+// the player uses.
+export type ActionFn = (
+	player: Player,
+	scene: Scene,
+	target?: string, // the target is the target of the action.
+	object?: string // the object is an item that is used with the action on the target.
+) => string; // should return a description of the result for the player.
+
+// Constant is a specific word that the engine knows about. If the player uses a word
+// that is not a Constant, then it should be a variable that represents something that
+// is defined in the writer's story files.
+export interface Constant {
+	constant: string;
+	category: Category;
+}
+
+// Category is a union of the different kinds of Constants.
+export type Category = "action" | "direction" | "position" | "interaction";
+
+// Term describes a known word that the player can use. It tells the parse what kinds of
+// words can come after it and what it means.
+export interface Term {
+	precedesCategories: Category[]; // categories of Constant that can follow this Term
+	precedesConstants: Constant[]; // Constants that can follow this Term
+	category: Category; // the category of this Terms Constant is part of
+	constant: string; // the name of the Constant that this Term describes
+	canPrecedeVariable: boolean; // indicates that what follows might be a variable
+}
+
+// ActionTerm extends Term, adding the function needed to execute a verb. It also
+// guarentees that the category is "action".
+export interface ActionTerm extends Term {
+	action: ActionFn;
+	category: "action";
+}

terms/actions.ts

@@ -0,0 +1,157 @@
+/**
+ * actions.ts contains the logic for various ActionTerms. Exported should match the
+ * signature for "ActionFn," which is:
+ *
+ * (player: Player, scene: Scene, target?: string, object?: string) => string
+ *
+ * The returned string should be a message for the user that describes the result of the
+ * action.
+ */
+
+import type Scene from "../Scene.ts";
+import type Player from "../Player.ts";
+import type { ApplyEffectArgs, ItemAction } from "../types.ts";
+
+const ITEM_MISSING = "I can't find that.";
+const ITEM_UNUSABLE = "I don't know how to use that.";
+const ITEM_ALREADY_USED = "I already did that.";
+const DIRECTION_INACCESSIBLE = "I can't go that way.";
+
+// look is what happens when a player uses the "look" verb. It gets a description of the
+// scene.
+export function look(player: Player, scene: Scene): string {
+	return scene.look(player.activeEffects, scene.activeEffects);
+}
+
+// pickUpItem is what happens when a player uses the "take" verb. It finds an item in the
+// scene and, if it's there, moves it into the players inventory.
+export function pickUpItem(
+	player: Player,
+	scene: Scene,
+	target?: string
+): string {
+	if (!target) {
+		return "What do you want me to get?";
+	}
+
+	// remove the item from the scene.
+	const item = scene.get(target);
+
+	// The item was found in the scene...
+	if (item !== null) {
+		player.put(item); // put it in the players inventory.
+
+		return "Taken.";
+	}
+
+	return ITEM_MISSING;
+}
+
+// checkInventory is what happens when a player uses the "inventory" verb. It gets the
+// description for what's in the player's inventory.
+export function checkInventory(player: Player): string {
+	return player.look();
+}
+
+// move changes the scene around the player
+export function move(_player: Player, scene: Scene, target?: string) {
+	if (!target) {
+		return "Where do you want me to go?";
+	}
+
+	const { exits } = scene.properties;
+
+	if (!exits) {
+		return DIRECTION_INACCESSIBLE;
+	}
+
+	const exit = exits.find(
+		({ direction }) => direction === target.toLocaleLowerCase()
+	);
+
+	if (!exit) {
+		return DIRECTION_INACCESSIBLE;
+	}
+
+	const { scene: identifier } = exit;
+	scene.changeScene(identifier);
+
+	const { description, name } = scene.properties;
+
+	return `${name}\n${description}`;
+}
+
+// use is what happens when a player uses the "use" verb. Different items do different
+// things when they are used, so the function needs to figure out what kind of usage an
+// item is written for, and then execute that logic with whatever arguments the
+// storywriter has provided for that item.
+export function use(player: Player, scene: Scene, target?: string): string {
+	if (!target) {
+		return "What do you want to use?";
+	}
+
+	const { inventory } = player;
+
+	// if there is no inventory...
+	if (inventory === null) {
+		return ITEM_MISSING;
+	}
+
+	// search the inventory for the named item
+	const item = inventory.find((i) => i.name === target);
+
+	if (item === undefined) {
+		return ITEM_MISSING;
+	}
+
+	const itemAction = item?.actions?.["use"];
+
+	// The item doesn't have action defined for "use", so it cannot be used.
+	if (itemAction === undefined) {
+		return ITEM_UNUSABLE;
+	}
+
+	return executeItemAction(player, scene, itemAction);
+}
+
+// executeItemAction figures out what kind of action an item supports and then executes
+// the logic for it.
+function executeItemAction(
+	player: Player,
+	scene: Scene,
+	{ args, type }: ItemAction
+): string {
+	switch (type) {
+		case "applyEffect":
+			return applyEffect(player, scene, args as ApplyEffectArgs);
+		default:
+			return ITEM_UNUSABLE;
+	}
+}
+
+// applyEffect contains the logic for applying an effect to the player or the scene.
+function applyEffect(
+	player: Player,
+	scene: Scene,
+	{ applyTo, effect, reverseResult, reversible, result }: ApplyEffectArgs
+): string {
+	const target = applyTo === "player" ? player : scene;
+
+	// if true, effect is already present.
+	const isApplied = target.hasActiveEffect(effect);
+
+	if (!reversible && isApplied) {
+		// effect is present already and can't be reversed
+		return ITEM_ALREADY_USED;
+	}
+
+	if (isApplied) {
+		// effect is present...
+		target.removeEffect(effect);
+		return reverseResult || result;
+	} else {
+		// effect is not present...
+		target.addEffect(effect);
+		return result;
+	}
+}

terms/terms.ts

@@ -0,0 +1,60 @@
+/**
+ * terms.ts contains the Terms that describe the Constants that the player can use.
+ * See Terms.ts for more information about what these terms can be.
+ */
+
+import * as actions from "./actions.ts";
+import { ActionTerm } from "./Term.ts";
+
+const inventory: ActionTerm = {
+	action: actions.checkInventory,
+	canPrecedeVariable: false,
+	category: "action",
+	constant: "inventory",
+	precedesCategories: [],
+	precedesConstants: [],
+};
+
+const look: ActionTerm = {
+	action: actions.look,
+	canPrecedeVariable: false,
+	category: "action",
+	constant: "look",
+	precedesCategories: [],
+	precedesConstants: [],
+};
+
+const move: ActionTerm = {
+	action: actions.move,
+	canPrecedeVariable: true,
+	category: "action",
+	constant: "go",
+	precedesCategories: [],
+	precedesConstants: [],
+};
+
+const take: ActionTerm = {
+	action: actions.pickUpItem,
+	canPrecedeVariable: true,
+	category: "action",
+	constant: "take",
+	precedesCategories: [],
+	precedesConstants: [],
+};
+
+const use: ActionTerm = {
+	action: actions.use,
+	canPrecedeVariable: true,
+	category: "action",
+	constant: "use",
+	precedesCategories: [],
+	precedesConstants: [],
+};
+
+export const terms = {
+	[inventory.constant]: inventory,
+	[look.constant]: look,
+	[move.constant]: move,
+	[take.constant]: take,
+	[use.constant]: use,
+};

types.ts

@@ -0,0 +1,90 @@
+// ActionArgs is a union of all the different argument types that can be used with an
+// action
+export type ActionArgs = ApplyEffectArgs;
+
+// ApplyEffectArgs are the arguments required to apply effects to a player or scene
+export interface ApplyEffectArgs extends Args {
+	effect: string;
+	applyTo: "player" | "scene";
+	reversible: boolean;
+	reverseResult?: string;
+}
+
+// ApplyEffectItemAction describes the apply effect action
+export interface ApplyEffectItemAction extends ItemAction {
+	type: "applyEffect";
+	args: ApplyEffectArgs;
+}
+
+// Args is a base interface for the various argument interfaces
+export interface Args {
+	result: string;
+}
+
+// Conditions contains story elements that can be conditionally applied to the scene or
+// player
+export interface Conditions<T extends EntityProperties> {
+	effects: Effect<T>[];
+}
+
+// Direction is union that contains the different directions a player can go
+export type Direction =
+	| "north"
+	| "northeast"
+	| "east"
+	| "southest"
+	| "south"
+	| "southwest"
+	| "west"
+	| "northwest"
+	| "up"
+	| "down";
+
+// Effect represents some effect that may be applied to a scene or player
+export interface Effect<T extends EntityProperties> {
+	name: string;
+	properties: T;
+	source: "player" | "scene"; // where the effect is applied
+}
+
+// Exit describes the exits that the user can take to go to another scene
+export interface Exit {
+	description: string;
+	direction: Direction;
+	scene: string;
+}
+
+// Item represents some item either in the scene or in the player's inventory
+export interface Item {
+	name: string;
+	actions?: { [name: string]: ItemAction };
+}
+
+// ItemActionType is a union of all the types of action that can be used
+export type ItemActionType = "applyEffect";
+
+// ItemAction represents an action that can be taken by an item
+export interface ItemAction {
+	type: ItemActionType;
+	args: ActionArgs;
+}
+
+// SceneProperties are the properties (in addition to the EntityProperties) that are
+// needed by the scene
+export interface SceneProperties extends EntityProperties {
+	description?: string;
+	exits?: Exit[];
+	name?: string;
+}
+
+// StoryScene holds both the conditions and properties for a scene.
+export interface StoryScene<T extends EntityProperties> {
+	identifier: string;
+	conditions: Conditions<T>;
+	properties: SceneProperties;
+}
+
+// EntityProperties is a base interface for the properties that a user or scene might have
+export interface EntityProperties {
+	items?: Item[];
+}