diff --git a/bin/canadyarn.js b/bin/canadyarn.js
new file mode 100644
index 0000000..93d757c
--- /dev/null
+++ b/bin/canadyarn.js
@@ -0,0 +1,2 @@
+#!/usr/bin/env node
+require("../dist/canadyarn");
diff --git a/src/canadyarn.ts b/src/canadyarn.ts
new file mode 100644
index 0000000..93f0289
--- /dev/null
+++ b/src/canadyarn.ts
@@ -0,0 +1,92 @@
+import {parse} from "@yarnpkg/lockfile";
+import {bold as cb} from "chalk";
+import {existsSync, readFileSync, statSync} from "fs";
+import {join} from "path";
+import {checkYarnLock} from "./check-yarn-lock";
+import {ERROR_UNKNOWN_EXIT_CODE, ExecutionError, MultipleVersionError} from "./error";
+import type {DependencyName, MultipleVersionErrorMessage, ParseResult} from "./types";
+
+function execute(): void {
+    const checkSingleVersionDependencies: DependencyName[] = (function () {
+        const filepath = join(process.cwd(), "package.json");
+        if (!(existsSync(filepath) && statSync(filepath).isFile())) {
+            throw new ExecutionError("cannot find package.json");
+        }
+        const packageJson = JSON.parse(readFileSync(filepath, {encoding: "utf8"}));
+        if (!("checkSingleVersionDependencies" in packageJson && Array.isArray(packageJson.checkSingleVersionDependencies))) {
+            throw new ExecutionError("package.json#checkSingleVersionDependencies must be an array");
+        }
+        return packageJson.checkSingleVersionDependencies;
+    })();
+    const lockfile: ParseResult = (function () {
+        const filepath = join(process.cwd(), "yarn.lock");
+        if (!(existsSync(filepath) && statSync(filepath).isFile())) {
+            throw new Error("[check-yarn-lock]: cannot find yarn.lock");
+        }
+        return parse(readFileSync(filepath, {encoding: "utf8"}));
+    })();
+    const errorMessages: MultipleVersionErrorMessage[] = checkYarnLock(checkSingleVersionDependencies, lockfile);
+    if (errorMessages.length > 0) {
+        throw new MultipleVersionError(errorMessages);
+    }
+}
+
+try {
+    execute();
+} catch (error) {
+    const buffer: string[] = [];
+    let processExitCode: number;
+    buffer.push(`${cb.yellowBright("[check-yarn-lock-single-version]")}: `);
+    if (ExecutionError.isInstance(error)) {
+        const {exitCode, message} = error;
+        processExitCode = exitCode;
+        buffer.push("an execution error occurred ", cb.whiteBright(":("));
+        buffer.push("\n");
+        buffer.push(" > ", cb.whiteBright(message));
+        buffer.push("\n");
+    } else if (MultipleVersionError.isInstance(error)) {
+        const {exitCode, errorMessages} = error;
+        processExitCode = exitCode;
+        buffer.push(cb.whiteBright(`found ${cb.red(errorMessages.length)} modules with non-singular version resolutions`));
+        buffer.push("\n");
+        errorMessages.forEach(errorMessage => {
+            buffer.push(errorMessage);
+            buffer.push("\n");
+        });
+        buffer.push("\n");
+        buffer.push(Array(40).fill("-").join(""));
+        buffer.push("\n");
+        buffer.push("\n");
+        buffer.push(`The above list ${cb.whiteBright(`might contain ${cb.redBright("false")} positives`)} `);
+        buffer.push("if the resolution of dependency version range is removed after upgrading, ");
+        buffer.push(`but yarn does not remove the resolution from ${cb.whiteBright("yarn.lock")}.`);
+        buffer.push("\n");
+        buffer.push("\n");
+        buffer.push(`Try running "${cb.blueBright("$ yarn install")}" again in the workspace root to update ${cb.whiteBright("yarn.lock")}.`);
+        buffer.push("\n");
+        buffer.push("\n");
+        buffer.push(`Run "${cb.blueBright("$ yarn why <PACKAGE_NAME>")}" to find out where the duplicate dependencies come from.`);
+        buffer.push("\n");
+        buffer.push("\n");
+        buffer.push(`If you don't know what to do, `);
+        buffer.push(cb.whiteBright("ask the "), "🍁", cb.whiteBright("Canadian"), "🍁", cb.whiteBright(" guy"), " (he knows how to fix this) ", cb.whiteBright(":)"));
+        buffer.push("\n");
+    } else {
+        processExitCode = ERROR_UNKNOWN_EXIT_CODE;
+        buffer.push(cb.yellowBright("[check-yarn-lock-single-version]"));
+        buffer.push(": an unknown error occurred");
+        buffer.push("\n");
+        let message: string;
+        if (typeof error === "string") {
+            message = error;
+        } else if (typeof error === "object" && "message" in error && typeof error.message === "string") {
+            message = error.message;
+        }
+        if (message) {
+            buffer.push(" > ", cb.whiteBright(error.mess));
+            buffer.push("\n");
+        }
+    }
+    console.warn(buffer.join(""));
+    process.exit(processExitCode);
+}
diff --git a/src/check-yarn-lock.ts b/src/check-yarn-lock.ts
new file mode 100644
index 0000000..1e2b637
--- /dev/null
+++ b/src/check-yarn-lock.ts
@@ -0,0 +1,42 @@
+import {bold as cb} from "chalk";
+import {ExecutionError} from "./error";
+import type {CheckableDependencies, DependencyName, MultipleVersionErrorMessage, ParseResult} from "./types";
+
+export function checkYarnLock(checkSingleVersionDependencies: ReadonlyArray<DependencyName>, lockfile: Readonly<ParseResult>): MultipleVersionErrorMessage[] {
+    const checkableDeps: CheckableDependencies = (function () {
+        const checkableDeps = {};
+        checkSingleVersionDependencies.forEach(dep => {
+            checkableDeps[dep] = {};
+        });
+        return checkableDeps;
+    })();
+
+    if (lockfile.type !== "success") {
+        throw new ExecutionError("failed to parse yarn.lock");
+    }
+
+    const lockfileEntries = Object.entries(lockfile.object);
+
+    lockfileEntries.forEach(([qualifiedName, resolution]) => {
+        const dep = checkSingleVersionDependencies.find(dep => qualifiedName.startsWith(`${dep}@`));
+        if (dep !== undefined) {
+            checkableDeps[dep][resolution.version] = checkableDeps[dep][resolution.version] || [];
+            checkableDeps[dep][resolution.version].push(resolution); // only need resolution.version for now
+        }
+    });
+
+    const errorMessages: MultipleVersionErrorMessage[] = [];
+
+    Object.entries(checkableDeps).forEach(([dep, versionMap]) => {
+        const versions: string[] = Object.keys(versionMap);
+        if (versions.length !== 1) {
+            const buffer: string[] = [];
+            buffer.push(" > ", `${cb.red(versions.length)} versions of ${cb.blueBright(`"${dep}"`)} detected`);
+            buffer.push("\n");
+            buffer.push("   ", versions.join(", "));
+            errorMessages.push(buffer.join(""));
+        }
+    });
+
+    return errorMessages;
+}
diff --git a/src/error.ts b/src/error.ts
new file mode 100644
index 0000000..a9d0d36
--- /dev/null
+++ b/src/error.ts
@@ -0,0 +1,33 @@
+import {MultipleVersionErrorMessage} from "./types";
+
+export const ERROR_EXECUTION_EXIT_CODE = 1;
+export const ERROR_MULTIPLE_VERSION_EXIT_CODE = 2;
+export const ERROR_UNKNOWN_EXIT_CODE = 3;
+
+export class ExecutionError extends Error {
+    static isInstance(error: unknown): error is ExecutionError {
+        return typeof error === "object" && "__type__" in error && error["__type__"] === "ExecutionError";
+    }
+
+    readonly exitCode = ERROR_EXECUTION_EXIT_CODE;
+    private readonly __type__ = "ExecutionError";
+
+    constructor(message?: string) {
+        super(message);
+    }
+}
+
+export class MultipleVersionError extends Error {
+    static isInstance(error: unknown): error is MultipleVersionError {
+        return typeof error === "object" && "__type__" in error && error["__type__"] === "MultipleVersionError";
+    }
+
+    readonly exitCode = ERROR_MULTIPLE_VERSION_EXIT_CODE;
+    readonly errorMessages: MultipleVersionErrorMessage[];
+    private readonly __type__ = "MultipleVersionError";
+
+    constructor(errorMessages: MultipleVersionErrorMessage[]) {
+        super();
+        this.errorMessages = errorMessages;
+    }
+}
diff --git a/src/types.ts b/src/types.ts
new file mode 100644
index 0000000..e755101
--- /dev/null
+++ b/src/types.ts
@@ -0,0 +1,21 @@
+export type DependencyName = ({__type__: "DependencyName"} & string) | string;
+
+export type MultipleVersionErrorMessage = ({__type__: "MultipleVersionErrorMessage"} & string) | string;
+
+export type QualifiedName = ({__type__: "QualifiedName"} & string) | string;
+
+export type Version = ({__type__: "Version"} & string) | string;
+
+export type DependencyResolution = {
+    version: string;
+    resolved: string;
+    integrity: string;
+    dependencies?: Record<string, string>;
+};
+
+export type ParseResult = {
+    type: "merge" | "success" | "conflict";
+    object: Record<QualifiedName, DependencyResolution>;
+};
+
+export type CheckableDependencies = Record<DependencyName, Record<Version, DependencyResolution[] | undefined>>;