diff --git a/src/util/compat.js b/src/util/compat.js
new file mode 100644
index 0000000..5422985
--- /dev/null
+++ b/src/util/compat.js
@@ -0,0 +1,45 @@
+// @flow
+
+export opaque type Compatible<T> = [CompatInfo, T];
+type CompatInfo = {|
+  +type: string,
+  +version: string,
+|};
+
+export function toCompat<T>(compatInfo: CompatInfo, obj: T): Compatible<T> {
+  return [compatInfo, obj];
+}
+
+/**
+ * Load an object from compatibilized state created by `toCompat`.
+ * The object has an expected type and version, and may optionally have
+ * handler functions for transforming previous versions into a canonical state.
+ * If a handler is present for the current version, it will be applied.
+ * Throws an error if the compatibilized object is the wrong type, or if its version
+ * is not current and there was no handler for its version.
+ */
+export function fromCompat<T>(
+  expectedCompatInfo: CompatInfo,
+  obj: Compatible<any>,
+  handlers: ?{[version: string]: (any) => T}
+): T {
+  if (!Array.isArray(obj) || obj.length !== 2) {
+    throw new Error(
+      "Tried to load object that didn't have compatibility defined"
+    );
+  }
+  const {type, version} = obj[0];
+  let result = obj[1];
+
+  const {type: expectedType, version: expectedVersion} = expectedCompatInfo;
+  if (type !== expectedType) {
+    throw new Error(`Expected type to be ${expectedType} but got ${type}`);
+  }
+
+  if (handlers != null && handlers[version] != null) {
+    result = handlers[version](result);
+  } else if (version !== expectedVersion) {
+    throw new Error(`${type}: tried to load unsupported version ${version}`);
+  }
+  return result;
+}
diff --git a/src/util/compat.test.js b/src/util/compat.test.js
new file mode 100644
index 0000000..69be93b
--- /dev/null
+++ b/src/util/compat.test.js
@@ -0,0 +1,168 @@
+// @flow
+
+import {toCompat, fromCompat} from "./compat";
+import type {Compatible} from "./compat";
+
+describe("compat utilities", () => {
+  type FooV1 = {foo: number};
+  type FooV2 = {bar: {foo: number}};
+  const type = "Foo Plugin's Experiment";
+  const v1 = "v1";
+  const v2 = "v2";
+  function fooV1ToV2(x: FooV1): FooV2 {
+    return {bar: x};
+  }
+  const example = () => {
+    const dataV1: FooV1 = {foo: 1};
+    const dataV2: FooV2 = {bar: {foo: 1}};
+    const compatV1 = toCompat({type, version: v1}, dataV1);
+    const compatV2 = toCompat({type, version: v2}, dataV2);
+    return {dataV1, dataV2, compatV1, compatV2};
+  };
+
+  it("toCompat doesn't fails on primitives", () => {
+    expect(
+      fromCompat({type, version: v1}, toCompat({type, version: v1}, 3))
+    ).toBe(3);
+  });
+
+  it("toCompat -> fromCompat is identity", () => {
+    const {dataV1, compatV1} = example();
+    expect(fromCompat({type, version: v1}, compatV1)).toEqual(dataV1);
+  });
+
+  it("fromCompat fails if compatibility undefined", () => {
+    const dataV1: any = example().dataV1;
+    expect(() => fromCompat({type, version: v1}, dataV1)).toThrowError(
+      "didn't have compatibility defined"
+    );
+  });
+
+  it("fromCompat fails if type is wrong", () => {
+    const {compatV1} = example();
+    expect(() =>
+      fromCompat({type: "who is Foo?", version: v1}, compatV1)
+    ).toThrowError("Expected type");
+  });
+
+  it("fromCompat fails if version is wrong", () => {
+    const {compatV1} = example();
+    expect(() => fromCompat({type, version: v2}, compatV1)).toThrowError(
+      "unsupported version"
+    );
+  });
+
+  it("handlers can load older versions", () => {
+    const {compatV1, dataV2} = example();
+    const handlers = {[v1]: fooV1ToV2};
+    expect(fromCompat({type: type, version: v2}, compatV1, handlers)).toEqual(
+      dataV2
+    );
+  });
+
+  it("handlers activate even on current version", () => {
+    const {compatV1} = example();
+    const handlers = {
+      [v1]: () => ({
+        hello: "world",
+      }),
+    };
+    expect(fromCompat({type, version: v1}, compatV1, handlers)).toEqual({
+      hello: "world",
+    });
+  });
+
+  describe("composable versioning", () => {
+    class InnerV1 {
+      x: number;
+      constructor(x: number) {
+        this.x = x;
+      }
+      toJSON(): Compatible<FooV1> {
+        return toCompat({type: "inner", version: v1}, {foo: this.x});
+      }
+      static fromJSON(json): InnerV1 {
+        const from: FooV1 = fromCompat({type: "inner", version: v1}, json);
+        return new InnerV1(from.foo);
+      }
+    }
+
+    class InnerV2 {
+      x: number;
+      constructor(x: number) {
+        this.x = x;
+      }
+      toJSON(): Compatible<FooV2> {
+        return toCompat({type: "inner", version: v2}, {bar: {foo: this.x}});
+      }
+      static fromJSON(json): InnerV2 {
+        const from: FooV2 = fromCompat({type: "inner", version: v2}, json, {
+          [v1]: fooV1ToV2,
+        });
+        return new InnerV2(from.bar.foo);
+      }
+    }
+
+    class OuterV1 {
+      platypus: InnerV1 | InnerV2;
+      constructor(i: InnerV1 | InnerV2) {
+        this.platypus = i;
+      }
+      toJSON() {
+        return toCompat(
+          {type: "outer", version: v1},
+          {platypus: this.platypus.toJSON()}
+        );
+      }
+      fromJSON(json: any): OuterV1 {
+        return fromCompat({type: "outer", version: v1}, json, {
+          [v1]: function(x) {
+            return new OuterV1(InnerV2.fromJSON(x.platypus));
+          },
+        });
+      }
+    }
+
+    class OuterV2 {
+      // Naming this property "platypus" in the previous version was silly
+      inner: InnerV1 | InnerV2;
+      constructor(i: InnerV1 | InnerV2) {
+        this.inner = i;
+      }
+      toJSON() {
+        return toCompat(
+          {type: "outer", version: v2},
+          {inner: this.inner.toJSON()}
+        );
+      }
+      static fromJSON(json: any): OuterV2 {
+        return fromCompat({type: "outer", version: v2}, json, {
+          [v1]: function(x) {
+            return new OuterV2(InnerV2.fromJSON(x.platypus));
+          },
+          [v2]: function(x) {
+            return new OuterV2(InnerV2.fromJSON(x.inner));
+          },
+        });
+      }
+    }
+
+    const canonical = () => new OuterV2(new InnerV2(1));
+    it("loads OuterV1<InnerV1>", () => {
+      const json = new OuterV1(new InnerV1(1)).toJSON();
+      expect(OuterV2.fromJSON(json)).toEqual(canonical());
+    });
+    it("loads OuterV1<InnerV2>", () => {
+      const json = new OuterV1(new InnerV2(1)).toJSON();
+      expect(OuterV2.fromJSON(json)).toEqual(canonical());
+    });
+    it("loads OuterV2<InnerV1>", () => {
+      const json = new OuterV2(new InnerV1(1)).toJSON();
+      expect(OuterV2.fromJSON(json)).toEqual(canonical());
+    });
+    it("loads OuterV2<InnerV2>", () => {
+      const json = new OuterV2(new InnerV2(1)).toJSON();
+      expect(OuterV2.fromJSON(json)).toEqual(canonical());
+    });
+  });
+});