diff --git a/src/Err.ts b/src/Err.ts
index 9f130ba..0b0fa4c 100644
--- a/src/Err.ts
+++ b/src/Err.ts
@@ -40,7 +40,7 @@ class Err<E, A> extends Result<E, A> {
 
   public assign<K extends string, B>(
     k: K,
-    other: Result<E, B> | ((a: A) => Result<E, B>),
+    other: Result<E, B> | ((a: A) => Result<E, B>)
   ): Result<E, A & { [k in K]: B }> {
     return new Err<E, A & { [k in K]: B }>(this.error);
   }
@@ -48,6 +48,11 @@ class Err<E, A> extends Result<E, A> {
   public do(fn: (a: A) => void): Result<E, A> {
     return new Err<E, A>(this.error);
   }
+
+  public elseDo(fn: (err: E) => void): Result<E, A> {
+    fn(this.error);
+    return this;
+  }
 }
 
 /**
diff --git a/src/Ok.ts b/src/Ok.ts
index 1cdc512..d380b4c 100644
--- a/src/Ok.ts
+++ b/src/Ok.ts
@@ -34,17 +34,9 @@ class Ok<E, A> extends Result<E, A> {
     return matcher.Ok(this.value);
   }
 
-  public ap<B, C>(result: Result<E, B>): Result<E, C> {
-    if (typeof this.value !== 'function') {
-      throw new TypeError(`'ap' can only be applied to functions: ${JSON.stringify(this.value)}`);
-    }
-
-    return result.map(this.value);
-  }
-
   public assign<K extends string, B>(
     k: K,
-    other: Result<E, B> | ((a: A) => Result<E, B>),
+    other: Result<E, B> | ((a: A) => Result<E, B>)
   ): Result<E, A & { [k in K]: B }> {
     const result = other instanceof Result ? other : other(this.value);
     return result.map<A & { [k in K]: B }>(b => ({
@@ -57,6 +49,10 @@ class Ok<E, A> extends Result<E, A> {
     fn(this.value);
     return new Ok<E, A>(this.value);
   }
+
+  public elseDo(fn: (err: E) => void): Result<E, A> {
+    return this;
+  }
 }
 
 /**
diff --git a/src/Result.ts b/src/Result.ts
index 71ad41c..822a5e5 100644
--- a/src/Result.ts
+++ b/src/Result.ts
@@ -48,13 +48,6 @@ abstract class Result<E, A> {
    */
   public abstract cata<B>(matcher: Catamorphism<E, A, B>): B;
 
-  /**
-   * Apply the value of a successful result to a function result. If this
-   * result is an Err, nothing happens. If this result is NOT a function,
-   * a TypeError is raised.
-   */
-  public abstract ap<B, C>(result: Result<E, B>): Result<E, C>;
-
   /**
    * Encapsulates a common pattern of needing to build up an Object from
    * a series of Result values. This is often solved by nesting `andThen` calls
@@ -69,7 +62,7 @@ abstract class Result<E, A> {
    */
   public abstract assign<K extends string, B>(
     k: K,
-    other: Result<E, B> | ((a: A) => Result<E, B>),
+    other: Result<E, B> | ((a: A) => Result<E, B>)
   ): Result<E, A & { [k in K]: B }>;
 
   /**
@@ -90,6 +83,22 @@ abstract class Result<E, A> {
    *
    */
   public abstract do(fn: (a: A) => void): Result<E, A>;
+
+  /**
+   * Inject a side-effectual operation into a chain of Result computations.
+   *
+   * The side effect only runs when there is an error (Err).
+   *
+   * The value will remain unchanged during the `elseDo` operation.
+   *
+   *    ok({})
+   *      .assign('foo', ok(42))
+   *      .assign('bar', ok('hello'))
+   *      .do(scope => console.log('Scope: ', JSON.stringify(scope)))
+   *      .map(doSomethingElse)
+   *
+   */
+  public abstract elseDo(fn: (err: E) => void): Result<E, A>;
 }
 
 export default Result;
diff --git a/tests/Err.test.ts b/tests/Err.test.ts
index e6703d0..39986a4 100644
--- a/tests/Err.test.ts
+++ b/tests/Err.test.ts
@@ -3,7 +3,10 @@ import { err, ok } from './../src/index';
 
 test('Err.getOrElse', t => {
   const result = err<string, string>('foo');
-  t.equal('bar', result.getOrElse(() => 'bar'));
+  t.equal(
+    'bar',
+    result.getOrElse(() => 'bar')
+  );
   t.end();
 });
 
@@ -35,37 +38,44 @@ test('Err.cata', t => {
 });
 
 test('Err.mapError', t => {
-  err('foo').mapError(m => m.toUpperCase()).cata({
-    Err: err => t.equal('FOO', err),
-    Ok: v => t.fail('should not have passed'),
-  });
+  err('foo')
+    .mapError(m => m.toUpperCase())
+    .cata({
+      Err: err => t.equal('FOO', err),
+      Ok: v => t.fail('should not have passed'),
+    });
   t.end();
 });
 
-test('Err.ap', t => {
-  const fn = (a: string) => (b: number) => ({ a, b });
-
-  err('oops!').ap(ok('hi')).ap(ok(42)).cata({
-    Err: m => t.pass(`Failed as expected: ${m}`),
-    Ok: v => t.fail(`Should have failed: ${JSON.stringify(v)}`),
-  });
-
+test('Err.assign', t => {
+  ok({})
+    .assign('x', ok(42))
+    .assign('y', () => err('ooops!'))
+    .cata({
+      Err: m => t.pass(`Failed as expected: ${m}`),
+      Ok: v => t.fail(`Should have failed: ${JSON.stringify(v)}`),
+    });
   t.end();
 });
 
-test('Err.assign', t => {
-  ok({}).assign('x', ok(42)).assign('y', () => err('ooops!')).cata({
-    Err: m => t.pass(`Failed as expected: ${m}`),
-    Ok: v => t.fail(`Should have failed: ${JSON.stringify(v)}`),
-  });
+test('Err.do', t => {
+  err('oops!')
+    .do(v => t.fail(`Should NOT run side effect: ${v}`))
+    .cata({
+      Err: m => t.pass(`Should be an error: ${m}`),
+      Ok: v => t.fail(`Should not succeeded: ${JSON.stringify(v)}`),
+    });
+
   t.end();
 });
 
-test('Err.do', t => {
-  err('oops!').do(v => t.fail(`Should NOT run side effect: ${v}`)).cata({
-    Err: m => t.pass(`Should be an error: ${m}`),
-    Ok: v => t.fail(`Should not succeeded: ${JSON.stringify(v)}`),
-  });
+test('Err.elseDo', t => {
+  err('oops!')
+    .elseDo(v => t.pass(`Error side effect ran: ${v}`))
+    .cata({
+      Err: m => t.pass(`Should be an error: ${m}`),
+      Ok: v => t.fail(`Should not succeed: ${JSON.stringify(v)}`),
+    });
 
   t.end();
 });
diff --git a/tests/Ok.test.ts b/tests/Ok.test.ts
index 3eda344..93cbcf7 100644
--- a/tests/Ok.test.ts
+++ b/tests/Ok.test.ts
@@ -1,15 +1,21 @@
 import * as test from 'tape';
-import { err, ok } from './../src/index';
+import { ok } from './../src/index';
 
 test('Ok.getOrElse', t => {
   const result = ok<string, string>('foo');
-  t.equal('foo', result.getOrElse(() => 'bar'));
+  t.equal(
+    'foo',
+    result.getOrElse(() => 'bar')
+  );
   t.end();
 });
 
 test('OK.map', t => {
   const result = ok<string, string>('foo');
-  t.equal('FOO', result.map(s => s.toUpperCase()).getOrElse(() => ''));
+  t.equal(
+    'FOO',
+    result.map(s => s.toUpperCase()).getOrElse(() => '')
+  );
   t.end();
 });
 
@@ -35,43 +41,46 @@ test('Ok.cata', t => {
 });
 
 test('Ok.mapError', t => {
-  ok<string, string>('foo').mapError(m => m.toUpperCase()).cata({
-    Err: err => t.fail('should have passed'),
-    Ok: v => t.pass('Worked!'),
-  });
+  ok<string, string>('foo')
+    .mapError(m => m.toUpperCase())
+    .cata({
+      Err: err => t.fail('should have passed'),
+      Ok: v => t.pass('Worked!'),
+    });
 
   t.end();
 });
 
-test('Ok.ap', t => {
-  const fn = (a: string) => (b: number) => ({ a, b });
-
-  ok(fn).ap(ok('hi')).ap(ok(42)).cata({
-    Err: m => t.fail(`Should have passed: ${m}`),
-    Ok: v => t.pass(`Worked!: ${JSON.stringify(v)}`),
-  });
-
-  ok(fn).ap(ok('hi')).ap(err('oops!')).cata({
-    Err: m => t.pass(`ap failed: ${m}`),
-    Ok: v => t.fail(`should have failed: ${v}`),
-  });
-
+test('Ok.assign', t => {
+  ok({})
+    .assign('x', ok(42))
+    .assign('y', v => ok(String(v.x + 8)))
+    .cata({
+      Err: m => t.fail(`Should have succeeded: ${m}`),
+      Ok: v => t.deepEqual(v, { x: 42, y: '50' }),
+    });
   t.end();
 });
 
-test('Ok.assign', t => {
-  ok({}).assign('x', ok(42)).assign('y', v => ok(String(v.x + 8))).cata({
-    Err: m => t.fail(`Should have succeeded: ${m}`),
-    Ok: v => t.deepEqual(v, { x: 42, y: '50' }),
-  });
+test('Ok.do', t => {
+  ok({})
+    .assign('x', ok(42))
+    .do(scope => t.pass(`'do' should run: ${JSON.stringify(scope)}`))
+    .cata({
+      Err: m => t.fail(`Should have succeeded: ${m}`),
+      Ok: v => t.deepEqual(v, { x: 42 }),
+    });
+
   t.end();
 });
 
-test('Ok.do', t => {
-  ok({}).assign('x', ok(42)).do(scope => t.pass(`'do' should run: ${JSON.stringify(scope)}`)).cata({
-    Err: m => t.fail(`Should have succeeded: ${m}`),
-    Ok: v => t.deepEqual(v, { x: 42 }),
-  });
+test('Ok.elseDo', t => {
+  ok({ x: 42 })
+    .elseDo(err => t.fail(`Error side effect should not run: ${err}`))
+    .cata({
+      Err: m => t.fail(`Should have succeeded: ${m}`),
+      Ok: v => t.deepEqual(v, { x: 42 }),
+    });
 
   t.end();
 });