diff --git a/README.md b/README.md
index 4f9e195..5565b64 100644
--- a/README.md
+++ b/README.md
@@ -168,6 +168,15 @@ delete(key)
 get(key)
 ```
 
+```js
+// Fetch the value from a Map located at the corresponding key.
+// If the key is missing, the key is set to the result of function fun.
+// key: *
+// fun: Function () -> value
+// returns: value
+getOrCompute(key, fun)
+```
+
 ```js
 // Set a value on the Map stored at the `key` location.
 // key: *
diff --git a/src/ChainedMap.js b/src/ChainedMap.js
index 00822de..7777415 100644
--- a/src/ChainedMap.js
+++ b/src/ChainedMap.js
@@ -72,6 +72,13 @@ module.exports = class extends Chainable {
     return this.store.get(key);
   }
 
+  getOrCompute(key, fun) {
+    if (!this.has(key)) {
+      this.set(key, fun());
+    }
+    return this.get(key);
+  }
+
   has(key) {
     return this.store.has(key);
   }
diff --git a/src/Config.js b/src/Config.js
index 487061b..f7f4505 100644
--- a/src/Config.js
+++ b/src/Config.js
@@ -95,19 +95,11 @@ module.exports = class extends ChainedMap {
   }
 
   entry(name) {
-    if (!this.entryPoints.has(name)) {
-      this.entryPoints.set(name, new ChainedSet(this));
-    }
-
-    return this.entryPoints.get(name);
+    return this.entryPoints.getOrCompute(name, () => new ChainedSet(this));
   }
 
   plugin(name) {
-    if (!this.plugins.has(name)) {
-      this.plugins.set(name, new Plugin(this, name));
-    }
-
-    return this.plugins.get(name);
+    return this.plugins.getOrCompute(name, () => new Plugin(this, name));
   }
 
   toConfig() {
diff --git a/src/Module.js b/src/Module.js
index 3f247a6..0199322 100644
--- a/src/Module.js
+++ b/src/Module.js
@@ -10,19 +10,11 @@ module.exports = class extends ChainedMap {
   }
 
   defaultRule(name) {
-    if (!this.defaultRules.has(name)) {
-      this.defaultRules.set(name, new Rule(this, name));
-    }
-
-    return this.defaultRules.get(name);
+    return this.defaultRules.getOrCompute(name, () => new Rule(this, name));
   }
 
   rule(name) {
-    if (!this.rules.has(name)) {
-      this.rules.set(name, new Rule(this, name));
-    }
-
-    return this.rules.get(name);
+    return this.rules.getOrCompute(name, () => new Rule(this, name));
   }
 
   toConfig() {
diff --git a/src/Resolve.js b/src/Resolve.js
index ed94243..dc1a5a1 100644
--- a/src/Resolve.js
+++ b/src/Resolve.js
@@ -25,11 +25,7 @@ module.exports = class extends ChainedMap {
   }
 
   plugin(name) {
-    if (!this.plugins.has(name)) {
-      this.plugins.set(name, new Plugin(this, name));
-    }
-
-    return this.plugins.get(name);
+    return this.plugins.getOrCompute(name, () => new Plugin(this, name));
   }
 
   toConfig() {
diff --git a/src/Rule.js b/src/Rule.js
index f845961..45e1695 100644
--- a/src/Rule.js
+++ b/src/Rule.js
@@ -31,19 +31,11 @@ module.exports = class Rule extends ChainedMap {
   }
 
   use(name) {
-    if (!this.uses.has(name)) {
-      this.uses.set(name, new Use(this, name));
-    }
-
-    return this.uses.get(name);
+    return this.uses.getOrCompute(name, () => new Use(this, name));
   }
 
   oneOf(name) {
-    if (!this.oneOfs.has(name)) {
-      this.oneOfs.set(name, new Rule(this, name));
-    }
-
-    return this.oneOfs.get(name);
+    return this.oneOfs.getOrCompute(name, () => new Rule(this, name));
   }
 
   pre() {
diff --git a/test/ChainedMap.js b/test/ChainedMap.js
index 2ca69ae..07558a8 100644
--- a/test/ChainedMap.js
+++ b/test/ChainedMap.js
@@ -28,6 +28,14 @@ test('get', t => {
   t.is(map.get('a'), 'alpha');
 });
 
+test('getOrCompute', t => {
+  const map = new ChainedMap();
+
+  t.is(map.get('a'), undefined);
+  t.is(map.getOrCompute('a', () => 'alpha'), 'alpha');
+  t.is(map.get('a'), 'alpha');
+});
+
 test('clear', t => {
   const map = new ChainedMap();