diff --git a/src/algebra/reader.js b/src/algebra/reader.js
index 16448b8..875a969 100644
--- a/src/algebra/reader.js
+++ b/src/algebra/reader.js
@@ -1,7 +1,7 @@
 import { id } from './fn.js'
 import { Algebra, Monad } from './index.js'
 
-/** @import { InferredMorphism } from './types.js' */
+/** @import { InferredMorphism, Morphism } from './types.js' */
 
 /**
  * @template T
@@ -12,9 +12,7 @@ import { Algebra, Monad } from './index.js'
 export class Reader extends Algebra(Monad) {
   #run
 
-  /**
-   * @param {InferredMorphism<T>} run 
-   */
+  /** @param {InferredMorphism<T>} run */
   constructor(run) {
     super()
 
@@ -51,6 +49,19 @@ export class Reader extends Algebra(Monad) {
     return new Reader(env => f(this.#run(env)).run(env))
   }
 
+  /**
+   * @template R
+   * @param {Reader<(v: T) => R>} other
+   * @returns {Reader<R>}
+   */
+  ap(other) {
+    return new Reader(
+      // FIXME: type errors
+      // @ts-ignore
+      env => other.run(env)(this.run(env))
+    )
+  }
+
   /**
    * @param {InferredMorphism<T>} f 
    * @returns {Reader<T>}