diff --git a/src/algebra/fn.js b/src/algebra/fn.js
index 3a877d0..75e3f0d 100644
--- a/src/algebra/fn.js
+++ b/src/algebra/fn.js
@@ -1,5 +1,6 @@
 import { curry } from '../curry.js'
-import { Suspend, suspend, pure } from './free.js'
+import { concat } from '../iter.js'
+import { Impure, pure, impure } from './free.js'
 
 /** @import {InferredMorphism, Morphism, ChainConstructor} from './types.js' */
 
@@ -32,12 +33,14 @@ export const kleisliCompose = curry(
   (f, g, x) => f(x).chain(g)
 )
 
+export const lift = (...args) => args
+
 /**
  * @template T
  * @param {T} x
- * @returns {Suspend<T>}
+ * @returns {Impure<T>}
  */
-export const liftF = x => suspend(
+export const liftF = x => impure(
   x,
   /** @type {InferredMorphism<T>} */(pure)
 )
@@ -47,5 +50,18 @@ export const liftF = x => suspend(
  * @param {number} b
  * @returns {number}
  */
-export const sum = (a, b) => a + b
+export const add = (a, b) => a + b
+
+/**
+ * @template T
+ * @param {T} x
+ * @returns {() => T}
+ */
+export const thunk = x => () => x
+
+export const prepend = (x, xs) => concat([x], xs)
+
+export const type = x => typeof x
+export const is = (ctr, x) => x.constructor === ctr
+
 
diff --git a/src/algebra/free.js b/src/algebra/free.js
index be54b1e..002ab0b 100644
--- a/src/algebra/free.js
+++ b/src/algebra/free.js
@@ -1,164 +1,163 @@
-import { liftF } from './fn.js'
-import { Algebra, BaseSet, Monad, Semigroupoid } from './index.js'
+import { Option } from './option.js'
+import { thunk } from './fn.js'
+/** @import { Morphism } from './types.js' */
 
-/** @import { InferredMorphism, Morphism } from './types.js' */
 
 /**
- * @template T
- * @extends BaseSet
- * @mixes Monad
+ * @template A
+ * @typedef {Pure<A> | Impure<A>} Free
  */
-export class Suspend extends Algebra(Semigroupoid, Monad) {
-  _value
-  #fn
+
+/** @template A */
+export class Pure {
+  #value
 
   /**
-   * @param {T} value 
-   * @param {InferredMorphism<T>} fn
+   * @param {A} value 
    */
-  constructor(value, fn) {
-    super()
-    this._value = value
-    this.#fn = fn
-  }
-
-  /** @returns {this is Pure<T>} */
-  isPure() { return false }
-
-  /** @returns {this is Suspend<T>} */
-  isSuspend() { return true }
-
-  /**
-   * @template U
-   * @param {Morphism<T, Free<U>>} f 
-   * @returns {Suspend<T>}
-   */
-  chain(f) {
-    return new Suspend(this._value, x => this.#fn(x).chain(f))
-  }
-
-  /**
-   * @template U
-   * @param {Morphism<T, U>} g
-   * @returns {Suspend<T>}
-   */
-  map(g) {
-    return new Suspend(this._value, x => this.#fn(x).map(g))
-  }
-
-  /**
-   * @template U
-   * @param {Morphism<T, U>} g
-   * @returns {Suspend<T>}
-   */
-  then(g) {
-    return this.map(g)
-  }
-
-  /**
-   * @template T
-   * @param {InferredMorphism<T>} other 
-   */
-  compose(other) {
-    return this.chain(() => other)
-  }
-
-  step() {
-    return this.#fn(this._value)
-  }
-
-  run() {
-    return this.step().run()
-  }
-}
-
-/** @template T */
-export class Pure extends Algebra(Monad) {
-  _value
-
-  /** @param {T} value */
   constructor(value) {
-    super()
-    this._value = value
+    this.#value = value
   }
 
-  /** @returns {this is Pure<T>} */
-  isPure() { return true }
-
-  /** @returns {this is Suspend<T>} */
-  isSuspend() { return false }
-
   /**
-   * @template U
-   * @param {Morphism<T, Pure<U>>} f 
-   * @returns {Pure<U>}
+   * @template {Free<B>} B
+   * @param {Morphism<A, B>} f
+   * @returns {B}
    */
   chain(f) {
-    return f(this._value)
+    console.log('Pure.chain', this.#value)
+    return f(this.#value)
   }
 
   /**
-   * @template U
-   * @param {Morphism<T, U>} f 
-   * @returns {Pure<U>}
+   * @template B
+   * @param {Morphism<A, B>} f
+   * @returns {Free<B>}
    */
   map(f) {
-    return this.chain(x => pure(f(x)))
+    console.log(`Pure.map ${f} ${this.#value}`)
+    // @ts-ignore
+    return pure(f(this.#value))
   }
 
   /**
-   * @template R
-   * @param {Morphism<T, R>} f 
-   * @returns {Pure<R>}
+   * @template B, C
+   * @param {Free<B>} b 
+   * @returns {Free<C>}
    */
-  then(f) {
-    return this.map(f)
+  ap(b) {
+    console.log('Pure.ap', b, this.#value)
+    return b.map(this.#value)
+  }
+
+  traverse(f, A) {
+    return A.of(this.map(f))
   }
 
   /**
-   * @template T
-   * @param {InferredMorphism<T>} other 
+   * @template B
+   * @param {(acc: B, value: A) => B} f 
+   * @param {B} init
+   * @returns {B}
    */
-  compose(other) {
-    return this.chain(() => pure(other))
+  reduce(f, init) {
+    console.log('Pure.reduce', init, this.#value)
+    return f(init, this.#value)
   }
 
-  step() {
-    return this._value
+}
+
+/**
+ * @template A
+ * @typedef {<B>(value: A) => Free<B>} Computation
+ */
+
+/** @template A */
+export class Impure {
+  #next
+
+  /**
+   * @param {() => Free<any>} next 
+   */
+  constructor(next) {
+    this.#next = next
   }
 
-  run() {
-    return this._value
+  /**
+   * @template {Free<B>} B
+   * @param {Morphism<A, B>} f
+   * @returns {B}
+   */
+  chain(f) {
+    console.log('Impure.chain')
+    // @ts-ignore
+    return new Impure(
+      // @ts-ignore
+      () => {
+        console.log(`Impure.chain<${f}>`, this.#next())
+        this.#next().chain(f)
+      }
+      //() => this.#next().chain(f)
+    )
+  }
+
+  /**
+   * @template B
+   * @param {Morphism<A, B>} f
+   * @returns {Free<B>}
+   */
+  map(f) {
+    console.log(`Impure.map ${f}`)
+    return new Impure(
+      // @ts-ignore
+      () => {
+        console.log(`Impure.map<${f}>`, this.#next())
+        return this.#next().map(f)
+      }
+      //() => this.#next().map(f)
+    )
+  }
+
+  /**
+   * @template B
+   * @param {Free<any>} b 
+   * @returns {Free<B>}
+   */
+  ap(b) {
+    console.log('Impure.ap', b)
+    return new Impure(
+      // @ts-ignore
+      () => b.map(this.#next())
+    )
+  }
+
+  traverse(f, A) {
+    return A.of(liftF(
+      this.#next().traverse(f, A)
+    ))
+  }
+
+  /**
+   * @template B
+   * @param {(acc: B, value: A) => B} f 
+   * @param {B} acc
+   * @returns {B}
+   */
+  reduce(f, acc) {
   }
 }
 
 /**
- * @template T
- * @type {Pure<T> | Suspend<T>} Free
+ * @template A
+ * @param {A} x
+ * @returns {Free<A, A>}
  */
-export class Free {
-  /**
-   * @template T
-   * @param {T} value
-   * @returns {Free<T>}
-   */
-  static of(value) {
-    return liftF(value)
-  }
-}
+export const pure = x => new Pure(x)
 
 /**
- * @template T
- * @param {T} value
- * @returns {Pure<T>}
+ * @template A, B
+ * @param {Computation<A, B>} f
  */
-export const pure = value => new Pure(value)
-
-/**
- * @template T
- * @param {T} value
- * @param {InferredMorphism<T>} f
- * @returns {Suspend<T>}
- */
-export const suspend = (value, f) => new Suspend(value, f)
+export const impure = f => new Impure(f)
+export const liftF = effect => impure(thunk(effect))
 
diff --git a/src/algebra/io.js b/src/algebra/io.js
index a5e0782..3fa4971 100644
--- a/src/algebra/io.js
+++ b/src/algebra/io.js
@@ -7,7 +7,7 @@ export class IO extends Algebra(Monad) {
   _effect
 
   /**
-   * @type {T} effect
+   * @param {T} effect
    */
   constructor(effect) {
     super()
@@ -38,12 +38,11 @@ export class IO extends Algebra(Monad) {
 
   /**
    * @template {Fn} U
-   * @this {IO<T>}
    * @param {IO<Morphism<T, U>>} other
-   * @returns {IO<U>}
+   * @returns {IO<T>}
    */
   ap(other) {
-    return IO.of(() => other.run()(this.run()))
+    return /** @type {IO<T>} */ (IO.of((() => other.run()(this.run()))))
   }
 
   run() {
diff --git a/src/algebra/list.js b/src/algebra/list.js
index bc84dae..ec6da93 100644
--- a/src/algebra/list.js
+++ b/src/algebra/list.js
@@ -1,112 +1,110 @@
 import { Pure, Suspend } from './free.js'
-import { AlgebraWithBase, Category, Comonad, Foldable, Monoid } from './index.js'
+import { Algebra, AlgebraWithBase, Comonad, Foldable, Monoid, Semigroup } from './index.js'
 
 /** @import { InferredMorphism, Morphism } from './types.js' */
 
-const Nil = Symbol('Nil')
-
-/**
- * @template T
- * @extends {Pure<T>}
- */
-class ListPure extends AlgebraWithBase(Pure)(Category, Foldable, Monoid) {
-  static empty() {
-    return List.empty()
-  }
-
-  static id() {
-    return List.empty()
-  }
-
-  head() {
-    return this._value
-  }
-
-  tail() {
-    return Empty
-  }
-
-  /**
-   * @template U
-   * @this {ListPure<T>}
-   * @param {Morphism<T, ListPure<U>>} f 
-   * @returns {ListPure<U>}
-   */
-  chain(f) {
-    return f(this.head())
-  }
-
-  /**
-   * @template U
-   * @this {ListPure<T>}
-   * @param {Morphism<T, U>} f 
-   * @returns {ListPure<U>}
-   */
-  map(f) {
-    return new ListPure(f(this._value))
-  }
-
-
-  /**
-   * @template U
-   * @param {(acc: U, value: T) => U} f
-   * @param {U} init
-   * @returns {U}
-   */
-  reduce(f, init) {
-    return f(init, this._value)
-  }
-
-  count() {
-    return this.isEmpty() ? 0 : 1
-  }
-
-  /**
-   * @this {List<any>}
-   * @returns {this is Empty}
-   */
-  isEmpty() {
-    return this === Empty
-  }
-}
-
-/**
- * @template T
- * @extends {Suspend<T>}
- */
-class ListSuspend extends AlgebraWithBase(Suspend)(Foldable, Monoid, Comonad) {
-  /**
-   * @param {T} head
-   * @param {() => List<T>} tail
-   */
-  constructor(head, tail) {
-    super(
-      head,
-      /** @type {InferredMorphism<T>} */(tail)
-    )
-  }
-
-
-  static empty() {
-    return List.empty()
+class Empty extends Algebra(Semigroup, Foldable, Monoid, Comonad) {
+  constructor() {
+    super()
   }
 
   /**
    * @template T
-   * @this {ListSuspend<T>}
+   * @this {Empty}
    * @param {T} value 
-   * @returns {ListSuspend<T>}
+   * @returns {List<T>}
    */
-  cons(value) {
-    return new ListSuspend(value, () => this)
+  prepend(value) {
+    return new Element(value, () => this)
   }
 
   head() {
-    return this._value
+    return empty
   }
 
   tail() {
-    return this.step()
+    return empty
+  }
+
+  /**
+   * @template T
+   * @param {List<Element<T>>} other 
+   * @returns {List<Element<T>>}
+   */
+  concat(other) {
+    return other
+  }
+
+  chain(f) {
+    return this
+  }
+
+  map(f) {
+    return this
+  }
+
+}
+
+/**
+ * @template T
+ */
+class Element extends Algebra(Semigroup, Foldable, Monoid, Comonad) {
+  #head
+  #tail
+
+  /**
+   * @param {T} head
+   * @param {() => List<T>} tail
+   */
+  constructor(head, tail = List.empty) {
+    super()
+    this.#head = head
+    this.#tail = tail
+  }
+
+  /**
+   * @template T
+   * @this {Empty}
+   * @param {T} value 
+   * @returns {List<T>}
+   */
+  prepend(value) {
+    return new Element(value, () => this)
+  }
+
+  head() {
+    return this.#head
+  }
+
+  tail() {
+    return this.#tail()
+  }
+
+  /**
+   * @param {List<T>} other 
+   * @returns {List<T>}
+   */
+  concat(other) {
+    if (this.isEmpty()) {
+      return other
+    } else {
+      return new Element(
+        this.head(),
+        () => this.tail().concat(other)
+      )
+    }
+  }
+
+  /**
+   * @template U
+   * @param {Morphism<T, List<U>>} f 
+   * @returns {List<U>}
+   */
+  chain(f) {
+    return new Element(
+      f(this.head()),
+      this.tail().chain(f)
+    )
   }
 
   /**
@@ -136,7 +134,7 @@ class ListSuspend extends AlgebraWithBase(Suspend)(Foldable, Monoid, Comonad) {
 
 /**
  * @template T
- * @type {ListPure<T> | ListSuspend<T>} List
+ * @type {Element<T> | Empty}
  */
 export class List {
   /**
@@ -145,7 +143,7 @@ export class List {
    * @returns {List<T>}
    */
   static of(value) {
-    return new ListSuspend(value, List.empty)
+    return empty.prepend(value)
   }
 
   /**
@@ -169,16 +167,26 @@ export class List {
     if (next.done) {
       return List.empty()
     } else {
-      return new ListSuspend(next.value, () => List.fromIterator(iterator))
+      return new Element(next.value, () => List.fromIterator(iterator))
     }
   }
 
+  /**
+   * @template T
+   * @param {T} head 
+   * @param {List<T>} tail 
+   * @returns {List<T>}
+   */
+  static cons(head, tail) {
+    return new Element(head, () => tail)
+  }
+
   static empty() {
     return Empty
   }
 }
 
-const Empty = new ListPure(Nil)
+const empty = new Empty()
 
 /**
  * @template T
@@ -188,3 +196,7 @@ const Empty = new ListPure(Nil)
  */
 const reduceArray = (acc, value) => acc.concat(value)
 
+const arr = Array.from({ length: 10 })
+const list = List.from(arr)
+list.map(x => x * 2)
+