From 50f3cff3d9146da49e037531e0790ada85bef6cd Mon Sep 17 00:00:00 2001
From: rowan <rowan@kitsu.cafe>
Date: Mon, 7 Apr 2025 02:30:49 -0500
Subject: [PATCH] List.count

---
 src/algebra/fn.js   |  7 ++++
 src/algebra/list.js | 79 ++++++++++++++++++++++++++++++++-------------
 2 files changed, 64 insertions(+), 22 deletions(-)

diff --git a/src/algebra/fn.js b/src/algebra/fn.js
index ddf626f..3a877d0 100644
--- a/src/algebra/fn.js
+++ b/src/algebra/fn.js
@@ -42,3 +42,10 @@ export const liftF = x => suspend(
   /** @type {InferredMorphism<T>} */(pure)
 )
 
+/**
+ * @param {number} a
+ * @param {number} b
+ * @returns {number}
+ */
+export const sum = (a, b) => a + b
+
diff --git a/src/algebra/list.js b/src/algebra/list.js
index 4b8c83c..171cbd7 100644
--- a/src/algebra/list.js
+++ b/src/algebra/list.js
@@ -1,5 +1,3 @@
-import { concat } from '../iter.js'
-import { liftF } from './fn.js'
 import { Pure, Suspend } from './free.js'
 import { AlgebraWithBase, Category, Comonad, Foldable, Monoid } from './index.js'
 
@@ -37,6 +35,18 @@ class ListPure extends AlgebraWithBase(Pure)(Category, Foldable, Monoid) {
   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
+  }
 }
 
 /**
@@ -45,24 +55,29 @@ class ListPure extends AlgebraWithBase(Pure)(Category, Foldable, Monoid) {
  */
 class ListSuspend extends AlgebraWithBase(Suspend)(Foldable, Monoid, Comonad) {
   /**
-   * @param {Iterator<T>} iter
+   * @param {T} head
+   * @param {() => List<T>} tail
    */
-  constructor(iter) {
-    const next = iter.next()
-    const value = /** @type {T} */ (next.value)
+  constructor(head, tail) {
+    super(
+      head,
+      /** @type {InferredMorphism<T>} */(tail)
+    )
+  }
 
-    const fn = /** @type {InferredMorphism<T>} */ (next.done ? List.empty : () => new ListSuspend(iter))
-    super(value, fn)
+
+  static empty() {
+    return List.empty()
   }
 
   /**
-   * @template {Iterable<T>} T
-   * @this {ListSuspend<Iterator<T>>}
-   * @param {Iterator<T>} value 
+   * @template T
+   * @this {ListSuspend<T>}
+   * @param {T} value 
    * @returns {ListSuspend<T>}
    */
   cons(value) {
-    return new ListSuspend(concat(value, this._value))
+    return new ListSuspend(value, () => this)
   }
 
   head() {
@@ -88,8 +103,13 @@ class ListSuspend extends AlgebraWithBase(Suspend)(Foldable, Monoid, Comonad) {
     return this.reduce(reduceArray, [])
   }
 
-  static empty() {
-    return List.empty()
+  count() {
+    return this.tail().count() + 1
+  }
+
+  /** @returns {this is Empty} */
+  isEmpty() {
+    return false
   }
 }
 
@@ -99,22 +119,37 @@ class ListSuspend extends AlgebraWithBase(Suspend)(Foldable, Monoid, Comonad) {
  */
 export class List {
   /**
-   * @template {Iterable<T>} T
+   * @template  T
    * @param {T} value
-   * @returns {ListSuspend<T>}
+   * @returns {List<T>}
    */
   static of(value) {
-    // @ts-ignore
-    return new ListSuspend(liftF(value))
+    return new ListSuspend(value, List.empty)
   }
 
   /**
    * @template T
-   * @param {Iterable<T>} iterator
-   * @returns {ListSuspend<T>}
+   * @param {Iterable<T>} iterable
+   * @returns {List<T>}
    */
-  static from(iterator) {
-    return new ListSuspend(Iterator.from(iterator))
+  static from(iterable) {
+    const iterator = Iterator.from(iterable)
+    return List.fromIterator(iterator)
+  }
+
+  /**
+   * @template T
+   * @param {Iterator<T>} iterator
+   * @returns {List<T>}
+   */
+  static fromIterator(iterator) {
+    const next = iterator.next()
+
+    if (next.done) {
+      return List.empty()
+    } else {
+      return new ListSuspend(next.value, () => List.fromIterator(iterator))
+    }
   }
 
   static empty() {