From 62d0cebb97b1257aeada5f386e99c9e288acd82d Mon Sep 17 00:00:00 2001
From: Amaras <amaras@vivaldi.net>
Date: Fri, 13 Nov 2020 00:08:29 +0100
Subject: [PATCH] Forward Euler method, version 2.2: as functional as I could
 make it (for now)

---
 .../code/coconut/euler.coco                     | 17 +++++++++--------
 .../forward_euler_method.md                     |  2 ++
 2 files changed, 11 insertions(+), 8 deletions(-)

diff --git a/contents/forward_euler_method/code/coconut/euler.coco b/contents/forward_euler_method/code/coconut/euler.coco
index 2ffe29647..7297e9c51 100644
--- a/contents/forward_euler_method/code/coconut/euler.coco
+++ b/contents/forward_euler_method/code/coconut/euler.coco
@@ -1,18 +1,19 @@
 import math
 
 def forward_euler(time_step, n):
-    y = 1
-    for _ in range(n):
-        yield y
-        y *= (1 - 3 * time_step)
+    factors = [1] + [1 - 3 * time_step] * (n-1)
+    # We want all the cumulative values, thus the use of scan
+    return scan((*), factors)
+
 
 
 def check(result, threshold, time_step):
     approx = True
-    for i, y in enumerate(result):
-        solution = math.exp(-3 * i * time_step)
-        if not math.isclose(y, solution, abs_tol=threshold):
-            print(y, solution)
+    # A scan object has a len if the underlying iterable has a len
+    solution = range(len(result)) |> map$(i -> math.exp(-3*i*time_step))
+    for y, sol in zip(result, solution):
+        if not math.isclose(y, sol, abs_tol=threshold):
+            print(y, sol)
             approx = False
     return approx
 
diff --git a/contents/forward_euler_method/forward_euler_method.md b/contents/forward_euler_method/forward_euler_method.md
index dfb185431..3547deb1c 100644
--- a/contents/forward_euler_method/forward_euler_method.md
+++ b/contents/forward_euler_method/forward_euler_method.md
@@ -146,6 +146,8 @@ Full code for the visualization follows:
 [import, lang:"nim"](code/nim/forwardeuler.nim)
 {% sample lang="lisp" %}
 [import, lang="lisp"](code/clisp/euler.lisp)
+{%sample lang="coco" %}
+[import, lang:"coconut"](code/coconut/euler.coco)
 {% endmethod %}
 
 <script>