List functions

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diff --git a/lessons/src/lesson020_lists/code.gleam b/lessons/src/lesson020_lists/code.gleam
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+import gleam/io
+
+pub fn main() {
+  let ints = [1, 2, 3]
+
+  io.debug(ints)
+
+  // Immutably prepend
+  io.debug([-1, 0, ..ints])
+
+  // Uncomment this to see the error
+  // io.debug(["zero", ..ints])
+
+  // The original lists are unchanged
+  io.debug(ints)
+}
diff --git a/lessons/src/lesson020_lists/text.html b/lessons/src/lesson020_lists/text.html
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+<p>
+  Lists are ordered collections of values.
+</p>
+<p>
+  <code>List</code> is a generic type, having a type parameter
+  for the type of values it contains. A list of ints has the type
+  <code>List(Int)</code>, and a list of strings has the type
+  <code>List(String)</code>.
+</p>
+<p>
+  Lists are immutable single-linked lists, meaning they are very efficient to
+  add and remove elements from the front of the list.
+</p>
+<p>
+  Counting the length of a list or getting elements from other positions in the
+  list is expensive and rarely done. It is rare to write algorithms that index
+  into sequences in Gleam, but but when they are written a list is not the right
+  choice of data structure.
+</p>
diff --git a/lessons/src/lesson021_list_functions/code.gleam b/lessons/src/lesson021_list_functions/code.gleam
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+import gleam/io
+import gleam/list
+
+pub fn main() {
+  let ints = [0, 1, 2, 3, 4, 5]
+
+  let doubled = list.map(ints, fn(x) { x * 2 })
+  io.debug(doubled)
+
+  let even = list.filter(ints, fn(x) { x % 2 == 0 })
+  io.debug(even)
+
+  let total = list.fold(ints, from: 0, with: fn(count, e) { count + e })
+  io.debug(total)
+}
diff --git a/lessons/src/lesson021_list_functions/text.html b/lessons/src/lesson021_list_functions/text.html
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+<p>
+  The <a href="https://hexdocs.pm/gleam_stdlib/gleam/list.html"><code>gleam/list</code></a>
+  standard library module contains functions for working with lists. A Gleam
+  program will likely make heavy use of this module.
+</p>
+
+<p>
+  <a href="https://hexdocs.pm/gleam_stdlib/gleam/list.html#map"><code>map</code></a> 
+  makes a new list by running a function on each element in a list.
+</p>
+<p>
+  <a href="https://hexdocs.pm/gleam_stdlib/gleam/list.html#filter"><code>filter</code></a> 
+  makes a new list containing only the elements for which a function returns
+  true.
+</p>
+<p>
+  <a href="https://hexdocs.pm/gleam_stdlib/gleam/list.html#fold"><code>fold</code></a> 
+  combines all the elements in a list into a single value by running a function
+  left-to-right on each element, passing the result of the previous call to the
+  next call.
+</p>
+<p>
+  It's worth getting familiar with all the functions in this module when writing
+  Gleam code.
+</p>