Document opaque types

Closes https://github.com/gleam-lang/language-tour/issues/63

2 files changed, 31 insertions, 0 deletions

diff --git a/src/content/chapter5_advanced_features/lesson05_let_assert/code.gleam b/src/content/chapter5_advanced_features/lesson05_let_assert/code.gleam
new file mode 100644
index 0000000..2ba907a
--- /dev/null
+++ b/src/content/chapter5_advanced_features/lesson05_let_assert/code.gleam
@@ -0,0 +1,16 @@
+import gleam/io
+
+pub fn main() {
+  let a = unsafely_get_first_element([123])
+  io.debug(a)
+
+  let b = unsafely_get_first_element([])
+  io.debug(b)
+}
+
+pub fn unsafely_get_first_element(items: List(a)) -> a {
+  // This will panic if the list is empty.
+  // A regular `let` would not permit this partial pattern
+  let assert [first, ..] = items
+  first
+}
diff --git a/src/content/chapter5_advanced_features/lesson05_let_assert/en.html b/src/content/chapter5_advanced_features/lesson05_let_assert/en.html
new file mode 100644
index 0000000..29fd06c
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+++ b/src/content/chapter5_advanced_features/lesson05_let_assert/en.html
@@ -0,0 +1,15 @@
+<p>
+  <code>let assert</code> is the final way to intentionally crash your Gleam
+  program. It is similar to the <code>panic</code> keyword in that it crashes
+  when the program has reached a point that should never be reached.
+</p>
+<p>
+  <code>let assert</code> is similar to <code>let</code> in that it is a way to
+  assign values to variables, but it is different in that the pattern can be
+  <i>partial</i>. The pattern does not need to match every possible value of the
+  type being assigned.
+</p>
+<p>
+  Like <code>panic</code> this feature should be used sparingly, and likely not
+  at all in libraries.
+</p>