From 938f49fc7835501192ccc7db7c1ab3174f050b34 Mon Sep 17 00:00:00 2001
From: Louis Pilfold <louis@lpil.uk>
Date: Tue, 20 Feb 2024 12:20:16 +0000
Subject: Let assert

---
 .../lesson05_externals/code.gleam                  | 17 +++++++++++++++
 .../lesson05_externals/en.html                     | 25 ++++++++++++++++++++++
 2 files changed, 42 insertions(+)
 create mode 100644 src/content/chapter5_advanced_features/lesson05_externals/code.gleam
 create mode 100644 src/content/chapter5_advanced_features/lesson05_externals/en.html

(limited to 'src/content/chapter5_advanced_features/lesson05_externals')

diff --git a/src/content/chapter5_advanced_features/lesson05_externals/code.gleam b/src/content/chapter5_advanced_features/lesson05_externals/code.gleam
new file mode 100644
index 0000000..1101b82
--- /dev/null
+++ b/src/content/chapter5_advanced_features/lesson05_externals/code.gleam
@@ -0,0 +1,17 @@
+import gleam/io
+
+// A type with no Gleam constructors
+pub type DateTime
+
+// An external function that creates an instance of the type
+@external(javascript, "./my_package_ffi.mjs", "now")
+pub fn now() -> DateTime
+
+// The `now` function in `./my_package_ffi.mjs` looks like this:
+// export function now() {
+//   return new Date();
+// }
+
+pub fn main() {
+  io.debug(now())
+}
diff --git a/src/content/chapter5_advanced_features/lesson05_externals/en.html b/src/content/chapter5_advanced_features/lesson05_externals/en.html
new file mode 100644
index 0000000..8815fa7
--- /dev/null
+++ b/src/content/chapter5_advanced_features/lesson05_externals/en.html
@@ -0,0 +1,25 @@
+<p>
+  Sometimes in our projects we want to use code written in other languages, most
+  commonly Erlang and JavaScript, depending on which runtime is being used.
+  Gleam's <em>external functions</em> and <em>external types</em> allow us to
+  import and use this non-Gleam code.
+</p>
+<p>
+  An external type is one that has no constructors. Gleam doesn't know what
+  shape it has or how to create one, it only knows that it exists.
+</p>
+<p>
+  An external function is one that has the <code>@external</code> attribute on
+  it, directing the compiler to use the specified module function as the
+  implementation, instead of Gleam code.
+</p>
+<p>
+  The compiler can't tell the types of functions written in other languages, so
+  when the external attribute is given type annotations must be provided. Gleam
+  trusts that the type given is correct so an inaccurate type annotation can
+  result in unexpected behaviour and crashes at runtime. Be careful!
+</p>
+<p>
+  External functions are useful but should be used sparingly. Prefer to write
+  Gleam code where possible.
+</p>
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