Document opaque types

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

1 files changed, 17 insertions, 0 deletions

diff --git a/src/content/chapter5_advanced_features/lesson06_externals/code.gleam b/src/content/chapter5_advanced_features/lesson06_externals/code.gleam
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index 0000000..1101b82
--- /dev/null
+++ b/src/content/chapter5_advanced_features/lesson06_externals/code.gleam
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+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())
+}