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/lesson08_external_gleam_fallbacks/code.gleam b/src/content/chapter5_advanced_features/lesson08_external_gleam_fallbacks/code.gleam
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+++ b/src/content/chapter5_advanced_features/lesson08_external_gleam_fallbacks/code.gleam
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+import gleam/io
+
+@external(erlang, "lists", "reverse")
+pub fn reverse_list(items: List(e)) -> List(e) {
+  tail_recursive_reverse(items, [])
+}
+
+fn tail_recursive_reverse(items: List(e), reversed: List(e)) -> List(e) {
+  case items {
+    [] -> reversed
+    [first, ..rest] -> tail_recursive_reverse(rest, [first, ..reversed])
+  }
+}
+
+pub fn main() {
+  io.debug(reverse_list([1, 2, 3, 4, 5]))
+  io.debug(reverse_list(["a", "b", "c", "d", "e"]))
+}
diff --git a/src/content/chapter5_advanced_features/lesson08_external_gleam_fallbacks/en.html b/src/content/chapter5_advanced_features/lesson08_external_gleam_fallbacks/en.html
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+++ b/src/content/chapter5_advanced_features/lesson08_external_gleam_fallbacks/en.html
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+<p>
+  It's possible for a function to have both a Gleam implementation and an
+  external implementation. If there exists an external implementation for the
+  currently compiled-for target then it will be used, otherwise the Gleam
+  implementation is used.
+</p>
+<p>
+  This may be useful if you have a function that can be implemented in Gleam,
+  but there is an optimised implementation that can be used for one target. For
+  example, the Erlang virtual machine has a built-in list reverse function that
+  is implemented in native code. The code here uses this implementation when
+  running on Erlang, as it is then available.
+</p>