Mutable references

5 files changed, 50 insertions, 1 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1ce8036..be86f89 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,7 +3,8 @@
 ## Unreleased
 
 - The `javascript` module was created with the `TypeOf`, and `Symbol` types, and
-  `find_symbol`, and `type_of` functions.
+  `find_symbol`, `make_reference`, `set_reference`, `update_reference`,
+  `dereference` and `type_of` functions.
 - The `javascript/array` module was created with the `Array(element)` type and
   `to_list`, `from_list`, `map`, `fold`, `fold_right`, `get`, and `length`
   functions.
diff --git a/README.md b/README.md
index 1875e5c..bdd1887 100644
--- a/README.md
+++ b/README.md
@@ -5,5 +5,7 @@ Extra code for when running on JavaScript
 ## Features
 
 - Arrays
+- Mutable references
+- Promises
 - Symbols
 - `typeof`
diff --git a/src/ffi.js b/src/ffi.js
index d3ee8ce..d7b6bf9 100644
--- a/src/ffi.js
+++ b/src/ffi.js
@@ -98,3 +98,23 @@ export function map_promise(promise, fn) {
 export function rescue(promise, fn) {
   return promise.catch((error) => fn(error));
 }
+
+class Reference {
+  constructor(value) {
+    this.value = value;
+  }
+}
+
+export function dereference(reference) {
+  return reference.value;
+}
+
+export function make_reference(value) {
+  return new Reference(value);
+}
+
+export function set_reference(ref, value) {
+  let previous = ref.value;
+  ref.value = value;
+  return previous;
+}
diff --git a/src/gleam/javascript.gleam b/src/gleam/javascript.gleam
index 590dece..f4af2ed 100644
--- a/src/gleam/javascript.gleam
+++ b/src/gleam/javascript.gleam
@@ -17,3 +17,21 @@ pub external fn type_of(value) -> TypeOf =
 
 pub external fn get_symbol(String) -> Symbol =
   "../ffi.js" "get_symbol"
+
+pub external type Reference(value)
+
+pub external fn dereference(Reference(a)) -> a =
+  "../ffi.js" "dereference"
+
+pub external fn set_reference(Reference(a), a) -> a =
+  "../ffi.js" "set_reference"
+
+pub external fn make_reference(a) -> Reference(a) =
+  "../ffi.js" "make_reference"
+
+// returns the old value
+pub fn update_reference(ref: Reference(a), f: fn(a) -> a) -> a {
+  let value = dereference(ref)
+  set_reference(ref, f(value))
+  value
+}
diff --git a/test/gleam/javascript_test.gleam b/test/gleam/javascript_test.gleam
index 81477d8..4b0fc9e 100644
--- a/test/gleam/javascript_test.gleam
+++ b/test/gleam/javascript_test.gleam
@@ -23,3 +23,11 @@ pub fn find_symbol_test() {
   assert True = javascript.get_symbol("Gleam") == javascript.get_symbol("Gleam")
   assert False = javascript.get_symbol("Gleam") == javascript.get_symbol("Lua")
 }
+
+pub fn reference_test() {
+  let ref = javascript.make_reference(1)
+  assert 1 = javascript.update_reference(ref, fn(a) { a + 1 })
+  assert 2 = javascript.dereference(ref)
+  assert 2 = javascript.set_reference(ref, 3)
+  assert 3 = javascript.dereference(ref)
+}