From e57c18156cf11aaeae89e66828811c9fcc42fa81 Mon Sep 17 00:00:00 2001
From: Louis Pilfold <louis@lpil.uk>
Date: Sun, 29 Aug 2021 18:44:55 +0100
Subject: Mutable references

---
 src/ffi.js                 | 20 ++++++++++++++++++++
 src/gleam/javascript.gleam | 18 ++++++++++++++++++
 2 files changed, 38 insertions(+)

(limited to 'src')

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
+}
-- 
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