From fe9aa89e6d595599a8c33b50cb206cb71b4d8a19 Mon Sep 17 00:00:00 2001
From: Louis Pilfold <louis@lpil.uk>
Date: Sat, 11 Sep 2021 15:00:27 +0100
Subject: JS decode field for maps

---
 src/gleam_stdlib.js           | 13 ++++++++---
 test/gleam/dynamic_test.gleam | 50 +++++++++++++++++++++----------------------
 2 files changed, 34 insertions(+), 29 deletions(-)

diff --git a/src/gleam_stdlib.js b/src/gleam_stdlib.js
index b7173c2..158a571 100644
--- a/src/gleam_stdlib.js
+++ b/src/gleam_stdlib.js
@@ -556,7 +556,14 @@ export function decode_option(data, decoder) {
 }
 
 export function decode_field(value, name) {
-  return name in value
-    ? new Ok(value[name])
-    : decoder_error(`Value with field ${inspect(name)}`, value);
+  let error = () => decoder_error(`Value with field ${inspect(name)}`, value);
+  if (value instanceof Map) {
+    let entry = value.get(name);
+    return entry.isOk() ? entry : error();
+  }
+  try {
+    return name in value ? new Ok(value[name]) : error();
+  } catch {
+    return error();
+  }
 }
diff --git a/test/gleam/dynamic_test.gleam b/test/gleam/dynamic_test.gleam
index ae3285b..d39d4ef 100644
--- a/test/gleam/dynamic_test.gleam
+++ b/test/gleam/dynamic_test.gleam
@@ -246,36 +246,34 @@ if javascript {
   }
 }
 
-if erlang {
-  pub fn field_test() {
-    map.new()
-    |> map.insert("ok", 1)
-    |> dynamic.from
-    |> dynamic.field("ok")
-    |> should.equal(Ok(dynamic.from(1)))
+pub fn field_test() {
+  map.new()
+  |> map.insert("ok", 1)
+  |> dynamic.from
+  |> dynamic.field("ok")
+  |> should.equal(Ok(dynamic.from(1)))
 
-    map.new()
-    |> map.insert("ok", 3)
-    |> map.insert("error", 1)
-    |> dynamic.from
-    |> dynamic.field("ok")
-    |> should.equal(Ok(dynamic.from(3)))
+  map.new()
+  |> map.insert("ok", 3)
+  |> map.insert("error", 1)
+  |> dynamic.from
+  |> dynamic.field("ok")
+  |> should.equal(Ok(dynamic.from(3)))
 
-    map.new()
-    |> dynamic.from
-    |> dynamic.field("ok")
-    |> should.be_error
+  map.new()
+  |> dynamic.from
+  |> dynamic.field("ok")
+  |> should.be_error
 
-    1
-    |> dynamic.from
-    |> dynamic.field("ok")
-    |> should.be_error
+  1
+  |> dynamic.from
+  |> dynamic.field("ok")
+  |> should.be_error
 
-    []
-    |> dynamic.from
-    |> dynamic.field([])
-    |> should.be_error
-  }
+  []
+  |> dynamic.from
+  |> dynamic.field([])
+  |> should.be_error
 }
 
 pub fn element_test() {
-- 
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