Add basic parsing JSON example

Author: leejarvis

universal/test/formats/parsing_json.gleam

@@ -0,0 +1,40 @@
+//// # Parsing JSON
+////
+//// The gleam_json library can be used to decode JSON.
+////
+//// ## Dependencies
+////
+//// - https://hex.pm/packages/gleam_json
+
+import gleam/dict
+import gleam/dynamic
+import gleam/json
+import gleeunit/should
+
+type User(name, age, is_admin) {
+  User(name, age, is_admin)
+}
+
+pub fn main_test() {
+  // Basic string key-value pair
+  "{\"name\": \"cookbook\"}"
+  |> json.decode(dynamic.dict(dynamic.string, dynamic.string))
+  |> should.equal(Ok(dict.from_list([#("name", "cookbook")])))
+
+  // String key-value pair with list of integers
+  "{\"scores\": [1, 2, 3]}"
+  |> json.decode(dynamic.dict(dynamic.string, dynamic.list(of: dynamic.int)))
+  |> should.equal(Ok(dict.from_list([#("scores", [1, 2, 3])])))
+
+  // Custom decoder
+  let decoder =
+    dynamic.decode3(
+      User,
+      dynamic.field("name", dynamic.string),
+      dynamic.field("age", dynamic.int),
+      dynamic.field("is_admin", dynamic.bool),
+    )
+  "{\"name\": \"Alice\", \"age\": 42, \"is_admin\": true}"
+  |> json.decode(decoder)
+  |> should.equal(Ok(User("Alice", 42, True)))
+}