From abd66e4f2fd96b0194d1f81929cca13382a5653c Mon Sep 17 00:00:00 2001
From: Gavin Morrow <gavinfmorrow@gmail.com>
Date: Fri, 19 Sep 2025 20:34:46 -0400
Subject: [PATCH] Create protobuf `Decoder`

---
 src/protobuf_decode_gleam.gleam       | 14 ++++++++++++++
 test/protobuf_decode_gleam_test.gleam | 27 +++++++++++++--------------
 2 files changed, 27 insertions(+), 14 deletions(-)

diff --git a/src/protobuf_decode_gleam.gleam b/src/protobuf_decode_gleam.gleam
index 7bd65fb..a6861be 100644
--- a/src/protobuf_decode_gleam.gleam
+++ b/src/protobuf_decode_gleam.gleam
@@ -14,6 +14,20 @@ pub fn decode(
   decode.run(data, decoder) |> result.map_error(UnableToDecode)
 }
 
+pub fn decoder(
+  using decoder: fn() -> Decoder(t),
+  named name: String,
+  default default: t,
+) -> Decoder(t) {
+  use bits <- decode.then(decode.bit_array)
+
+  let value = decode(from: bits, using: decoder())
+  case value {
+    Ok(value) -> decode.success(value)
+    Error(_) -> decode.failure(default, name)
+  }
+}
+
 type WireType {
   VarInt
   I64
diff --git a/test/protobuf_decode_gleam_test.gleam b/test/protobuf_decode_gleam_test.gleam
index 776ef60..9858708 100644
--- a/test/protobuf_decode_gleam_test.gleam
+++ b/test/protobuf_decode_gleam_test.gleam
@@ -1,7 +1,9 @@
 import gleam/dynamic/decode.{type Decoder}
 import gleam/option
 import gleeunit
-import protobuf_decode_gleam.{decode, decode_fixed, decode_uint}
+import protobuf_decode_gleam.{
+  decode, decode_fixed, decode_string, decode_uint, decoder,
+}
 import simplifile as file
 
 pub fn main() -> Nil {
@@ -12,20 +14,17 @@ type Person {
   Person(id: Int, age: Int, score: Int, self: option.Option(Person), day: Int)
 }
 
-fn person_decoder() -> Decoder(Person) {
-  let person_inner_decoder = {
-    use bits <- decode.then(decode.bit_array)
+const default_person = Person(
+  id: 0,
+  age: 0,
+  score: 0,
+  self: option.None,
+  day: 0,
+)
 
-    let person = decode(from: bits, using: person_decoder())
-    case person {
-      Ok(person) -> decode.success(person)
-      Error(_) ->
-        decode.failure(
-          Person(id: 0, age: 0, score: 0, self: option.None, day: 0),
-          "Person",
-        )
-    }
-  }
+fn person_decoder() -> Decoder(Person) {
+  let person_inner_decoder =
+    decoder(using: person_decoder, named: "Person", default: default_person)
 
   use id <- decode.field(3, decode_fixed(64))
   use age <- decode.field(1, decode_uint())
-- 
2.51.2