From bfd1e9c99c16169600f42a988b1ac4fb0452f749 Mon Sep 17 00:00:00 2001
From: Simon Lydell <simon.lydell@gmail.com>
Date: Sun, 29 Oct 2023 16:44:45 +0100
Subject: [PATCH] Remove the second type variable of `Decoder` (#39)

---
 CHANGELOG.md                      | 18 ++++++++++++++++++
 README.md                         | 10 ----------
 examples/type-annotations.test.ts |  4 ++--
 index.ts                          |  2 +-
 tests/decoders.test.ts            |  4 ++--
 5 files changed, 23 insertions(+), 15 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index eb1dad7..f2594e8 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,23 @@
 Note: I’m currently working on several breaking changes to tiny-decoders, but I’m trying out releasing them piece by piece. The idea is that you can either upgrade version by version only having to deal with one or a few breaking changes at a time, or wait and do a bunch of them at the same time.
 
+### Version 18.0.0 (unreleased)
+
+This release removes the second type variable from `Decoder`.
+
+Before:
+
+```ts
+type Decoder<T, U = unknown> = (value: U) => DecoderResult<T>;
+```
+
+After:
+
+```ts
+type Decoder<T> = (value: unknown) => DecoderResult<T>;
+```
+
+This change unlocks further changes that will come in future releases.
+
 ### Version 17.0.1 (2023-10-29)
 
 Fixed: `fieldsAuto` now reports the correct field name when there’s an error in a renamed field.
diff --git a/README.md b/README.md
index c2b2367..86fd3e5 100644
--- a/README.md
+++ b/README.md
@@ -106,16 +106,6 @@ That’s it!
 
 tiny-decoders ships with a bunch of decoders, and a few functions to combine decoders. This way you can describe the shape of any data!
 
-### Advanced variant
-
-```ts
-type Decoder<T, U = unknown> = (value: U) => DecoderResult<T>;
-```
-
-The above is the _full_ definition of a decoder. The input value can be some other type (`U`) than `unknown` if you want.
-
-Most of the time you don’t need to think about this, though!
-
 ## Decoders
 
 Here’s a summary of all decoders (with slightly simplified type annotations):
diff --git a/examples/type-annotations.test.ts b/examples/type-annotations.test.ts
index e57d00f..6e1b62c 100644
--- a/examples/type-annotations.test.ts
+++ b/examples/type-annotations.test.ts
@@ -41,8 +41,8 @@ test("type annotations", () => {
 
   // The way to make the above type error more clear is to provide an explicit type
   // annotation, so that TypeScript knows what you’re trying to do.
-  // @ts-expect-error Type 'Decoder<{ name: string; aye: number; }, unknown>' is not assignable to type 'Decoder<Person>'.
-  //   Property 'age' is missing in type '{ name: string; aye: number; }' but required in type 'Person'.ts(2322)
+  // @ts-expect-error Type 'Decoder<{ name: string; aye: number; }>' is not assignable to type 'Decoder<Person>'.
+  //   Property 'age' is missing in type '{ name: string; aye: number; }' but required in type 'Person'.
   const personDecoder2: Decoder<Person> = fieldsAuto({
     name: string,
     aye: number,
diff --git a/index.ts b/index.ts
index 1823823..7c43bf2 100644
--- a/index.ts
+++ b/index.ts
@@ -2,7 +2,7 @@
 // No `any` “leaks” when _using_ the library, though.
 /* eslint-disable @typescript-eslint/no-explicit-any */
 
-export type Decoder<T, U = unknown> = (value: U) => DecoderResult<T>;
+export type Decoder<T> = (value: unknown) => DecoderResult<T>;
 
 export type DecoderResult<T> =
   | {
diff --git a/tests/decoders.test.ts b/tests/decoders.test.ts
index 78209fc..6c16924 100644
--- a/tests/decoders.test.ts
+++ b/tests/decoders.test.ts
@@ -1022,9 +1022,9 @@ describe("tag", () => {
 });
 
 describe("tuple", () => {
-  // @ts-expect-error Argument of type '{}' is not assignable to parameter of type 'readonly Decoder<unknown, unknown>[]'.
+  // @ts-expect-error Argument of type '{}' is not assignable to parameter of type 'Decoder<unknown>[]'.
   tuple({});
-  // @ts-expect-error Argument of type '(value: unknown) => number' is not assignable to parameter of type 'readonly Decoder<unknown, unknown>[]'.
+  // @ts-expect-error Argument of type '(value: unknown) => DecoderResult<number>' is not assignable to parameter of type 'Decoder<unknown>[]'.
   tuple(number);
 
   test("0 items", () => {