feat: preprocess type

commit 12f3253892db42e7ce5b9c1ec3da962df0651dc1
Author: skarab42 <skarab@bluewin.ch>
Date:   Sun Mar 13 07:27:10 2022 +0100

    docs: add preprocess type

commit 4f81fbab67620358540fad689c96f1c28f87ed6d
Author: skarab42 <skarab@bluewin.ch>
Date:   Sun Mar 13 07:15:26 2022 +0100

    test: preprocess type

commit 9bc691945f60e60cf642db5fec4335ccbd6f6959
Author: skarab42 <skarab@bluewin.ch>
Date:   Sun Mar 13 07:15:09 2022 +0100

    feat: preprocess type

README.md

@@ -119,6 +119,7 @@ type User = t.infer<typeof user>;
   - [function(args)](#functionargs)
   - [function(args, returns)](#functionargs-returns)
   - [function(args, returns, implement)](#functionargs-returns-implement)
+  - [preprocess(filter, type)](#preprocessfilter-type)
 - [Contributing 💜](#contributing-)
 
 # API
@@ -494,6 +495,17 @@ const func = t.function(args, returns, (input, toInt) => {
 type Func = t.infer<typeof func>; // (arg_0: string, arg_1: boolean) => string | number
 ```
 
+## preprocess(filter, type)
+
+If you want to modify the input before it is parsed you can use the `preprocess` type as follows.
+
+```ts
+const toString = t.preprocess((input) => String(input), t.string());
+
+toString.parse("42"); // => "42"
+toString.parse(42); // => "42"
+```
+
 # Contributing 💜
 
 See [CONTRIBUTING.md](https://github.com/skarab42/tson/blob/main/CONTRIBUTING.md)

src/type/index.ts

@@ -18,6 +18,7 @@ export { nullableType as nullable } from "./nullable";
 export { numberType as number } from "./number";
 export { objectType as object } from "./object";
 export { optionalType as optional } from "./optional";
+export { preprocessType as preprocess } from "./preprocess";
 export { promiseType as promise } from "./promise";
 export { recordType as record } from "./record";
 export { setType as set } from "./set";

src/type/preprocess.ts

@@ -0,0 +1,13 @@
+import { InferType, Type } from "../types";
+
+export function preprocessType<
+  TFunc extends (input: unknown) => unknown,
+  TType extends Type<InferType<TType>>,
+>(filter: TFunc, type: TType): TType {
+  return {
+    ...type,
+    parse(input: unknown) {
+      return type.parse(filter(input));
+    },
+  };
+}

test/preprocess.test.ts

@@ -0,0 +1,18 @@
+import { expect, test } from "vitest";
+import { t } from "../src";
+
+test("preprocess() string", () => {
+  const preprocess = t.preprocess((input) => String(input), t.string());
+  expect(preprocess.parse("42")).toBe("42");
+  expect(preprocess.parse(42)).toBe("42");
+});
+
+test("preprocess() number", () => {
+  const preprocess = t.preprocess((input) => Number(input), t.number());
+  expect(preprocess.parse("42")).toBe(42);
+  expect(preprocess.parse(42)).toBe(42);
+
+  expect(() => preprocess.parse(undefined)).toThrow(
+    "expected 'number' got 'NaN'",
+  );
+});