fix(types): align isPromise return type with its logic (#175)

docs/typed/isPromise.mdx

@@ -1,16 +1,28 @@
 ---
 title: isPromise
-description: 'Determine if a value is a Promise'
+description: 'Determine if a value is a Promise or has a `then` method'
 ---
 
 ### Usage
 
-Pass in a value and get a boolean telling you if the value is a Promise. This function is not _"bullet proof"_ because determining if a value is a Promise in javascript is not _"bullet proof"_. The standard/recommended method is to use `Promise.resolve` to essentially cast any value, promise or not, into an awaited value. However, this may do in a pinch.
+The `isPromise` function checks if a value is "Promise-like" by determining if it has a `then` method.
 
 ```ts
 import * as _ from 'radashi'
 
+_.isPromise({ then: () => {} }) // => true
+_.isPromise(new Promise(() => {})) // => true
+_.isPromise(Promise.resolve(1)) // => true
+_.isPromise(Promise.reject(new Error('nope'))) // => true
+
 _.isPromise('hello') // => false
-_.isPromise(['hello']) // => false
-_.isPromise(new Promise(res => res())) // => true
+_.isPromise({}) // => false
 ```
+
+This approach is useful for identifying objects that conform to the Promise interface without actually being instances of `Promise`. It's particularly helpful in scenarios where:
+
+1. You need to quickly check if a value is thenable without resolving it.
+2. Performance is critical, and you want to avoid the overhead of `Promise.resolve`.
+3. You're working with custom Promise implementations or third-party libraries that use Promise-like objects.
+
+While `Promise.resolve` is generally recommended for handling both Promise and non-Promise values uniformly, `isPromise` can be preferable when you need to make decisions based on whether a value is Promise-like without actually resolving or chaining it. This can be especially useful in type-checking scenarios or when implementing control flow that depends on whether a value is immediately available or needs to be awaited.

src/async/tryit.ts

@@ -1,4 +1,4 @@
-import { isPromise } from 'radashi'
+import { isPromise, type Result, type ResultPromise } from 'radashi'
 
 /**
  * The result of a `tryit` function.
@@ -18,29 +18,36 @@ import { isPromise } from 'radashi'
  * })
  * ```
  */
-export type TryitResult<Return> = Return extends Promise<any>
-  ? Promise<[Error, undefined] | [undefined, Awaited<Return>]>
-  : [Error, undefined] | [undefined, Return]
+export type TryitResult<
+  TReturn,
+  TError extends Error = Error,
+> = TReturn extends PromiseLike<infer TResult>
+  ? ResultPromise<TResult, TError>
+  : Result<TReturn, TError>
 
 /**
  * A helper to try an async function without forking the control flow.
  * Returns an error-first callback-_like_ array response as `[Error,
  * result]`
  */
-export function tryit<Args extends any[], Return>(
-  func: (...args: Args) => Return,
-): (...args: Args) => TryitResult<Return> {
-  return (...args) => {
+export function tryit<
+  TArgs extends any[],
+  TReturn,
+  TError extends Error = Error,
+>(
+  func: (...args: TArgs) => TReturn,
+): (...args: TArgs) => TryitResult<TReturn, TError> {
+  return (...args): any => {
     try {
       const result = func(...args)
-      if (isPromise(result)) {
-        return result
-          .then(value => [undefined, value])
-          .catch(err => [err, undefined]) as TryitResult<Return>
-      }
-      return [undefined, result] as TryitResult<Return>
+      return isPromise(result)
+        ? result.then(
+            value => [undefined, value],
+            err => [err, undefined],
+          )
+        : [undefined, result]
     } catch (err) {
-      return [err, undefined] as TryitResult<Return>
+      return [err, undefined]
     }
   }
 }

src/typed/isPromise.ts

@@ -11,6 +11,6 @@ import { isFunction } from 'radashi'
  * isPromise(1) // => false
  * ```
  */
-export function isPromise(value: any): value is Promise<any> {
+export function isPromise(value: any): value is PromiseLike<unknown> {
   return !!value && isFunction(value.then)
 }

tests/typed/isPromise.test.ts

@@ -1,12 +1,14 @@
 import * as _ from 'radashi'
 
 describe('isPromise', () => {
-  test('return true for Promise values', () => {
-    expect(_.isPromise(new Promise(res => res(0)))).toBeTruthy()
-    expect(_.isPromise(new Promise(res => res('')))).toBeTruthy()
+  test('return true for Promise-like values', () => {
+    expect(_.isPromise(new Promise(() => {}))).toBeTruthy()
+    expect(_.isPromise(Promise.resolve(1))).toBeTruthy()
     expect(_.isPromise((async () => {})())).toBeTruthy()
+    // biome-ignore lint/suspicious/noThenProperty:
+    expect(_.isPromise({ then: () => {} })).toBeTruthy()
   })
-  test('return false for non-Date values', () => {
+  test('return false for non-Promise-like values', () => {
     expect(_.isPromise(22)).toBeFalsy()
     expect(_.isPromise({ name: 'x' })).toBeFalsy()
     expect(_.isPromise('abc')).toBeFalsy()