feat: add errorFirst function

src/async/errorFirst.ts

@@ -0,0 +1,34 @@
+import { isFunction } from 'radashi'
+
+const left = <Error>(error: Error): [Error, null] => [error, null]
+const right = <T>(value: T): [null, T] => [null, value]
+
+export type ErrorFirst<T, Error = unknown> = [null, T] | [Error, null]
+
+/**
+ * Call an async function and return a promise that resolves to an
+ * array of the error and the value.
+ *
+ * - If the promise resolves, the result is `[null, value]`.
+ * - If the promise rejects, the result is `[error, null]`.
+ */
+export function errorFirst<T, Args extends any[]>(
+  fn: (...args: Args) => Promise<T>,
+  ...args: Args
+): Promise<ErrorFirst<T>>
+
+export function errorFirst<T>(promise: Promise<T>): Promise<ErrorFirst<T>>
+
+export function errorFirst<T>(
+  promise: Promise<T> | ((...args: any[]) => Promise<T>),
+  ...args: any[]
+): Promise<ErrorFirst<T>> {
+  if (isFunction(promise)) {
+    try {
+      promise = promise(...args)
+    } catch (error) {
+      promise = Promise.reject(error)
+    }
+  }
+  return promise.then(right, left)
+}

src/mod.ts

@@ -34,6 +34,7 @@ export * from './array/zipToObject.ts'
 export * from './async/AggregateError.ts'
 export * from './async/all.ts'
 export * from './async/defer.ts'
+export * from './async/errorFirst.ts'
 export * from './async/guard.ts'
 export * from './async/map.ts'
 export * from './async/parallel.ts'