From d9656076ec5c4138201080f0a250b57533357165 Mon Sep 17 00:00:00 2001
From: Elizabeth Hunt <me@liz.coffee>
Date: Sun, 17 Aug 2025 15:09:14 -0700
Subject: Add some retry behavior

---
 lib/trace/util.ts      |  1 -
 lib/types/fn/either.ts | 40 ++++++++++++++++++++++++++++++++++++++++
 package.json           |  2 +-
 tst/either.test.ts     | 45 +++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 86 insertions(+), 2 deletions(-)
 create mode 100644 tst/either.test.ts

diff --git a/lib/trace/util.ts b/lib/trace/util.ts
index ec67571..95f8d25 100644
--- a/lib/trace/util.ts
+++ b/lib/trace/util.ts
@@ -1,7 +1,6 @@
 import {
     IEither,
     IMetric,
-    isEither,
     ITraceable,
     ITraceWith,
     LogLevel,
diff --git a/lib/types/fn/either.ts b/lib/types/fn/either.ts
index 0f65859..ffe06d4 100644
--- a/lib/types/fn/either.ts
+++ b/lib/types/fn/either.ts
@@ -25,6 +25,7 @@ export interface IEither<E, T> extends Tagged<IEitherTag> {
     readonly flatMapAsync: <_T>(mapper: Mapper<T, Promise<IEither<E, _T>>>) => Promise<IEither<E, _T>>;
 
     readonly moveRight: <_T>(t: _T) => IEither<E, _T>;
+    readonly swap: Supplier<IEither<T, E>>;
     readonly fold: <_T>(leftFolder: Mapper<E, _T>, rightFolder: Mapper<T, _T>) => _T;
     readonly joinRight: <O, _T>(other: IEither<E, O>, mapper: (a: O, b: T) => _T) => IEither<E, _T>;
     readonly joinRightAsync: <O, _T>(
@@ -119,6 +120,11 @@ export class Either<E, T> extends _Tagged implements IEither<E, T> {
         });
     }
 
+    public swap(): IEither<T, E> {
+        if (isRight(this.self)) return Either.left(this.self.ok);
+        return Either.right(this.self.err);
+    }
+
     static left<E, T>(e: E): IEither<E, T> {
         return new Either({ err: e, _tag: ELeftTag });
     }
@@ -140,4 +146,38 @@ export class Either<E, T> extends _Tagged implements IEither<E, T> {
             .then((t: T) => Either.right<E, T>(t))
             .catch((e: E) => Either.left<E, T>(e));
     }
+
+    static async retrying<E, T>(
+        s: Supplier<Promise<IEither<E, T>>>,
+        attempts: number = 3,
+        interval: Mapper<number, Promise<void>> = (attempt) => Either.attemptWait(attempt),
+    ): Promise<IEither<E, T>> {
+        const res: IEither<T, E> = await Array(attempts)
+            .fill(0)
+            .reduce(
+                async (_result, _i, attempt) => {
+                    await interval(attempt);
+                    const result: IEither<T, E> = await _result;
+                    return result.joinRightAsync(
+                        () => s().then((s) => s.swap()),
+                        (res, _prevError) => res,
+                    );
+                },
+                Promise.resolve(Either.right<T, E>(<E>new Error('No attempts made'))),
+            );
+        return res.swap();
+    }
+
+    static attemptWait(
+        attempt: number,
+        backoffFactor: number = 500,
+        jitter: number = 300,
+        exponent: number = 1.3,
+    ): Promise<void> {
+        if (attempt === 0) {
+            return Promise.resolve();
+        }
+        const wait = Math.pow(exponent, attempt) * backoffFactor + jitter * Math.random() * Math.pow(exponent, attempt);
+        return new Promise((res) => setTimeout(res, wait));
+    }
 }
diff --git a/package.json b/package.json
index a77b9ae..d4bb7e5 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
     "name": "@emprespresso/pengueno",
-    "version": "0.0.9",
+    "version": "0.0.11",
     "description": "emprespresso pengueno utils",
     "main": "./dist/index.js",
     "types": "./dist/index.d.ts",
diff --git a/tst/either.test.ts b/tst/either.test.ts
new file mode 100644
index 0000000..790d392
--- /dev/null
+++ b/tst/either.test.ts
@@ -0,0 +1,45 @@
+import { Either } from '@emprespresso/pengueno';
+
+describe('Either.retrying', () => {
+    beforeEach(() => {
+        jest.clearAllMocks();
+    });
+
+    test('succeeds on first attempt', async () => {
+        const supplier = jest.fn().mockResolvedValue(Either.right<string, string>('success'));
+        const interval = jest.fn().mockResolvedValue(undefined);
+
+        const result = await Either.retrying(supplier, 3, interval);
+
+        expect(result.right().get()).toBe('success');
+        expect(supplier).toHaveBeenCalledTimes(1);
+    });
+
+    test('never succeeds after all attempts', async () => {
+        const supplier = jest.fn().mockResolvedValue(Either.left<string, string>('failed'));
+        const interval = jest.fn().mockResolvedValue(undefined);
+
+        const result = await Either.retrying(supplier, 3, interval);
+
+        expect(result.left().get()).toBe('failed');
+        expect(supplier).toHaveBeenCalledTimes(3);
+    });
+
+    test('attempts correct number of times and calls interval with backoff', async () => {
+        const supplier = jest.fn()
+            .mockResolvedValueOnce(Either.left<string, string>('attempt 1 failed'))
+            .mockResolvedValueOnce(Either.left<string, string>('attempt 2 failed'))
+            .mockResolvedValueOnce(Either.right<string, string>('attempt 3 success'));
+
+        const interval = jest.fn().mockResolvedValue(undefined);
+
+        const result = await Either.retrying(supplier, 3, interval);
+
+        expect(result.right().get()).toBe('attempt 3 success');
+        expect(supplier).toHaveBeenCalledTimes(3);
+        expect(interval).toHaveBeenCalledTimes(3);
+        expect(interval).toHaveBeenNthCalledWith(1, 0);
+        expect(interval).toHaveBeenNthCalledWith(2, 1);
+        expect(interval).toHaveBeenNthCalledWith(3, 2);
+    });
+});
-- 
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