# Haskell Seq Queues

This note covers `44-haskell-seq-queues/`, which uses `Data.Sequence` as a queue for retry work.

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## 1. Why `Seq` Instead of a Plain List

Lists are excellent for recursive processing, but queues want efficient work at both ends:

- append new retries to the back, and
- take the next retry from the front.

`Data.Sequence` provides those operations directly, so the example can talk about queue behavior without building a custom data structure first.

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## 2. What the Example Demonstrates

The queue starts with four retries, then appends one more with `enqueueRetry`.

After that, `takeBatch` removes the next `n` items in FIFO order and returns:

- the batch to execute now, and
- the remaining queue for later.

That keeps the example focused on the queue contract, not on concurrency or backoff policy.

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## 3. Why the Test Uses Rendered Queue State

The test checks:

- `api#2, worker#1` as the next batch, and
- `billing#3, search#1, auth#1` as the remaining queue.

That is enough to prove both important properties:

- newly enqueued work goes to the back, and
- batch selection keeps the original front-to-back order.

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## 4. Commands to Try

```bash
cd 44-haskell-seq-queues

nix develop
cabal run
cabal run -- 3
cabal test

nix build
./result/bin/mini-seq-queues 3

nix run . -- 3
nix flake check
```