nix-playgraound/notes/047-haskell-seq-queues.md

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

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