62 lines
1.5 KiB
Markdown
62 lines
1.5 KiB
Markdown
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# Haskell State
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This note covers `25-haskell-state/`, which plans a sequence of deployments by threading a build counter and per-environment wave numbers through
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`State`.
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---
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## 1. What the State Represents
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The planner keeps two changing values:
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- the next global build number, and
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- the next rollout wave number for each environment.
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That is a good fit for `State`, because each planned deployment needs to read the current values and write back updated ones.
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---
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## 2. Why This Stays Pure
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The planning logic does not perform I/O. It just transforms a list of deployment requests into a list of planned deployments.
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`State` keeps that transformation pure:
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```haskell
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planDeployment :: DeploymentRequest -> State PlannerState PlannedDeployment
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```
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The caller still gets a plain value at the end through `evalState`.
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That is the important intermediate Haskell idea here: stateful logic does not have to mean mutable variables or `IO`.
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---
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## 3. What the Example Allocates
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Each request receives:
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- a build number that increments globally, and
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- a wave number that increments separately per environment.
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That makes the output more interesting than a single counter example, while still teaching one concept.
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---
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## 4. Commands to Try
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```bash
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cd 25-haskell-state
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nix develop
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cabal run
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cabal run -- api:production:3 worker:staging:1 cache:production:2
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cabal test
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nix build
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./result/bin/mini-state-planner api:production:3 worker:staging:1 cache:production:2
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nix run . -- api:production:3 worker:staging:1 cache:production:2
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nix flake check
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```
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