64 lines
1.4 KiB
Markdown
64 lines
1.4 KiB
Markdown
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# Haskell Transformer Stacks
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This note covers `37-haskell-transformer-stack/`, which composes `ReaderT`, `ExceptT`, and `Writer` in one rollout workflow.
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---
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## 1. Why a Stack Helps Here
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The rollout workflow needs three independent capabilities:
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- configuration from an environment,
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- explicit business failures, and
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- ordered audit output.
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This example puts them together directly:
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```haskell
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type App = ReaderT Env (ExceptT RolloutError (Writer [String]))
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```
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That is the core teaching point. The effect requirements live in one concrete stack, while the workflow stays readable.
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---
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## 2. What the Workflow Actually Does
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The stack is not there for decoration. The rollout code:
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- reads cluster and policy settings,
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- rejects restricted or oversized production rollouts, and
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- records each successful step in an audit log.
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That gives each transformer a concrete job.
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---
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## 3. Why This Complements the Earlier Examples
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Earlier notes introduced these pieces separately:
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- `ReaderT` and `Except` in `10-haskell-effects/`, and
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- `Writer` in `31-haskell-writer-audit/`.
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This example shows the next practical step: combining them when one workflow needs all three.
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---
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## 4. Commands to Try
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```bash
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cd 37-haskell-transformer-stack
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nix develop
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cabal run
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cabal run -- api:production:4
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cabal test
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nix build
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./result/bin/mini-stack api:production:4
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nix run . -- api:production:4
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nix flake check
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```
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