70 lines
1.9 KiB
Markdown
70 lines
1.9 KiB
Markdown
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# Haskell Traverse for Batch Resolution
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This note covers `30-haskell-traverse-resolution/`, which resolves a batch of deployment requests against a service catalog with `traverse`.
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---
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## 1. Why `traverse` Is the Point
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Each deployment request needs an effectful resolution step:
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- parse the environment,
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- look up the service in the catalog,
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- choose the right image tag, and
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- reject environment and release-track combinations that are not allowed.
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For one request, that is just `Either String ResolvedDeployment`.
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For a whole batch, the example uses:
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```haskell
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resolveRequests :: Catalog -> [DeploymentRequest] -> Either String [ResolvedDeployment]
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resolveRequests serviceCatalog = traverse (resolveRequest serviceCatalog)
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```
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That is the core lesson. `traverse` sequences the per-item effect and preserves the list structure.
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---
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## 2. What Makes the Example Non-Trivial
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The resolution step is more than one lookup.
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It decides:
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- which image tag to use for `stable` versus `candidate`,
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- whether the service even publishes a candidate image, and
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- whether the service is allowed in production at all.
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That makes the batch resolution behavior worth teaching on its own.
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---
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## 3. Why the Batch Fails as a Unit
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This example uses `Either`, so the whole batch fails on the first invalid request.
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That is intentional. The goal here is not accumulated validation. The goal is sequencing several effectful resolutions through one structure with
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`traverse`.
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`28-haskell-applicative-validation/` already covers the accumulated-error case.
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---
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## 4. Commands to Try
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```bash
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cd 30-haskell-traverse-resolution
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nix develop
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cabal run
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cabal run -- api:production:stable worker:staging:candidate auth:production:stable
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cabal test
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nix build
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./result/bin/mini-resolution api:production:stable worker:staging:candidate auth:production:stable
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nix run . -- api:production:stable worker:staging:candidate auth:production:stable
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nix flake check
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```
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