66 lines
1.7 KiB
Markdown
66 lines
1.7 KiB
Markdown
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# Haskell Monad Chaining
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This note covers `35-haskell-monad-chaining/`, which sequences several dependent rollout checks with `Either` and `do` notation.
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---
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## 1. Why Monad Chaining Matters
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Some workflows cannot be expressed as independent field checks.
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In this example, later steps depend on earlier successful results:
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- find the service profile first,
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- then check whether the environment is allowed,
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- then validate the production change ticket,
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- then choose the image tag for the requested track, and
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- finally choose the approver.
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That dependency chain is the point. Each step needs the result of the previous one.
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---
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## 2. Why `Either` Still Fits
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This example does not need accumulated errors. It needs short-circuiting business logic.
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That makes `Either String` a good fit:
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```haskell
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approveRollout :: Catalog -> RolloutRequest -> Either String ApprovedRollout
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```
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`do` notation keeps the happy path readable while preserving the fail-fast semantics.
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---
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## 3. How This Complements the Validation Example
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`28-haskell-applicative-validation/` teaches independent checks that all run so several errors can be reported together.
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This example teaches the opposite shape:
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- one decision unlocks the next, and
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- the workflow stops once a prerequisite fails.
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That contrast is useful. It shows why “Applicative versus Monad” is not just theory. The control flow shape changes the design.
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---
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## 4. Commands to Try
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```bash
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cd 35-haskell-monad-chaining
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nix develop
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cabal run
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cabal run -- api:production:stable:CHG-2048
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cabal test
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nix build
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./result/bin/mini-monad-chain api:production:stable:CHG-2048
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nix run . -- api:production:stable:CHG-2048
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nix flake check
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```
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