nix-playgraound/notes/031-haskell-applicative-validation.md

Haskell Applicative Validation

This note covers 28-haskell-applicative-validation/, which validates a release manifest with a custom Validation type that accumulates several field errors at once.

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1. Why This Is Not Just Either

Either stops at the first failure. That is useful for many workflows, but it is not always the best fit for validating a form-like input.

This example wants a different behavior:

• validate service,
• validate env,
• validate owners,
• validate replicas,
• validate strategy, and
• validate window,

and report every invalid field in one pass.

That is why the example uses a Validation type with an Applicative instance that combines errors through Semigroup.

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2. Where the Accumulation Happens

The important function is:

validateManifest :: RawManifest -> Validation [String] ReleaseManifest

It builds the final manifest with applicative style:

ReleaseManifest
  <$> validateService ...
  <*> validateEnvironment ...
  <*> validateOwners ...
  <*> validateReplicas ...
  <*> validateStrategy ...
  <*> validateWindow ...

Each field validator can fail independently, and the Applicative instance combines all the resulting error lists.

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3. Why the Input Is Split into Two Stages

The example still parses key=value assignments with Either.

That keeps syntax errors separate from semantic validation:

• malformed assignments such as service or owners= fail immediately, and
• well-shaped assignments become a RawManifest that the validation layer can inspect field by field.

This separation keeps the example focused. The interesting part is accumulated validation, not ad hoc string parsing.

---

4. Commands to Try

cd 28-haskell-applicative-validation

nix develop
cabal run
cabal run -- service=api-gateway env=production owners=platform,security replicas=3 strategy=canary:20 window=22-24
cabal test

nix build
./result/bin/mini-validation service=api-gateway env=production owners=platform,security replicas=3 strategy=canary:20 window=22-24

nix run . -- service=api-gateway env=production owners=platform,security replicas=3 strategy=canary:20 window=22-24
nix flake check