# 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.

---

## 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`.

---

## 2. Where the Accumulation Happens

The important function is:

```haskell
validateManifest :: RawManifest -> Validation [String] ReleaseManifest
```

It builds the final manifest with applicative style:

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

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

---

## 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

```bash
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
```