# Haskell Maybe and Either

This note covers `23-haskell-maybe-either/`, which builds a release request from `key=value` inputs by using `Maybe` for optional data and `Either`
for validation failures.

---

## 1. Why Both Types Matter

`Maybe` answers one question: is a value present or absent?

`Either` answers a different question: if something failed, why?

This example uses both on purpose:

- optional fields such as `owner` stay as `Maybe`,
- required fields start as lookups that may be absent, and
- missing or invalid required data becomes `Either String ...` with an error message.

That gives the program a clean progression from raw input toward a validated domain value.

---

## 2. Where `Maybe` Shows Up

The input is a flat list of assignments such as `service=api` and `owner=platform`.

`lookupOptional` searches that list and returns `Maybe String`:

```haskell
lookupOptional :: String -> [(String, String)] -> Maybe String
```

That is the right level for values that are genuinely optional. In the example, `owner` can be absent without making the whole request invalid.

---

## 3. Where `Either` Takes Over

Required fields cannot stay as `Maybe`, because the rest of the program needs a reason when construction fails.

The example upgrades a missing field into an error:

```haskell
lookupRequired :: String -> [(String, String)] -> Either String String
```

It then adds more validation:

- environment parsing,
- replica count parsing, and
- canary percentage rules for the rollout strategy.

That keeps absence and validation separate, which makes the control flow easier to read.

---

## 4. Commands to Try

```bash
cd 23-haskell-maybe-either

nix develop
cabal run
cabal run -- service=api env=production replicas=3 strategy=canary canary=10 owner=platform
cabal test

nix build
./result/bin/mini-release-request service=api env=production replicas=3 strategy=canary canary=10 owner=platform

nix run . -- service=api env=production replicas=3 strategy=canary canary=10 owner=platform
nix flake check
```