# `Attesto.Federation.MetadataPolicy`
[🔗](https://github.com/XukuLLC/attesto/blob/v2.1.0/lib/attesto/federation/metadata_policy.ex#L1)

OpenID Federation 1.0 metadata-policy resolution and application.

Policies use string-keyed maps. `apply/2` accepts either a complete
`metadata_policy` Claim together with complete Entity metadata, or one
Entity Type's parameter policy together with that Entity Type's metadata.
`merge/2` accepts the same two policy levels; its first argument is the
policy of the more superior Entity.

Unknown, non-critical extension operators are ignored, as required by the
Federation specification. Critical extension operators are handled while
validating an Entity Statement and are outside this core seven-operator API.

# `error`

```elixir
@type error() :: :policy_error
```

# `policy`

```elixir
@type policy() :: %{optional(String.t()) => term()}
```

# `apply`

```elixir
@spec apply(policy(), map()) :: {:ok, map()} | {:error, error()}
```

Apply a metadata policy in the standard operator order.

The return value contains no `null` metadata parameters. Unsupported value
types, illegal operator combinations, and failed value checks are policy
errors.

# `merge`

```elixir
@spec merge(policy(), policy()) :: {:ok, policy()} | {:error, error()}
```

Merge a superior policy with the next lower policy in a Trust Chain.

The result is at least as restrictive as both inputs. Incompatible
same-operator values or newly illegal operator combinations fail closed.

---

*Consult [api-reference.md](api-reference.md) for complete listing*