# `X402.Client.Policy`
[🔗](https://github.com/cardotrejos/x402/blob/v0.9.0/lib/x402/client/policy.ex#L1)

Ready-made selection policies for `X402.Client.select_requirements/2`.

A policy is a 2-arity function receiving a candidate requirements entry
and the decoded `PaymentRequired` map it came from (`nil` when selecting
from a bare list). It returns `true` to accept the entry, `false` to skip
it, or `{:error, reason}` to abort selection with that error. Policies are
passed with the `:policies` option and every one of them must accept an
entry for it to be selected:

    X402.Client.Finch.request(MyApp.Finch, url,
      signer: signer,
      policies: [
        X402.Client.Policy.max_amount("1000000"),
        X402.Client.Policy.networks(["eip155:8453", "solana:*"]),
        X402.Client.Policy.assets(["0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913"])
      ]
    )

Custom policies are plain functions and compose the same way:

    fn requirements, _payment_required ->
      requirements["payTo"] in MyApp.trusted_receivers()
    end

# `t`

```elixir
@type t() :: (map(), map() | nil -> boolean() | {:error, term()})
```

A selection policy.

# `assets`
*since 0.9.0* 

```elixir
@spec assets([String.t()]) :: t()
```

Accepts entries paying with one of the given assets (compared
case-insensitively).

## Examples

    iex> policy = X402.Client.Policy.assets(["0x036CbD53842c5426634e7929541eC2318f3dCF7e"])
    iex> policy.(%{"asset" => "0x036cbd53842c5426634e7929541ec2318f3dcf7e"}, nil)
    true
    iex> policy.(%{"asset" => "0x0000000000000000000000000000000000000000"}, nil)
    false

# `max_amount`
*since 0.9.0* 

```elixir
@spec max_amount(String.t() | non_neg_integer()) :: t()
```

Accepts entries whose `amount` (atomic units) does not exceed `limit`.

Entries with a missing or unparsable amount are skipped.

## Examples

    iex> policy = X402.Client.Policy.max_amount("10000")
    iex> policy.(%{"amount" => "10000"}, nil)
    true
    iex> policy.(%{"amount" => "10001"}, nil)
    false
    iex> policy.(%{"amount" => "lots"}, nil)
    false

    iex> X402.Client.Policy.max_amount(500).(%{"amount" => "499"}, nil)
    true

# `networks`
*since 0.9.0* 

```elixir
@spec networks([String.t()]) :: t()
```

Accepts entries on one of the given CAIP-2 networks.

A trailing `*` acts as a prefix wildcard (for example `"eip155:*"`).

## Examples

    iex> policy = X402.Client.Policy.networks(["eip155:8453", "solana:*"])
    iex> policy.(%{"network" => "eip155:8453"}, nil)
    true
    iex> policy.(%{"network" => "solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp"}, nil)
    true
    iex> policy.(%{"network" => "eip155:1"}, nil)
    false

# `schemes`
*since 0.9.0* 

```elixir
@spec schemes([String.t()]) :: t()
```

Accepts entries using one of the given schemes.

## Examples

    iex> policy = X402.Client.Policy.schemes(["exact"])
    iex> policy.(%{"scheme" => "exact"}, nil)
    true
    iex> policy.(%{"scheme" => "upto"}, nil)
    false

---

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