# `Riddler.Screens.Type`
[🔗](https://github.com/riddler/riddler-ex/blob/v0.4.0/lib/riddler/screens/type.ex#L1)

What a node type has to tell the document about itself.

A type is a module. It declares the fields a node of that type may carry,
and it checks the ones only it knows how to check - a heading's level, a
button's writes, a variant's candidates. Everything a node needs that is
not particular to its type - the key, the condition, the templates, the
duplicate check - belongs to `Riddler.Screens.Document` and is done once
for every node whatever its type is.

The split is what keeps a sixth type from being a change to the document.
Adding one is a module implementing this behaviour and a line in
`Riddler.Screens.Registry`; nothing in the document's own checks moves.

## `fields/0`

Returns `%{required: [atom()], optional: [atom()]}`. The names are the
field names as a document spells them, as atoms. A required field the node
does not carry is a finding the document raises; an optional one is not.
A field named by neither list is not part of this version of the type, and
the admitted node does not carry it.

## `validate/1`

Takes the admitted node - an atom-keyed map, with the type's declared
fields under their own names - and returns the findings that are this
type's to raise, in source order, or `[]`. It is never asked about a field
the node does not carry: a missing required field has already been
reported, and there is nothing further to say about its value.

# `node_t`

```elixir
@type node_t() :: %{:type => term(), optional(atom()) => term()}
```

An admitted node: `:type` and, where the document declared them, `:key`,
`:condition` and the fields the node's type names.

# `fields`

```elixir
@callback fields() :: %{required: [atom()], optional: [atom()]}
```

The fields a node of this type may carry, split into required and optional.

# `validate`

```elixir
@callback validate(node_t()) :: [Riddler.Finding.t()]
```

The findings only this type can raise about an admitted node of it.

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