# `F3.Schema`

Logical Arrow schema read from an F3 footer.

A field type is a nested term. Primitive widths that the committed
fixtures use are `{:int, width, signed?}` and `{:float, precision}`.
`{:list, field}` and `{:struct, fields}` carry their children. Any other
Arrow type id is `{:unsupported, id}` and does not by itself reject the file.

Row values are not part of this struct. `F3.read/2` returns batches whose columns use these field names.

# `arrow_type`

```elixir
@type arrow_type() ::
  :null
  | :bool
  | :utf8
  | :binary
  | {:int, pos_integer(), boolean()}
  | {:float, :half | :single | :double}
  | {:list, field()}
  | {:struct, [field()]}
  | {:unsupported, non_neg_integer()}
```

# `field`

```elixir
@type field() :: %{name: String.t(), type: arrow_type(), nullable: boolean()}
```

One logical field.

`name` is the footer name. `type` is the nested Arrow term. `nullable`
is true when the field accepts nulls. A writer keyword `:int64` becomes
`nullable: false`. `{:int64, null: true}` becomes `nullable: true`.

# `t`

```elixir
@type t() :: %F3.Schema{fields: [field()]}
```

`fields` is the top-level schema, in footer order.

# `field_map`

```elixir
@spec field_map(field()) :: map()
```

JSON-ready form of one field. Keys are strings. A nested type is a map.
A primitive type is the same string as `type_label/1`.

## Examples

    iex> F3.Schema.field_map(%{name: "id", type: {:int, 64, true}, nullable: false})
    %{"name" => "id", "nullable" => false, "type" => "int64"}

# `type_label`

```elixir
@spec type_label(arrow_type()) :: String.t()
```

Short type name used by `F3.inspect/2`.

`{:int, width, true}` is `int{width}`. `{:int, width, false}` is
`uint{width}`. Float precisions are `float16`, `float32`, and `float64`.
Lists and structs include their children. This does not raise.

## Examples

    iex> F3.Schema.type_label({:int, 64, true})
    "int64"

    iex> F3.Schema.type_label({:list, %{name: "item", type: :utf8, nullable: true}})
    "list<utf8>"

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