# Auras

Auras are system of nested "soft types" on atoms that are used to track metadata about how a particular atom is to be interpreted. This is used for type checking as well as pretty printing.

You can learn more about auras in [Hoon school](https://docs.urbit.org/build-on-urbit/hoon-school/b-syntax#nouns).

## Table of Auras <a href="#table-of-auras" id="table-of-auras"></a>

```
Aura         Meaning                                 Example Literal Syntax
-------------------------------------------------------------------------
@            empty aura
@c           UTF-32                                  ~-~45fed
@d           date
  @da        absolute date                           ~2018.5.14..22.31.46..1435
  @dr        relative date (ie, timespan)            ~h5.m30.s12
@f           Loobean (for compiler, not castable)    &
@i           Internet address
  @if        IPv4 address                            .195.198.143.90
  @is        IPv6 address                            .0.0.0.0.0.1c.c3c6.8f5a
@n           nil (for compiler, not castable)        ~
@p           phonemic base (ship name)               ~sorreg-namtyv
@q           phonemic base, unscrambled              .~litsyn-polbel
@r           IEEE 754 floating-point
  @rh        half precision (16 bits)                .~~3.14
  @rs        single precision (32 bits)              .6.022141e23
  @rd        double precision (64 bits)              .~6.02214085774e23
  @rq        quad precision (128 bits)               .~~~6.02214085774e23
@s           signed integer, sign bit low
  @sb        signed binary                           --0b11.1000
  @sd        signed decimal                          --1.000.056
  @si        signed decimal, no separator            --1000056
  @sv        signed base32                           -0v1df64.49beg
  @sw        signed base64                           --0wbnC.8haTg
  @sx        signed hexadecimal                      -0x5f5.e138
@t           UTF-8 text (cord)                       'howdy'
  @ta        URL-safe path element text (knot)       ~.howdy
    @tas     short name text (term)                  %howdy
@u           unsigned integer
  @ub        unsigned binary                         0b11.1000
  @ud        unsigned decimal                        1.000.056
  @ui        unsigned decimal, no separator          1000056
  @uv        unsigned base32                         0v1df64.49beg
  @uw        unsigned base64                         0wbnC.8haTg
  @ux        unsigned hexadecimal                    0x5f5.e138
@x           unsigned hexadecimal, no separator      0xf5fe138
```

## Bitwidth <a href="#bitwidth" id="bitwidth"></a>

Capital letters at the end of auras indicate the bitwidth in binary powers of two, starting from A.

```
        @udD    unsigned single-byte (8-bit) decimal
        @tD     8-bit UTF-8 text
        @rhE    half-precision (16-bit) floating-point number
        @uxG    unsigned 64-bit hexadecimal
        @uvJ    unsigned, 512-bit integer (frequently used for entropy)
```

## Nesting <a href="#nesting" id="nesting"></a>

A given aura nests under any aura whose name is a substring or extension of the given aura:

```
> :-  (~(nest ut [%atom %ud ~]) | [%atom %u ~])
      (~(nest ut [%atom %ud ~]) | [%atom %udx ~])
[%.y %.y]
```

but does not nest "sideways":

```
>  (~(nest ut [%atom %ud ~]) | [%atom %ux ~])
%.n
```

We call auras "soft types" since this nesting behavior can be ignored. Auras are non-coercive, but conversions may have to go via the empty aura, e.g.

```
> ^-(@ud ^-(@ 'foo'))
7.303.014
```

This is implicitly done by the irregular form of `^-`.

```
> `@ud`'foo'
7.303.014
```

## Bunting <a href="#bunting" id="bunting"></a>

The bunt value for all auras is 0 except for `@da`.

```
> *@da
~2000.1.1
```

A bunt value of 0 is helpful when working with loobeans.