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.
Table of Auras
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@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@uv unsigned base32 0v1df64.49beg@uw unsigned base64 0wbnC.8haTg@ux unsigned hexadecimal 0x5f5.e138
Bitwidth
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 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
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.