wing.md

description	Reference for wings in Hoon, which are lists of limbs forming search paths into the subject to access nested data and computations.
layout	title description tableOfContents outline pagination visible true visible visible true visible true visible true

Wings

[%wing p=(list limb)]

A wing is a limb search path into the subject.

Produces {#produces}

A wing is a list of limbs (including a trivial list of one limb). The limbs are resolved in succession. The result of the last limb resolution is the value produced by the wing expression.

Syntax {#syntax}

Irregular: a.b.c. Read this as 'a in b in c'. Finds limb a within limb b within limb c of the subject.

Discussion {#discussion}

Intuitively, Hoon wings are written in the opposite order from attribute dot-paths in most languages. Hoon a.b.c is Java's c.b.a; it means "a within b within c."

Any item in the wing can resolve to a leg (fragment) or arm (computation). But if a non-terminal item in the wing would resolve to an arm, it resolves instead to the subject of the arm -- in other words, the core exporting that name.

The mysterious idiom ..b produces the leg .b if .b is a leg; the core exporting .b if .b is an arm. Since . is the same limb as +, ..b is the same wing as +1.foo.

Examples {#examples}

~zod:dojo> =a [fod=3 bat=[baz=1 moo=2]]

~zod:dojo> bat.a
[baz=1 moo=2]

~zod:dojo> moo.bat.a
2

Wing Resolution {#wing-resolution}

There are two common syntaxes used to resolve a wing path into the current subject: . dot and : col.

• . dot syntax, as c.b.a, resolves the wing path into the subject at the right hand using Nock 0 (or possibly Nock 9 or Nock 10 depending on the expression).

  > !,(*hoon c.b.a)
  [%wing p=~[%c %b %a]]

• The : col operator expands to a => tisgar to resolve the wing path against its right-hand side as the subject. This can be a Nock 7 or possibly optimized by the compiler to a Nock 0.

  > !,(*hoon c:b:a)
  [%tsgl p=[%wing p=~[%c]] q=[%tsgl p=[%wing p=~[%b]] q=[%wing p=~[%a]]]]