Urbit / Docs

Limb Expressions

A limb is an attribute of subject.


There are two kinds of limbs: arms and legs. An arm is a computation of some core. A leg is a piece of data in the subject.

If a limb expression resolves to a leg, the leg is produced. If a limb expression resolves to an arm -- in particular, by way of an arm name -- then the arm is computed with its parent core as the subject. The result of that computation is produced.


Irregular: +15 is slot 15. The value at address 15 of the subject is produced.

Irregular: . is slot 1, the whole subject.

Irregular: &4 is slot 30, the 4th element of a tuple with at least 5 elements.

Irregular: |4 is slot 31, the elements that follow &4.

Irregular: abc is a name that can resolve either to an arm or a leg of the subject.

Irregular: ^^^abc is the name abc, but which will skip the first three name matches in the subject.

Irregular: +<- is “take the tail, then take the head of that, then the head of that.” + and > mean “tail” while - and < mean “head.” This limb syntax starts on + or - and alternates with > or < for readability.


Name resolution happens by way of a search through the subject. The search traverse takes a name q and a skip count p.

The search product may be an arm or a leg. A leg is a subtree of the subject. An arm is a Nock formula paired with a core to compute the result. You can think of the limb as an attribute -- computation or subtree, “synthesized” or “natural” -- of the subject.

We search the subject type headfirst, depth-first, and pre-order. If we descend to a %face type whose label matches the limb symbol, the descent address is a leg. The type is the type beneath the label. But if the %face label differs, the search skips this whole subtree.

If we descend into a %core type in which the limb symbol is an arm, we produce that arm. If the limb symbol is not found, we descend into the payload (data) of the core.

If the skip count p is nonzero, we pretend our first p matches are actually mismatches. This lets the programmer “look through” an overriding label.


The Dojo prompt gives you a subject with a decent namespace. Try:

> now

You can also add variables to the subject, then use them. (Note that =variable value is not Hoon (language) syntax; it's Dojo (shell) syntax.)

> =a 12

> a