UIP-0128.md

uip	title	description	author	status	type	category	created
0128	Seer	A monadic scry interface.	~fodwyt-ragful, ~mastyr-bottec	Draft	Standards Track	Kernel, Hoon	2023-10-04

Abstract

Seer is a monadic scry interface which allows application developers to read from the namespace without using dotket (.^) or its virtual Nock operator 12. Once we remove Nock 12, all code in the system can take advantage of persistent memoization.

Motivation

dotket and Nock 12, which dotket compiles to, makes Nock not referentially transparent. By consequence, memoization is less effective.

In the interest of removing dotket and Nock 12 in the future, a replacement must first be supplied. The replacement should attempt to be as ergonomic as dotket.

Specification

Here's a Gall agent snippet that scries with .^:

++  on-poke
  |=  [m=mark v=vase]
  ^-  (quip card _this)
  =/  foo-result  .^  @da  /cx/(now)/foo
  ::  do something with foo-result...
  [~ this]

Here's the same Gall agent snippet that scries with seer in the continuation style:

++  gear         ::  gall seer
  |*  a=mold
  (seer vase a)
++  on-poke
  |=  [m=mark v=vase]
  ^-  (gear (quip card _this))  ::  (seer vase (quip card _this))
  :+  %scry  /cx/(now)/foo  |=  foo=vase
  =/  foo-result  !<(@ foo)
  ::  do something with foo-result...
  [%done ~ this]

Here's an example of composing seer values with monadic bind:

++  foo  ^-  (seer vase @)  !!  ::  /foo
++  bar  ^-  (seer vase @)  !!  ::  /bar
++  on-poke
  |=  [m=mark v=vase]
  =*  r  (quip card _this)
  ^-  (gear r)
  =/  bind  (rapt vase r)
  ;<  foo=@  bind  foo
  ;<  bar=@  bind  bar
  [%done ~ this(some-state (add foo bar))]

A seer is the type of programs that scry:

++  seer
  |*  [r=mold a=mold]
  $~  [%done *a]
  $%  [%done p=a]
      [%scry p=path k=$-(r (seer r a))]
  ==

See the reference implementation for the monadic bind (rapt).

Rationale

We believe seer degrades ergonomics only slightly while removing an ugly wart that has plagued Urbit since its birth.

12 destroys the referential transparency of Nock, making every computation dependent on a hidden context. This prevents transparent, system-wide persistent memoization (among other things).

Backwards Compatibility

The seer scry system is intentionally incompatible with the .^ scry system.

We will support both scry styles for some time before deprecating and eventually removing .^. In the meantime, we shall provide a wrapper library which accepts an agent written in the seer-style and produces an agent that scries in the .^-style.

Reference Implementation

urbit/urbit#6842

Security Considerations

A simpler system is a more secure system.

Copyright

Copyright and related rights waived via CC0.