Seer: A Monadic Scry Interface

UIPS/SEER.md

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+---
+title: Seer
+description: A monadic scry interface.
+author: ~fodwyt-ragful, ~mastyr-bottec
+status: Draft
+type: Standards Track
+category: Kernel, Hoon
+created: 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 `.^`:
+```hoon
+++  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:
+```hoon
+++  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:
+```hoon
+++  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:
+```hoon
+++  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
+
+https://github.com/urbit/urbit/pull/6842
+
+## Security Considerations
+
+A simpler system is a more secure system.
+
+## Copyright
+
+Copyright and related rights waived via [CC0](../LICENSE.md).