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Diffstat (limited to '')
| -rw-r--r-- | docs/evaluators.org | 66 |
1 files changed, 26 insertions, 40 deletions
diff --git a/docs/evaluators.org b/docs/evaluators.org index 84bc493..b7ce3b8 100644 --- a/docs/evaluators.org +++ b/docs/evaluators.org @@ -1,7 +1,7 @@ #+TITLE: Evaluators * Position -RNT should supply objects, storage, transactions, and execution +RNT should supply objects, storage, and execution lifecycle, but not directly a sublanguage. That duty is then on the clients to implement one and register it, using the kernel's capabilities. @@ -25,17 +25,16 @@ get back B and C. * Protocols An evaluator is an object like any other, that can be used to execute -program with a context, exactly as an interpreter takes an expression -and an environment. The plan tree should already bear every value the -program needs, so there is nothing left to bind. Each invocation -carries independent state based atop of a database view of the world +a program, exactly as an interpreter takes an expression and an +environment. The plan tree should already bear every value and handle +the program needs, so there is nothing left to bind. Each invocation +carries independent state based atop of the handles it was given (being therefore hash dependent and isolated). #+begin_src ocaml module Make (P : PROGRAM) : sig class type implementation = object - method invoke : - program:P.t -> Context.t -> (Handle.t, condition) result + method invoke : P.t -> (Handle.t, condition) result end end #+end_src @@ -50,13 +49,12 @@ intends to speak to. The FOL evaluator, which lives under understands a single node: #+begin_src ocaml -type plan = Base of string +type plan = Base of Handle.t #+end_src -=Base= names a relation, which is resolved through the context and -enumerated to exhaustion. The name is resolved rather than an address -being followed, which keeps the evaluator free of storage and makes -every resolution happen under one snapshot. +=Base= carries the handle of a relation, which is enumerated to +exhaustion. The evaluator only invokes protocols on the handles it +receives, which keeps it free of storage and of name resolution. The returned handle carries a result protocol with =Protocols.Cursor=, giving it incremental consumption. Since executing a plan node yields @@ -69,38 +67,26 @@ sublanguages. That is not to say however that we cannot have a builtin catalog relation installed and configured by the client that enumerates them (that's a concern for the future). -* Context -=Protocols.Context.t= supplies the selected database state, the root -handle through which names are resolved (which assumes authorization, -since we are a capability based system), and the status of the -evaluation. Evaluators reach kernel resources *only* through it. +* Handles +There is no evaluation context. Since we are a capability based +system, holding a handle is what authorizes its use, so the caller +obtains every handle a program needs beforehand (through the +directories it was granted) and passes them in. Evaluators reach +kernel resources *only* through those handles. A program that refers +to things by name carries the directory they are resolved in, as the +lambda evaluator does: #+begin_src ocaml -type t = - { snapshot : Concepts.Hash.hash - ; root : Handle.t - ; status : unit -> [`Live | `Cancelled | `Exhausted] } +type program = Program of term * Directory.t Handle.interface #+end_src -The snapshot is the hash of a commit. Since every object is content -addressed and prolly trees do not depend on insertion order, equal -state yields an equal hash, so a commit hash is a frozen world rather -than a pointer into a changing one. A cursor therefore pins a hash -instead of an open transaction, and every node of a plan reads the -same state, which gives repeatable reads across a whole plan for free -and makes concurrent commits simply invisible to a running -evaluation. - -=status= collapses cancellation and resource exhaustion into the -single question an operator asks between tuples, namely whether it may -produce another tuple. The runtime owns how =Exhausted= is decided. A -nested invocation shares the snapshot, so nesting does not need to -resolve a branch tip again, and a successful evaluation and cursor -exhaustion never commit. Commit should be an explicit host operation. - -Budgets, authority narrowing, and resource accounting are not -implemented, and adding a fourth status case would break exhaustive -matches in consumers, which is the price of keeping this closed. +Since every object is content addressed and prolly trees do not +depend on insertion order, equal state yields an equal hash, so the +relation behind a handle does not change under a running evaluation. +A successful evaluation and cursor exhaustion never commit. Commit +should be an explicit host operation. + +Cancellation, budgets, and resource accounting are not implemented. * Composition Evaluators compose through invocation and result protocols alone. A |