path: root/lib/world.mli

(** Worlds: the store of relations, and the loader that builds the
    standalone one.

    Decision 4 of the integration document: the VM is parameterized
    over a world. This module gives the module type every host
    satisfies, the standalone in-memory world, and the loader that
    admits a parsed program into it. Admission is where totality is
    enforced: a program that gets a world is a program whose queries
    terminate. *)

(** A compiled relational expression: what rule bodies and queries
    are made of after names are resolved. *)
type expr =
  | Apply of string * Pattern.t Label.Map.t
  | And of expr * expr
  | Or of expr * expr

type rule = {
  rule_name : string;
  head : Pattern.t Label.Map.t;
  body : expr;
  witness : Label.t option;
      (** For a recursive rule, the label whose argument strictly
          decreases. The VM requires it ground before unfolding,
          which is what turns the admission proof into runtime
          termination. [None] on non-recursive rules. *)
}

(** The builtin domains a schema may constrain an attribute by. *)
type domain = D_integer | D_natural | D_rational | D_string

(** How a relation is defined. *)
type def =
  | Table of { schema : (Label.t * domain) list; facts : Term.t Label.Map.t list }
      (** Extensional: a schema and finite stored tuples. *)
  | Rules of rule list  (** Intensional: defined by admitted rules. *)

(** What a host must answer for the VM to run: which relation lives
    under a name. Builtins are the VM's own and are not consulted
    here. *)
module type WORLD = sig
  type t

  val find : t -> string -> def option
end

type error

val pp_error : Format.formatter -> error -> unit

(** The standalone world: an immutable catalog loaded from program
    text. This is Sol as a separate language; there are no branches,
    no transitions, only the pure fragment. *)
module Standalone : sig
  type t

  val find : t -> string -> def option

  val load : Ast.statement list -> (t * expr list, error) result
  (** Admits a program: registers relations, checks every fact
      against its schema and domains, compiles rules and queries,
      and enforces totality. Rejected, not deferred: a recursive
      rule without a structural decrease witness, a mutual
      recursion cycle, an unknown relation, or an ill-typed fact is
      an [error], and no world exists. Returns the world and the
      query expressions in program order. *)
end