package ogre

  1. Overview
  2. Docs

OGRE - Open Generic Representation.

OGRE is a self-describing data storage. It is open for extensibility, i.e., adding new types of knowledge doesn't break the storage. It also has a well-specified open representation, so that any tool written in any language (or even a human itself) can create, modify and understand the contents (like XML). Ogre provides data persitance and, more importantly, a type safe way of querying and updating the data. The query language is rich enough, and supports joins and boolean constraints

It can be seen as a document NoSQL database engine. As a backing storage Ogre uses S-Expressions, and the structure of a document is close to JSON. In fact it is a restricted subset of JSON, where only scalar values are allowed.

A database, called a "document" (or just "doc") in Ogre parlance, is a set of facts. Each fact is described with a proposition having the following syntax,

(<attribute-name> <v1> <v2> ... <vM> )

where <attribute-name> is a name of a proposition and <vN> is the value of N'th object (or subject) of a proposition. For example,

(student (name Joe) (gpa 3.5))

is a proposition that a student named Joe has a GPA rate 3.5. Thus a proposition is a tuple with named fields. All propositions must be well-typed, so the predicate student should be declared before used. The field values maybe stored (and are by default) without the names in the order in which they are specified in the declaration, e.g., the following definition is equivalent to the previous one:

(student Joe 3.5)

Given, that the predicate is declared as:

(declare student (name str) (gpa float))

where the declaration has the following syntax:

     declaration ::= ( declare <attribute-name> <field> <field> ... )
     field ::= ( <field-name> <field-type> )
     field-type ::= int | str | bool | float

Each declaration declare an attribute, that defines a type of the propositions. Unlike the SQL, we denote each tuple type with the word attribute, as under our model each document describes some knowledge (an attribute) about some abstract entity. For example, a document "college.ogre" that contains definitions of attributes named student, teacher, class, assignments is a set of knowledge about a college. Thus an attribute maps to a SQL notion of table (or a relvar). Correspondingly, a column of a table (that is usually referred as an attribute in the relational model), maps to Ogre's field.

type doc

the document

type ('a, 'k) typeinfo constraint 'k = _ -> _

type information associated with an attribute

type ('a, 'k) attribute = unit -> ('a, 'k) typeinfo

a descriptor of an attribute.

Used to construct attribute values, and to query documents. Created with declare function.

Note, that due to a value restriction, an attribute should be defined as a function returning a type information.

type 'a field

t field a descriptor of an attribute field.

Used to construct attributes, and to construct variables that reference particular fields of an attribute.

the type variable t range is float, int64, string or bool.

type 'a query

attrs query constructs a query type.

Created using the Query module. The attrs type variable encodes the types of requested attributes. For example,

((student -> teacher -> 'a) -> 'a) query

represents a query for two attributes of type student and teacher correspondingly. It is represented as a continuation, denoting the fact, that the query can be executed later for an arbitrary result.

type 'a seq = 'a Core_kernel.Sequence.t

a result of a selection.

type ('f, 'k) scheme constraint 'f = _ -> _ constraint 'k = _ -> _

type that describes an attribute.

The two type variables describe the constructor and destructor interface. The 'a variable, the accessor, describes how an attribute can be constructed. The 's variable, describes how an attribute can be packed in the database. These two types come along and differ only in a return type. The general form of a type variable is ('a -> 'r) -> 'r, where 'r is the return type (a type of attribute for instance), and 'a variable is extended every time a new field is added to a scheme.

val declare : name:string -> ('f -> 'a, 'k) scheme -> 'f -> ('a, 'k) typeinfo

let attr () = declare ~name scheme declares an attribute named with name, and having a type described by the scheme.

Due to a value restriction, each attribute should be defined as a thunk (a function).

module Type : sig ... end

Ogre type system.

module Query : sig ... end

Domain specific language for constructing queries.

module Doc : sig ... end

An Ogre document.

module type S = sig ... end

Monadic interface to the document.

module Make (M : Monads.Std.Monad.S) : S with type 'a m := 'a M.t

Make(M) returns an Ogre monad implementation wrapped in a monad M.

Default implementation of the Orge monad, that is not wrapped into any other monads (in other words, that is wrapped into the identity)

include S with type 'a m = 'a and type 'a t = 'a Make(Monads.Std.Monad.Ident).t and type 'a e = doc -> ('a * doc) Core_kernel.Or_error.t
include Monads.Std.Monad.S with type 'a t = 'a Make(Monads.Std.Monad.Ident).t
val void : 'a t -> unit t

void m computes m and discrards the result.

val sequence : unit t list -> unit t

sequence xs computes a sequence of computations xs in the left to right order.

val forever : 'a t -> 'b t

forever xs creates a computationt that never returns.

module Fn : sig ... end

Various function combinators lifted into the Kleisli category.

module Pair : sig ... end

The pair interface lifted into the monad.

module Triple : sig ... end

The triple interface lifted into a monad.

module Lift : sig ... end

Lifts functions into the monad.

module Exn : sig ... end

Interacting between monads and language exceptions

module Collection : sig ... end

Lifts collection interface into the monad.

module List : Collection.S with type 'a t := 'a list

The Monad.Collection.S interface for lists

module Seq : Collection.S with type 'a t := 'a Core_kernel.Sequence.t

The Monad.Collection.S interface for sequences

include Monads.Std.Monad.Syntax.S with type 'a t := 'a t
val (>=>) : ('a -> 'b t) -> ('b -> 'c t) -> 'a -> 'c t

f >=> g is fun x -> f x >>= g

val (!!) : 'a -> 'a t

!!x is return x

val (!$) : ('a -> 'b) -> 'a t -> 'b t

!$f is Lift.unary f

val (!$$) : ('a -> 'b -> 'c) -> 'a t -> 'b t -> 'c t

!$$f is Lift.binary f

val (!$$$) : ('a -> 'b -> 'c -> 'd) -> 'a t -> 'b t -> 'c t -> 'd t

!$$$f is Lift.ternary f

val (!$$$$) : ('a -> 'b -> 'c -> 'd -> 'e) -> 'a t -> 'b t -> 'c t -> 'd t -> 'e t

!$$$$f is Lift.quaternary f

val (!$$$$$) : ('a -> 'b -> 'c -> 'd -> 'e -> 'f) -> 'a t -> 'b t -> 'c t -> 'd t -> 'e t -> 'f t

!$$$$$f is Lift.quinary f

include Monads.Std.Monad.Syntax.Let.S with type 'a t := 'a t
val let* : 'a t -> ('a -> 'b t) -> 'b t

let* r = f x in b is f x >>= fun r -> b

val and* : 'a t -> 'b t -> ('a * 'b) t

monoidal product

val let+ : 'a t -> ('a -> 'b) -> 'b t

let+ r = f x in b is f x >>| fun r -> b

val and+ : 'a t -> 'b t -> ('a * 'b) t

monoidal product

include Core_kernel.Monad.S with type 'a t := 'a t
val (>>=) : 'a t -> ('a -> 'b t) -> 'b t

t >>= f returns a computation that sequences the computations represented by two monad elements. The resulting computation first does t to yield a value v, and then runs the computation returned by f v.

val (>>|) : 'a t -> ('a -> 'b) -> 'b t

t >>| f is t >>= (fun a -> return (f a)).

module Monad_infix : sig ... end
val bind : 'a t -> f:('a -> 'b t) -> 'b t

bind t ~f = t >>= f

val return : 'a -> 'a t

return v returns the (trivial) computation that returns v.

val map : 'a t -> f:('a -> 'b) -> 'b t

map t ~f is t >>| f.

val join : 'a t t -> 'a t

join t is t >>= (fun t' -> t').

val ignore_m : 'a t -> unit t

ignore_m t is map t ~f:(fun _ -> ()). ignore_m used to be called ignore, but we decided that was a bad name, because it shadowed the widely used Caml.ignore. Some monads still do let ignore = ignore_m for historical reasons.

val all : 'a t list -> 'a list t
val all_unit : unit t list -> unit t

Like all, but ensures that every monadic value in the list produces a unit value, all of which are discarded rather than being collected into a list.

module Let_syntax : sig ... end

These are convenient to have in scope when programming with a monad:

module Let : Monads.Std.Monad.Syntax.Let.S with type 'a t := 'a t

Monadic operators, see Monad.Syntax.S for more.

module Syntax : Monads.Std.Monad.Syntax.S with type 'a t := 'a t

Monadic operators, see Monad.Syntax.S for more.

include Monads.Std.Monad.Trans.S with type 'a t := 'a t with type 'a m = 'a with type 'a e = doc -> ('a * doc) Core_kernel.Or_error.t
type 'a m = 'a
type 'a e = doc -> ('a * doc) Core_kernel.Or_error.t
val lift : 'a m -> 'a t

lifts inner monad into the resulting monad

val require : ?that:('a -> bool) -> ('a, _) attribute -> 'a t

require a ~that:p requires that an attribute a has one and only one value that satisfies a predicate p. It is an error, if there are no such values, or if there are more than one value.

val request : ?that:('a -> bool) -> ('a, _) attribute -> 'a option t

request a ~that:p request no more than one value of an attribute a, that satisfies a predicate p. The returned value is wrapped in an option. If there are more than one satisfying value, then it is an error.

val foreach : ('a -> 'b) query -> f:'a -> 'b seq t

foreach query ~f:action applies an action for each value of an attributes specified in the query. The query value is built using a domain specific language embedded into OCaml. This language is very similar to SQL, and has join and where clauses, e.g.,

let better_than_average_students =
  foreach Query.(begin
        ~where:(students.(gpa) > float 3.5)
          [field classid];
            field teacher ~from:students;
            field id ~from:teachers
        (from students $ teachers)
    ~f:(fun s t -> return (s,t))

The type of the query value encodes the type of the function f. A well formed query has a type of form (t1 -> t2 -> .. -> tm -> 'a t) -> 'a t, where t1 till tm are types of attributes enumerated in the from clause (in that particular order).

See the Query module documentation for more information about the query EDSL.

val collect : (('a -> 'a) -> 'b) query -> 'b seq t

collect query is the same as foreach query

val provide : (_, 'a -> unit t) attribute -> 'a

provide attr v1 v2 ... vm stores the constituents of an attribute value in the document. An attribute type encodes not only the type of an attribute value, but also a type and the order of the fields. Thus, the attribute itself captures a format of the attribute representation, the same as format is used in printf-like functions. In that sense, the provide function is variadic, where the first argument (the attribute) defines the type and the arity of the function.

val fail : Core_kernel.Error.t -> 'a t

fail error aborts an inference process with the specified error.

val failf : ('a, Format.formatter, unit, unit -> 'b t) Core_kernel.format4 -> 'a

failf fmt args... () constructs an error based on the specified format fmt and arguments, terminated by the unit value (). Example:

failf "the file type %s is unsupported" name ()

Note: don't forget to terminate a sequence of arguments with an extra unit value. See the corresponding invalid_argf and failwithf function for the reason, why this extra argument is needed.

val eval : 'a t -> doc -> 'a Core_kernel.Or_error.t m

eval property document makes an inference of a property based on facts stored in a document. If all requirements are satisfied and no errors occurred the inferred result.

For example, given the property names_of_best_students, defined as,

let names_of_best_students =
  foreach Query.(select (from students)
                   ~where:(students.(gpa) > float 3.8))
    ~f:(fun s -> return ( s))

we can evaluate this property, with

eval names_of_best_students

to get a sequence (possibly empty) of all students that have the GPA score greater than 3.8.

val exec : 'a t -> doc -> doc Core_kernel.Or_error.t m

exec op doc executes an operation op that, presumably, updates the document doc, returns an updated version.

val run : 'a t -> doc -> ('a * doc) Core_kernel.Or_error.t m

run op doc runs an operation op that does some inference as well as may update the document. This function is a usual part of a generic state monad interface, and is provided for the consistency. Usually, it is a bad idea, or a notion of a bad style to use this function.