Dynamic types for OCaml
Switch branches/tags
Pull request Compare This branch is 64 commits behind mirage:master.
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Failed to load latest commit information.


Dyntype is a syntax extension which makes OCaml types and values easier to manipulate programmatically. It consists of two parts, to manipulate types and values separately.


You can download the latest distribution from Github at http://github.com/mirage/dyntype. It depends on:

The library installs an ocamlfind META file, so use it with the dyntype.syntax package. To compile a file foo.ml with Dyntype and findlib, do:

ocamlfind ocamlopt -syntax camlp4o -package dyntype.syntax -c t.ml

To link it into a standalone executable:

ocamlfind ocamlopt -syntax camlp4o -linkpkg -package dyntype.syntax t.ml

You can report issues using the Github issue tracker at http://github.com/mirage/dyntype/issues, or mail the authors at mailto:mirage@recoil.org. If you use Dyntype somewhere, feel free to drop us a short line and we can add your project to the Wiki as well.

Dynamic Types

The type library converts an ML type definition annotated with the keyword type_of into a finite ML value representing that type and usable at runtime. For a given type t, the library will generate a value type_of_t of type Type.t defined as:

module Type = struct
  type t =
    | Unit | Bool | Float | Char | String
    | Int of int option
    | Arrow of t * t
    | Option of t
    | Enum of t
    | Tuple of t list
    | Dict of ( string * [`RW|`RO] * t ) list
    | Sum of ( string * t list ) list
    | Ext of string * t
    | Rec of string * t
    | Var of string

This is a simpler representation than the full syntax exposed by camlp4 (e.g. objects and records are coalesced into a Dict value).

The basic types are similar to the usual OCaml basic types, i.e. Bool, Float, Char and String, which can be composed using Arrow. Integers have an additional bit-width range parameter that can be 31-, 32-, 63- or 64-bit depending on the exact OCaml type and host architecture, or unlimited for BigInt types. These basic types can be composed to form either a Tuple, a record or an object with Dict, or a (polymorphic) variant with Sum.

For example, consider the following code fragment:

type tuple = int32 * string with type_of
type record = { mutable foo : string } with type_of
type variant = Foo | Bar of bool with type_of

This fragment will generate the following additional values:

let type_of_tuple = Ext ( "tuple", Tuple [ Int (Some 32); String ] )
let type_of_record = Ext ( "record", Dict [ ("foo", `RW, String) ] )
let type_of_variant = Ext ( "variant", Sum [ ("Foo", []) ; ("Bar", [Bool]) ] )

Types variables are handled by induction on the type structure in which they are used. Hence, the type definition...

type t = x option with type_of

...will generate the ML expression:

let type_of_t = Ext ( "t", Option type_of_x)

In this case, type_of_x has to be defined for the program to compile. This definition may have either been automatically generated previously by type_of, or have been defined by the user. The latter option makes the type_of library easily extensible, especially for abstract types.

Recursive types

Recursive types are handled carefully in order to always keep a finite representation of the ML type. This is done using the constructors Rec and Var. Rec(v, t) is the binding of the type variable v to the type expression t. Var v always appears in the scope of a corresponding Rec(v,t) and is equivalent to the substitution of Var v by t in t.

The following example shows the automatically generated code for simple recursive types:

(* User-defined datatype *)
type t = { x : x } and x = { t : t } with type_of

(* Auto-generated code *)
let type_of_t = Rec ( "t", Dict [ "x", Ext ( "x", Dict [ "t", Var "t"] ) ] )
let type_of_x = Rec ( "x", Dict [ "t", Ext ( "t", Dict [ "x", Var "x"] ) ] )

Dynamic Values

The purpose of the value library is to make runtime value introspection available in OCaml. It works on any ML type definition annotated with the keyword value a pair of functions which marshall/unmarshall any value of that type into a simpler and well-defined ML value. Hence, for a given type t, the library generates two functions value_of_t : t -> Value.t and t_of_value : Value.t -> t, where Value.t is defined as:

module Value = struct
  type t =
    | Int of int64 | Bool of bool | Float of float | String of string
    | Arrow of string
    | Enum of t list
    | Tuple of t list
    | Dict of (string * t) list
    | Sum of string * t list
    | Null
    | Value of t
    | Ext of (string * int64) * t
    | Rec of (string * int64) * t
    | Var of (string * int64)

Values whose type uses a type variable are built by induction on that type, as with the type library described earlier. For a type variable t, the user can add the keyword value to the type definition of t, and let the value library generates the value_of_t and t_of_value functions. In the following example, value_of_x and x_of_value might either be auto-generated from the definition of t or be user-defined:

(* User-defined datatype *)
type t = x option with value

(* Auto-generated code *)
let value_of_t = function
  | None -> Ext (("t", 0), Null)
  | Some x -> Ext (("t", 0), Value (value_of_x x))

let t_of_value = function
  | Ext(("t", _), Null) -> None
  | Ext(("t", _), Value x) -> Some (x_of_value x)
  | _ -> failwith "runtime error"