depyt 0.3.0 · OCaml Package

Yet-an-other type combinator library

Depyt provides type combinators to define runtime representation for OCaml types and generic operations to manipulate values with a runtime type representation.

The type combinators supports all the usual type primitives but also compact definitions of records and variants. It also allows to define the runtime representation of recursive types.

Depyt is a modern reboot of Dyntype but using GADTs-based combinators instead of syntax-extensions. When we originally wrote Dyntype (in 2012) GADTs were not available in OCaml and camlp4 was everywhere -- this is not the case anymore. Finally, Depyt avoids some of the performance caveats present in Dyntype by avoiding allocating and converting between intermediate formats.

0.3.0 — homepage

Depyt

type 'a t

The type for runtime representation of values of type 'a.

Primitives

val unit : unit t

unit is a representation of the unit type.

val bool : bool t

bool is a representation of the boolean type.

val char : char t

char is a representation of the character type.

val int : int t

int is a representation of the integer type.

val int32 : int32 t

int32 is a representation of the 32-bit integers type.

val int64 : int64 t

int64 is a representation of the 64-bit integer type.

val float : float t

float is a representation of the float type.

val string : string t

string is a representation of the string type.

val list : 'a t -> 'a list t

list t is a representation of list of values of type t.

val array : 'a t -> 'a array t

array t is a representation of array of values of type t.

val option : 'a t -> 'a option t

option t is a representation of value of type t option.

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

pair x y is a representation of values of type x * y.

val triple : 'a t -> 'b t -> 'c t -> ('a * 'b * 'c) t

triple x y z is a representation of values of type x * y * z.

Records

type ('a, 'b) field

The type for fields holding values of type 'b and belonging to a record of type 'a.

val field : string -> 'a t -> ('b -> 'a) -> ('b, 'a) field

field n t g is the representation of the field n of type t with getter g.

For instance:

type t = { foo: string option }

let foo = field "foo" (option string) (fun t -> t.x)

type ('a, 'b, 'c) open_record

The type for representing open records of type 'a with constructors of type 'b. 'c represents the fields missings to the record, e.g. an open record initially holds 'c = 'b and it can can be sealed when 'c = 'a.

val sealr : ('a, 'b, 'a) open_record -> 'a t

sealr r seal the open record r.

val (|+) : 
  ('a, 'b, 'c -> 'd) open_record ->
  ('a, 'c) field ->
  ('a, 'b, 'd) open_record

r |+ f adds the field f to the open record r.

val record : string -> 'b -> ('a, 'b, 'b) open_record

record n f fs is the representation of the record called n of type 'a using f as constructor and with the fields fs.

Putting all together:

type t = { foo: string; bar = (int * string) list; }

let t =
  record "t" (fun foo -> { foo })
  |+ field "foo" string (fun t -> t.foo)
  |+ field "bar" (list (pair int string)) (fun t -> t.bar)
  |> sealr

Variants

type ('a, 'b) case

The type for representing variant cases of type 'a with patterns of type 'b.

type 'a case_p

The type for representing patterns for a variant of type 'a.

val case0 : string -> 'a -> ('a, 'a case_p) case

case0 n v is a representation of a variant case n with no argument and a singleton pattern. e.g.

type t = Foo

let foo = case0 "Foo" Foo

val case1 : string -> 'b t -> ('b -> 'a) -> ('a, 'b -> 'a case_p) case

case1 n t c is a representation of a variant case n with 1 argument of type t and a pattern c an function with one argument of type t. e.g.

type t = Foo of string

let foo = case1 "Foo" string (fun s -> Foo s)

type ('a, 'b, 'c) open_variant

The type for representing open variants of type 'a with pattern matching of type 'b. 'c represents the missing cases for the variant, e.g. initially variant hols c' = 'b and it can be sealed when 'c = 'a.

val sealv : ('a, 'b, 'a -> 'a case_p) open_variant -> 'a t

sealv v seals the open variant v.

val (|~) : 
  ('a, 'b, 'c -> 'd) open_variant ->
  ('a, 'c) case ->
  ('a, 'b, 'd) open_variant

v |~ c is v augmented with the case c.

val variant : string -> 'b -> ('a, 'b, 'b) open_variant

variant n c p is a representation of a variant type containing the cases c and using p to deconstruct values.

Putting all together:

type t = Foo | Bar of string

let t =
  variant "t" (fun foo bar -> function
    | Foo   -> foo
    | Bar s -> bar s)
  |~ case0 "Foo" Foo
  |~ case1 "Bar" string (fun x -> Bar x)
  |> sealr

val enum : string -> (string * 'a) list -> 'a t

enum n l is a representation of the variant type which has only constant variant case. e.g.

type t = Foo | Bar | Toto

let t = enum "t" ["Foo", Foo; "Bar", Bar; "Toto", Toto]

Recursive definitions

Depyt allows to create a limited form of recursive records and variants.

TODO: describe the limitations, e.g. only regular recursion and no use of the generics inside the mu* functions and the usual caveats with recursive values (such as infinite loops on most of the generics which don't check sharing).

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

mu f is the representation r such that r = mu r.

For instance:

type x = { x: x option }

let x = mu (fun x ->
    record "x" (fun x -> { x })
    |+ field "x" x (fun x -> x.x)
    |> sealr)

val mu2 : ('a t -> 'b t -> 'a t * 'b t) -> 'a t * 'b t

mu2 f is the representations r and s such that r, s = mu2 r s.

For instance:

type r = { foo: int; bar: string list; z: z option }
and z = { x: int; r: r list }

(* Build the representation of [r] knowing [z]'s. *)
let mkr z =
  record "r" (fun foo bar z -> { foo; bar; z })
  |+ field "foo" int (fun t -> t.foo)
  |+ field "bar" (list string) (fun t -> t.bar)
  |+ field "z" (option z) (fun t -> t.z)
  |> sealr

(* And the representation of [z] knowing [r]'s. *)
let mkz r =
  record "z" (fun x r -> { x; r })
  |+ field "x" int (fun t -> t.x)
  |+ field "r" (list r) (fun t -> t.r)
  |> sealr

(* Tie the loop. *)
let r, z = mu2 (fun r z -> mkr z, mkz y)

Bijections

Sometimes it is not always possible to describe precisely a type (or it could be too tedious) and it is easier to describe the relation with an other know type. This is what bijections are about.

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

like x f g is the description of a type which looks like x using the bijetion (f, g).

Generic Operations

Given a value 'a t, it is possible to define generic operations on value of type 'a such as pretty-printing, parsing and unparsing.

val dump : 'a t -> 'a Fmt.t

dump t dumps the values of type t as a parsable OCaml expression.

val equal : 'a t -> 'a -> 'a -> bool

equal t is the equality function between values of type t.

val compare : 'a t -> 'a -> 'a -> int

compare t compares values of type t.

Binary serialization

type buffer =

| C of Cstruct.t
| B of bytes

The type for buffers.

val size_of : 'a t -> 'a -> int

size_of t is the size needed to serialize values of type t.

val write : 'a t -> buffer -> pos:int -> 'a -> int

write t serializes values of type t. Use size_of to pre-determine the size of the buffer.

val read : 'a t -> buffer -> pos:int -> int * 'a

read t reads a serialization of a value of type t.

JSON converters

val pp_json : ?minify:bool -> 'a t -> 'a Fmt.t

Similar to dump but pretty-prints the JSON representation instead of the OCaml one. See encode_json for details about the encoding.

For instance:

type t = { foo: int option; bar: string list };;

let t =
  record "r" (fun foo bar -> { foo; bar })
  |+ field "foo" (option int) (fun t -> t.foo)
  |+ field "bar" (list string) (fun t -> t.bar)
  |> sealr

let s = Fmt.str "%a\n" (pp t) { foo = None; bar = ["foo"] }
(* s is "{ foo = None; bar = [\"foo\"]; }" *)

let j = Fmt.str "%a\n" (pp_json t) { foo = None; bar = ["foo"] }
(* j is "{ \"bar\":[\"foo\"] }" *)

NOTE: this will automatically convert JSON fragments to valid JSON objects by adding an enclosing array if necessary.

val encode_json : 'a t -> Jsonm.encoder -> 'a -> unit

encode_json t e encodes t into the jsonm encoder e. The encoding is a relatively straightforward translation of the OCaml structure into JSON. The main highlights are:

OCaml ints are translated into JSON floats.
OCaml strings are translated into JSON strings. You must then ensure that the OCaml strings contains only valid UTF-8 characters.
OCaml record fields of type 'a option are automatically unboxed in their JSON representation. If the value if None, the field is removed from the JSON object.
variant cases built using case0 are represented as strings.
variant cases built using case1 are represented as a record with one field; the field name is the name of the variant.

NOTE: this can be used to encode JSON fragments. That's the responsibility of the caller to ensure that the encoded JSON fragment fits properly into a well-formed JSON object.

val decode_json : 'a t -> Jsonm.decoder -> ('a, string) Result.t

decode_json t e decodes values of type t from the jsonm decoder e.

val decode_json_lexemes : 'a t -> Jsonm.lexeme list -> ('a, string) Result.t

decode_json_lexemes is similar to decode_json but use an already decoded list of JSON lexemes instead of a decoder.

package depyt

Depyt

Primitives

Records

Variants

Recursive definitions

Bijections

Generic Operations

Binary serialization

JSON converters