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Yet-an-other type combinator library

Type 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 the definition of run-time representations of recursive types.

Type Combinators

type 'a t

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

type len = [
  1. | `Int
  2. | `Int8
  3. | `Int16
  4. | `Int32
  5. | `Int64
  6. | `Fixed of int
]

The type of integer used to store buffers, list or array lengths.

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 integers. Binary serialization uses a varying-width representation.

val int32 : int32 t

int32 is a representation of the 32-bit integer 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 bytes : bytes t

bytes is a representation of the bytes type.

val string_of : len -> string t

Like string but with a given fixed size.

val bytes_of : len -> bytes t

Like bytes but with a given fixed size.

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

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

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

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

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

option t is a representation of values 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.

val result : 'a t -> 'b t -> ('a, 'b) result t

result a b is a representation of values of type (a, b) result.

Records

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

The type for representing open records of type 'a with a constructor of type 'b. 'c represents the remaining fields to be described using the (|+) operator. An open record initially satisfies 'c = 'b and can be sealed once 'c = 'a.

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

record n f is an incomplete representation of the record called n of type 'a with constructor f. To complete the representation, add fields with (|+) and then seal the record with sealr.

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 manuscript = { title : string option }

let manuscript = field "title" (option string) (fun t -> t.title)
val (|+) : ('a, 'b, 'c -> 'd) open_record -> ('a, 'c) field -> ('a, 'b, 'd) open_record

r |+ f is the open record r augmented with the field f.

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

sealr r seals the open record r.

Putting all together:

type menu = { restaurant : string; items : (string * int32) list }

let t =
  record "t" (fun restaurant items -> { restaurant; items })
  |+ field "restaurant" string (fun t -> t.restaurant)
  |+ field "items" (list (pair string int32)) (fun t -> t.items)
  |> sealr

Variants

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

The type for representing open variants of type 'a with pattern matching of type 'b. 'c represents the remaining constructors to be described using the (|~) operator. An open variant initially satisfies c' = 'b and can be sealed once 'c = 'a.

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

variant n p is an incomplete representation of the variant type called n of type 'a using p to deconstruct values. To complete the representation, add cases with (|~) and then seal the variant with sealv.

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 constructor v with no arguments and name n. 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 constructor c with an argument of type t and name n. e.g.

type t = Foo of string

let foo = case1 "Foo" string (fun s -> Foo s)
val (|~) : ('a, 'b, 'c -> 'd) open_variant -> ('a, 'c) case -> ('a, 'b, 'd) open_variant

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

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

sealv v seals the open variant v.

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

enum n cs is a representation of the variant type called n with singleton cases cs. e.g.

type t = Foo | Bar | Toto

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

Recursive definitions

Type allows a limited description 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))

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 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.

val short_hash : 'a t -> ?seed:int -> 'a -> int

hash t x is a short hash of x of type t.

type 'a pp = 'a Fmt.t

The type for pretty-printers.

type 'a of_string = string -> ('a, [ `Msg of string ]) result

The type for parsers.

val pp : 'a t -> 'a pp

pp t is the pretty-printer for values of type t.

val pp_ty : 'a t pp

The pretty printer for generics of type t.

val to_string : 'a t -> 'a -> string

to_string t is Fmt.to_to_string (pp t).

val of_string : 'a t -> 'a of_string

of_string t parses values of type t.

JSON converters

module Json : sig ... end

Overlay on top of Jsonm to work with rewindable streams.

type 'a encode_json = Jsonm.encoder -> 'a -> unit

The type for JSON encoders.

type 'a decode_json = Json.decoder -> ('a, [ `Msg of string ]) result

The type for JSON decoders.

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.strf "%a\n" (pp t) { foo = None; bar = [ "foo" ] }

(* s is "{ foo = None; bar = [\"foo\"]; }" *)

let j = Fmt.strf "%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. It'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, [ `Msg of string ]) result

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

val decode_json_lexemes : 'a t -> Jsonm.lexeme list -> ('a, [ `Msg of string ]) result

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

val to_json_string : ?minify:bool -> 'a t -> 'a -> string

to_json_string is encode_json with a string encoder.

val of_json_string : 'a t -> string -> ('a, [ `Msg of string ]) result

of_json_string is decode_json with a string decoder .

Binary Converters

type 'a bin_seq = 'a -> (string -> unit) -> unit
type 'a encode_bin = ?headers:bool -> 'a bin_seq

The type for binary encoders. If headers is not set, do not output extra length headers for buffers.

type 'a decode_bin = ?headers:bool -> string -> int -> int * 'a

The type for binary decoders. IF headers is not set, do not read extra length header for buffers and consider the whole buffer instead.

type 'a size_of = ?headers:bool -> 'a -> int option

The type for size function related to binary encoder/decoders.

val pre_hash : 'a t -> 'a bin_seq

pre_hash t x is the string representation of x, of type t, which will be used to compute the digest of the value. By default it's to_bin_string t x but it can be overriden by v, like and map operators.

val encode_bin : 'a t -> 'a encode_bin

encode_bin t is the binary encoder for values of type t.

val decode_bin : 'a t -> 'a decode_bin

decode_bin t is the binary decoder for values of type t.

val to_bin_string : 'a t -> 'a -> string

to_bin_string t x use encode_bin to convert x, of type t, to a string.

NOTE: When t is Type.string or Type.bytes, the original buffer x is not prefixed by its size as encode_bin would do. If t is Type.string, the result is x (without copy).

val of_bin_string : 'a t -> string -> ('a, [ `Msg of string ]) result

of_bin_string t s is v such that s = to_bin_string t v.

NOTE: When t is Type.string, the result is s (without copy).

val size_of : 'a t -> 'a size_of

size_of t x is either the size of encode_bin t x or the binary encoding of x, if the backend is not able to pre-compute serialisation lengths.

Customs converters

val v : cli:('a pp * 'a of_string) -> json:('a encode_json * 'a decode_json) -> bin:('a encode_bin * 'a decode_bin * 'a size_of) -> equal:('a -> 'a -> bool) -> compare:('a -> 'a -> int) -> short_hash:(?seed:int -> 'a -> int) -> pre_hash:'a bin_seq -> 'a t
val like : ?cli:('a pp * 'a of_string) -> ?json:('a encode_json * 'a decode_json) -> ?bin:('a encode_bin * 'a decode_bin * 'a size_of) -> ?equal:('a -> 'a -> bool) -> ?compare:('a -> 'a -> int) -> ?short_hash:('a -> int) -> ?pre_hash:'a bin_seq -> 'a t -> 'a t
val map : ?cli:('a pp * 'a of_string) -> ?json:('a encode_json * 'a decode_json) -> ?bin:('a encode_bin * 'a decode_bin * 'a size_of) -> ?equal:('a -> 'a -> bool) -> ?compare:('a -> 'a -> int) -> ?short_hash:('a -> int) -> ?pre_hash:'a bin_seq -> 'b t -> ('b -> 'a) -> ('a -> 'b) -> 'a t
type 'a ty = 'a t
module type S = sig ... end
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