RFC6901 JSON Pointer implementation in OCaml using jsont
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doc/dune
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doc/dune
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doc/tutorial.md
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doc/tutorial.md
···
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Each reference token becomes an `Index.t` value in the library:
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<!-- $MDX skip -->
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```ocaml
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type t =
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| Mem of string (* Object member access *)
···
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In the library, this is the `Jsont_pointer.get` function:
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<!-- $MDX skip -->
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```ocaml
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val get : t -> Jsont.json -> Jsont.json
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```
···
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The empty pointer returns the whole document. In OCaml, this is
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`Jsont_pointer.root`:
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<!-- $MDX skip -->
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```ocaml
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val root : t
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(** The empty pointer that references the whole document. *)
···
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**Important**: When using the OCaml library programmatically, you don't need
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to worry about escaping. The `Index.Mem` variant holds the literal key name:
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<!-- $MDX skip -->
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```ocaml
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(* To access the key "a/b", just use the literal string *)
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let pointer = Jsont_pointer.make [Mem "a/b"]
···
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The library provides both exception-raising and result-returning variants:
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<!-- $MDX skip -->
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```ocaml
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val get : t -> Jsont.json -> Jsont.json
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val get_result : t -> Jsont.json -> (Jsont.json, Jsont.Error.t) result
···
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In OCaml:
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<!-- $MDX skip -->
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```ocaml
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val add : t -> Jsont.json -> value:Jsont.json -> Jsont.json
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```
···
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to think about escaping. The `Index.Mem` variant stores unescaped strings,
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and escaping happens automatically during serialization:
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<!-- $MDX skip -->
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```ocaml
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(* Create a pointer to key "a/b" - no escaping needed *)
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let p = Jsont_pointer.make [Mem "a/b"]
···
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The `Token` module exposes the escaping functions if you need them:
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<!-- $MDX skip -->
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```ocaml
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module Token : sig
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val escape : string -> string (* "a/b" -> "a~1b" *)
···
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The library provides functions for URI fragment encoding:
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<!-- $MDX skip -->
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```ocaml
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val to_uri_fragment : t -> string
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val of_uri_fragment : string -> t
···
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## Jsont Integration
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-
The library integrates with the `Jsont` codec system for typed access:
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The library integrates with the `Jsont` codec system, allowing you to
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combine JSON Pointer navigation with typed decoding. This is powerful
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because you can point to a location in a JSON document and decode it
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directly to an OCaml type.
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Let's set up our OCaml environment and explore these features:
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```ocaml
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# open Jsont_pointer;;
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# let parse_json s =
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match Jsont_bytesrw.decode_string Jsont.json s with
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| Ok json -> json
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| Error e -> failwith e;;
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val parse_json : string -> Jsont.json = <fun>
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# let json_to_string json =
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match Jsont_bytesrw.encode_string ~format:Jsont.Minify Jsont.json json with
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| Ok s -> s
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| Error e -> failwith e;;
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val json_to_string : Jsont.json -> string = <fun>
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```
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### Working with JSON Values
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Let's create a sample configuration document:
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```ocaml
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# let config_json = parse_json {|{
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"database": {
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"host": "localhost",
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"port": 5432,
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"credentials": {"username": "admin", "password": "secret"}
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},
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"features": ["auth", "logging", "metrics"]
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}|};;
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val config_json : Jsont.json =
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Jsont.Object
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([(("database", <abstr>),
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Jsont.Object
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([(("host", <abstr>), Jsont.String ("localhost", <abstr>));
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(("port", <abstr>), Jsont.Number (5432., <abstr>));
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(("credentials", <abstr>),
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Jsont.Object
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([(("username", <abstr>), Jsont.String ("admin", <abstr>));
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(("password", <abstr>), Jsont.String ("secret", <abstr>))],
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<abstr>))],
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<abstr>));
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(("features", <abstr>),
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Jsont.Array
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([Jsont.String ("auth", <abstr>); Jsont.String ("logging", <abstr>);
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Jsont.String ("metrics", <abstr>)],
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<abstr>))],
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<abstr>)
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```
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### Creating and Using Pointers
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Create a pointer and use it to extract values:
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```ocaml
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# let host_ptr = of_string "/database/host";;
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val host_ptr : t = <abstr>
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# let host_value = get host_ptr config_json;;
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val host_value : Jsont.json = Jsont.String ("localhost", <abstr>)
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# match host_value with
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| Jsont.String (s, _) -> s
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| _ -> failwith "expected string";;
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- : string = "localhost"
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```
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### Building Pointers Programmatically
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Instead of parsing strings, you can build pointers from indices:
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```ocaml
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# let port_ptr = make [Mem "database"; Mem "port"];;
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val port_ptr : t = <abstr>
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# to_string port_ptr;;
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- : string = "/database/port"
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# match get port_ptr config_json with
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| Jsont.Number (n, _) -> int_of_float n
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| _ -> failwith "expected number";;
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- : int = 5432
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```
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For array access, use `Nth`:
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```ocaml
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# let first_feature_ptr = make [Mem "features"; Nth 0];;
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val first_feature_ptr : t = <abstr>
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# match get first_feature_ptr config_json with
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| Jsont.String (s, _) -> s
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| _ -> failwith "expected string";;
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- : string = "auth"
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```
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### Pointer Navigation
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You can build pointers incrementally using `append`:
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```ocaml
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# let db_ptr = of_string "/database";;
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val db_ptr : t = <abstr>
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# let creds_ptr = append db_ptr (Mem "credentials");;
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val creds_ptr : t = <abstr>
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# let user_ptr = append creds_ptr (Mem "username");;
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val user_ptr : t = <abstr>
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# to_string user_ptr;;
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- : string = "/database/credentials/username"
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# match get user_ptr config_json with
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| Jsont.String (s, _) -> s
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| _ -> failwith "expected string";;
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- : string = "admin"
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```
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### Safe Access with `find`
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Use `find` when you're not sure if a path exists:
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```ocaml
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# find (of_string "/database/timeout") config_json;;
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- : Jsont.json option = None
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# find (of_string "/database/host") config_json |> Option.is_some;;
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- : bool = true
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```
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### Typed Access with `path`
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The `path` combinator combines pointer navigation with typed decoding:
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```ocaml
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# let db_host =
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Jsont.Json.decode
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(path (of_string "/database/host") Jsont.string)
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config_json
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|> Result.get_ok;;
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val db_host : string = "localhost"
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# let db_port =
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Jsont.Json.decode
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(path (of_string "/database/port") Jsont.int)
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config_json
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|> Result.get_ok;;
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val db_port : int = 5432
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```
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Extract a list of strings:
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```ocaml
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# let features =
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Jsont.Json.decode
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(path (of_string "/features") Jsont.(list string))
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config_json
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|> Result.get_ok;;
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val features : string list = ["auth"; "logging"; "metrics"]
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```
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### Default Values with `~absent`
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Use `~absent` to provide a default when a path doesn't exist:
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```ocaml
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# let timeout =
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Jsont.Json.decode
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(path ~absent:30 (of_string "/database/timeout") Jsont.int)
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config_json
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|> Result.get_ok;;
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val timeout : int = 30
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```
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### Mutation Operations
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The library provides mutation functions for modifying JSON:
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```ocaml
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# let sample = parse_json {|{"name": "Alice", "scores": [85, 92, 78]}|};;
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val sample : Jsont.json =
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Jsont.Object
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([(("name", <abstr>), Jsont.String ("Alice", <abstr>));
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(("scores", <abstr>),
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Jsont.Array
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([Jsont.Number (85., <abstr>); Jsont.Number (92., <abstr>);
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Jsont.Number (78., <abstr>)],
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<abstr>))],
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<abstr>)
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```
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**Add** a new field:
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```ocaml
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# let with_email = add (of_string "/email") sample
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~value:(Jsont.Json.string "alice@example.com");;
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val with_email : Jsont.json =
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Jsont.Object
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([(("name", <abstr>), Jsont.String ("Alice", <abstr>));
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(("scores", <abstr>),
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Jsont.Array
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([Jsont.Number (85., <abstr>); Jsont.Number (92., <abstr>);
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Jsont.Number (78., <abstr>)],
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<abstr>));
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(("email", <abstr>), Jsont.String ("alice@example.com", <abstr>))],
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<abstr>)
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# json_to_string with_email;;
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- : string =
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"{\"name\":\"Alice\",\"scores\":[85,92,78],\"email\":\"alice@example.com\"}"
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```
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**Add** to an array using `-` (append):
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```ocaml
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# let with_new_score = add (of_string "/scores/-") sample
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~value:(Jsont.Json.number 95.);;
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val with_new_score : Jsont.json =
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Jsont.Object
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([(("name", <abstr>), Jsont.String ("Alice", <abstr>));
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(("scores", <abstr>),
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Jsont.Array
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([Jsont.Number (85., <abstr>); Jsont.Number (92., <abstr>);
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Jsont.Number (78., <abstr>); Jsont.Number (95., <abstr>)],
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<abstr>))],
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<abstr>)
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# json_to_string with_new_score;;
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- : string = "{\"name\":\"Alice\",\"scores\":[85,92,78,95]}"
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```
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**Replace** an existing value:
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```ocaml
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# let renamed = replace (of_string "/name") sample
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~value:(Jsont.Json.string "Bob");;
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val renamed : Jsont.json =
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Jsont.Object
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([(("name", <abstr>), Jsont.String ("Bob", <abstr>));
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(("scores", <abstr>),
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Jsont.Array
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([Jsont.Number (85., <abstr>); Jsont.Number (92., <abstr>);
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Jsont.Number (78., <abstr>)],
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<abstr>))],
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<abstr>)
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# json_to_string renamed;;
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- : string = "{\"name\":\"Bob\",\"scores\":[85,92,78]}"
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```
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**Remove** a value:
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```ocaml
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# let without_first = remove (of_string "/scores/0") sample;;
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val without_first : Jsont.json =
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Jsont.Object
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([(("name", <abstr>), Jsont.String ("Alice", <abstr>));
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(("scores", <abstr>),
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Jsont.Array
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([Jsont.Number (92., <abstr>); Jsont.Number (78., <abstr>)], <abstr>))],
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<abstr>)
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# json_to_string without_first;;
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- : string = "{\"name\":\"Alice\",\"scores\":[92,78]}"
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```
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### Nested Path Extraction
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You can extract values from deeply nested structures:
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```ocaml
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(* Codec for JSON Pointers as JSON strings *)
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val jsont : t Jsont.t
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# let org_json = parse_json {|{
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"organization": {
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"owner": {"name": "Alice", "email": "alice@example.com", "age": 35},
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"members": [{"name": "Bob", "email": "bob@example.com", "age": 28}]
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}
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}|};;
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val org_json : Jsont.json =
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Jsont.Object
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([(("organization", <abstr>),
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Jsont.Object
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([(("owner", <abstr>),
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Jsont.Object
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([(("name", <abstr>), Jsont.String ("Alice", <abstr>));
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(("email", <abstr>),
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Jsont.String ("alice@example.com", <abstr>));
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(("age", <abstr>), Jsont.Number (35., <abstr>))],
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<abstr>));
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(("members", <abstr>),
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Jsont.Array
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([Jsont.Object
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([(("name", <abstr>), Jsont.String ("Bob", <abstr>));
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(("email", <abstr>),
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Jsont.String ("bob@example.com", <abstr>));
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(("age", <abstr>), Jsont.Number (28., <abstr>))],
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<abstr>)],
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<abstr>))],
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<abstr>))],
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<abstr>)
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# Jsont.Json.decode
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(path (of_string "/organization/owner/name") Jsont.string)
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org_json
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|> Result.get_ok;;
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- : string = "Alice"
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# Jsont.Json.decode
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(path (of_string "/organization/members/0/age") Jsont.int)
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org_json
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|> Result.get_ok;;
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- : int = 28
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```
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(* Query combinators *)
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val path : ?absent:'a -> t -> 'a Jsont.t -> 'a Jsont.t
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val set_path : ?allow_absent:bool -> 'a Jsont.t -> t -> 'a -> Jsont.json Jsont.t
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val update_path : ?absent:'a -> t -> 'a Jsont.t -> Jsont.json Jsont.t
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val delete_path : ?allow_absent:bool -> t -> Jsont.json Jsont.t
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### Comparison: Raw vs Typed Access
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**Raw access** requires pattern matching:
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```ocaml
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# let raw_port =
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match get (of_string "/database/port") config_json with
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| Jsont.Number (f, _) -> int_of_float f
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| _ -> failwith "expected number";;
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val raw_port : int = 5432
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```
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These allow you to use JSON Pointers with typed codecs rather than raw
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`Jsont.json` values.
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**Typed access** is cleaner and type-safe:
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```ocaml
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# let typed_port =
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Jsont.Json.decode
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(path (of_string "/database/port") Jsont.int)
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+
config_json
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|> Result.get_ok;;
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val typed_port : int = 5432
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```
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The typed approach catches mismatches at decode time with clear errors.
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## Summary
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