# JSON Pointer Tutorial

This tutorial introduces JSON Pointer as defined in
[RFC 6901](https://www.rfc-editor.org/rfc/rfc6901), and demonstrates
the `jsont-pointer` OCaml library through interactive examples.

## Setup

First, let's set up our environment with helper functions:

```ocaml
# open Jsont_pointer;;
# #install_printer Jsont_pointer_top.printer;;
# #install_printer Jsont_pointer_top.json_printer;;
# #install_printer Jsont_pointer_top.error_printer;;
# let parse_json s =
    match Jsont_bytesrw.decode_string Jsont.json s with
    | Ok json -> json
    | Error e -> failwith e;;
val parse_json : string -> Jsont.json = <fun>
```

## What is JSON Pointer?

From RFC 6901, Section 1:

> JSON Pointer defines a string syntax for identifying a specific value
> within a JavaScript Object Notation (JSON) document.

In other words, JSON Pointer is an addressing scheme for locating values
inside a JSON structure. Think of it like a filesystem path, but for JSON
documents instead of files.

For example, given this JSON document:

```ocaml
# let users_json = parse_json {|{
    "users": [
      {"name": "Alice", "age": 30},
      {"name": "Bob", "age": 25}
    ]
  }|};;
val users_json : Jsont.json =
  {"users":[{"name":"Alice","age":30},{"name":"Bob","age":25}]}
```

The JSON Pointer `/users/0/name` refers to the string `"Alice"`:

```ocaml
# let ptr = of_string "/users/0/name";;
val ptr : t = [`Mem "users"; `Nth 0; `Mem "name"]
# get ptr users_json;;
- : Jsont.json = "Alice"
```

In OCaml, this is represented by the `Jsont_pointer.t` type - a sequence
of navigation steps from the document root to a target value.

## Syntax: Reference Tokens

RFC 6901, Section 3 defines the syntax:

> A JSON Pointer is a Unicode string containing a sequence of zero or more
> reference tokens, each prefixed by a '/' (%x2F) character.

The grammar is elegantly simple:

```
json-pointer    = *( "/" reference-token )
reference-token = *( unescaped / escaped )
```

This means:
- The empty string `""` is a valid pointer (it refers to the whole document)
- Every non-empty pointer starts with `/`
- Everything between `/` characters is a "reference token"

Let's see this in action:

```ocaml
# of_string "";;
- : t = []
```

The empty pointer has no reference tokens - it points to the root.

```ocaml
# of_string "/foo";;
- : t = [`Mem "foo"]
```

The pointer `/foo` has one token: `foo`. Since it's not a number, it's
interpreted as an object member name (`Mem`).

```ocaml
# of_string "/foo/0";;
- : t = [`Mem "foo"; `Nth 0]
```

Here we have two tokens: `foo` (a member name) and `0` (interpreted as
an array index `Nth`).

```ocaml
# of_string "/foo/bar/baz";;
- : t = [`Mem "foo"; `Mem "bar"; `Mem "baz"]
```

Multiple tokens navigate deeper into nested structures.

### The Index Type

Each reference token becomes an `Index.t` value in the library:

<!-- $MDX skip -->
```ocaml
type t = [
  | `Mem of string   (* Object member access *)
  | `Nth of int      (* Array index access *)
  | `End             (* The special "-" marker for append operations *)
]
```

The `Mem` variant holds the **unescaped** member name - you work with the
actual key string (like `"a/b"`) and the library handles any escaping needed
for the JSON Pointer string representation.

### Invalid Syntax

What happens if a pointer doesn't start with `/`?

```ocaml
# of_string "foo";;
Exception:
Jsont.Error Invalid JSON Pointer: must be empty or start with '/': foo.
```

The RFC is strict: non-empty pointers MUST start with `/`.

For safer parsing, use `of_string_result`:

```ocaml
# of_string_result "foo";;
- : (t, string) result =
Error "Invalid JSON Pointer: must be empty or start with '/': foo"
# of_string_result "/valid";;
- : (t, string) result = Ok [`Mem "valid"]
```

## Evaluation: Navigating JSON

Now we come to the heart of JSON Pointer: evaluation. RFC 6901, Section 4
describes how a pointer is resolved against a JSON document:

> Evaluation of a JSON Pointer begins with a reference to the root value
> of a JSON document and completes with a reference to some value within
> the document. Each reference token in the JSON Pointer is evaluated
> sequentially.

Let's use the example JSON document from RFC 6901, Section 5:

```ocaml
# let rfc_example = parse_json {|{
    "foo": ["bar", "baz"],
    "": 0,
    "a/b": 1,
    "c%d": 2,
    "e^f": 3,
    "g|h": 4,
    "i\\j": 5,
    "k\"l": 6,
    " ": 7,
    "m~n": 8
  }|};;
val rfc_example : Jsont.json =
  {"foo":["bar","baz"],"":0,"a/b":1,"c%d":2,"e^f":3,"g|h":4,"i\\j":5,"k\"l":6," ":7,"m~n":8}
```

This document is carefully constructed to exercise various edge cases!

### The Root Pointer

```ocaml
# get root rfc_example ;;
- : Jsont.json =
{"foo":["bar","baz"],"":0,"a/b":1,"c%d":2,"e^f":3,"g|h":4,"i\\j":5,"k\"l":6," ":7,"m~n":8}
```

The empty pointer (`root`) returns the whole document.

### Object Member Access

```ocaml
# get (of_string "/foo") rfc_example ;;
- : Jsont.json = ["bar","baz"]
```

`/foo` accesses the member named `foo`, which is an array.

### Array Index Access

```ocaml
# get (of_string "/foo/0") rfc_example ;;
- : Jsont.json = "bar"
# get (of_string "/foo/1") rfc_example ;;
- : Jsont.json = "baz"
```

`/foo/0` first goes to `foo`, then accesses index 0 of the array.

### Empty String as Key

JSON allows empty strings as object keys:

```ocaml
# get (of_string "/") rfc_example ;;
- : Jsont.json = 0
```

The pointer `/` has one token: the empty string. This accesses the member
with an empty name.

### Keys with Special Characters

The RFC example includes keys with `/` and `~` characters:

```ocaml
# get (of_string "/a~1b") rfc_example ;;
- : Jsont.json = 1
```

The token `a~1b` refers to the key `a/b`. We'll explain this escaping
[below](#escaping-special-characters).

```ocaml
# get (of_string "/m~0n") rfc_example ;;
- : Jsont.json = 8
```

The token `m~0n` refers to the key `m~n`.

**Important**: When using the OCaml library programmatically, you don't need
to worry about escaping. The `Mem` variant holds the literal key name:

```ocaml
# let slash_ptr = make [`Mem "a/b"];;
val slash_ptr : t = [`Mem "a/b"]
# to_string slash_ptr;;
- : string = "/a~1b"
# get slash_ptr rfc_example ;;
- : Jsont.json = 1
```

The library escapes it when converting to string.

### Other Special Characters (No Escaping Needed)

Most characters don't need escaping in JSON Pointer strings:

```ocaml
# get (of_string "/c%d") rfc_example ;;
- : Jsont.json = 2
# get (of_string "/e^f") rfc_example ;;
- : Jsont.json = 3
# get (of_string "/g|h") rfc_example ;;
- : Jsont.json = 4
# get (of_string "/ ") rfc_example ;;
- : Jsont.json = 7
```

Even a space is a valid key character!

### Error Conditions

What happens when we try to access something that doesn't exist?

```ocaml
# get_result (of_string "/nonexistent") rfc_example;;
- : (Jsont.json, Jsont.Error.t) result =
Error JSON Pointer: member 'nonexistent' not found
File "-":
# find (of_string "/nonexistent") rfc_example;;
- : Jsont.json option = None
```

Or an out-of-bounds array index:

```ocaml
# find (of_string "/foo/99") rfc_example;;
- : Jsont.json option = None
```

Or try to index into a non-container:

```ocaml
# find (of_string "/foo/0/invalid") rfc_example;;
- : Jsont.json option = None
```

The library provides both exception-raising and result-returning variants:

<!-- $MDX skip -->
```ocaml
val get : t -> Jsont.json -> Jsont.json
val get_result : t -> Jsont.json -> (Jsont.json, Jsont.Error.t) result
val find : t -> Jsont.json -> Jsont.json option
```

### Array Index Rules

RFC 6901 has specific rules for array indices. Section 4 states:

> characters comprised of digits [...] that represent an unsigned base-10
> integer value, making the new referenced value the array element with
> the zero-based index identified by the token

And importantly:

> note that leading zeros are not allowed

```ocaml
# of_string "/foo/0";;
- : t = [`Mem "foo"; `Nth 0]
```

Zero itself is fine.

```ocaml
# of_string "/foo/01";;
- : t = [`Mem "foo"; `Mem "01"]
```

But `01` has a leading zero, so it's NOT treated as an array index - it
becomes a member name instead. This protects against accidental octal
interpretation.

## The End-of-Array Marker: `-`

RFC 6901, Section 4 introduces a special token:

> exactly the single character "-", making the new referenced value the
> (nonexistent) member after the last array element.

This is primarily useful for JSON Patch operations (RFC 6902). Let's see
how it parses:

```ocaml
# of_string "/foo/-";;
- : t = [`Mem "foo"; `End]
```

The `-` is recognized as a special `End` index.

However, you cannot evaluate a pointer containing `-` because it refers
to a position that doesn't exist:

```ocaml
# find (of_string "/foo/-") rfc_example;;
- : Jsont.json option = None
```

The RFC explains this:

> Note that the use of the "-" character to index an array will always
> result in such an error condition because by definition it refers to
> a nonexistent array element.

But we'll see later that `-` is very useful for mutation operations!

## Mutation Operations

While RFC 6901 defines JSON Pointer for read-only access, RFC 6902
(JSON Patch) uses JSON Pointer for modifications. The `jsont-pointer`
library provides these operations.

### Add

The `add` operation inserts a value at a location:

```ocaml
# let obj = parse_json {|{"foo":"bar"}|};;
val obj : Jsont.json = {"foo":"bar"}
# add (of_string "/baz") obj ~value:(Jsont.Json.string "qux")
  ;;
- : Jsont.json = {"foo":"bar","baz":"qux"}
```

For arrays, `add` inserts BEFORE the specified index:

```ocaml
# let arr_obj = parse_json {|{"foo":["a","b"]}|};;
val arr_obj : Jsont.json = {"foo":["a","b"]}
# add (of_string "/foo/1") arr_obj ~value:(Jsont.Json.string "X")
  ;;
- : Jsont.json = {"foo":["a","X","b"]}
```

This is where the `-` marker shines - it appends to the end:

```ocaml
# add (of_string "/foo/-") arr_obj ~value:(Jsont.Json.string "c")
  ;;
- : Jsont.json = {"foo":["a","b","c"]}
```

### Remove

The `remove` operation deletes a value:

```ocaml
# let two_fields = parse_json {|{"foo":"bar","baz":"qux"}|};;
val two_fields : Jsont.json = {"foo":"bar","baz":"qux"}
# remove (of_string "/baz") two_fields ;;
- : Jsont.json = {"foo":"bar"}
```

For arrays, it removes and shifts:

```ocaml
# let three_elem = parse_json {|{"foo":["a","b","c"]}|};;
val three_elem : Jsont.json = {"foo":["a","b","c"]}
# remove (of_string "/foo/1") three_elem ;;
- : Jsont.json = {"foo":["a","c"]}
```

### Replace

The `replace` operation updates an existing value:

```ocaml
# replace (of_string "/foo") obj ~value:(Jsont.Json.string "baz")
  ;;
- : Jsont.json = {"foo":"baz"}
```

Unlike `add`, `replace` requires the target to already exist.
Attempting to replace a nonexistent path raises an error.

### Move

The `move` operation relocates a value:

```ocaml
# let nested = parse_json {|{"foo":{"bar":"baz"},"qux":{}}|};;
val nested : Jsont.json = {"foo":{"bar":"baz"},"qux":{}}
# move ~from:(of_string "/foo/bar") ~path:(of_string "/qux/thud") nested
  ;;
- : Jsont.json = {"foo":{},"qux":{"thud":"baz"}}
```

### Copy

The `copy` operation duplicates a value:

```ocaml
# let to_copy = parse_json {|{"foo":{"bar":"baz"}}|};;
val to_copy : Jsont.json = {"foo":{"bar":"baz"}}
# copy ~from:(of_string "/foo/bar") ~path:(of_string "/foo/qux") to_copy
  ;;
- : Jsont.json = {"foo":{"bar":"baz","qux":"baz"}}
```

### Test

The `test` operation verifies a value (useful in JSON Patch):

```ocaml
# test (of_string "/foo") obj ~expected:(Jsont.Json.string "bar");;
- : bool = true
# test (of_string "/foo") obj ~expected:(Jsont.Json.string "wrong");;
- : bool = false
```

## Escaping Special Characters

RFC 6901, Section 3 explains the escaping rules:

> Because the characters '\~' (%x7E) and '/' (%x2F) have special meanings
> in JSON Pointer, '\~' needs to be encoded as '\~0' and '/' needs to be
> encoded as '\~1' when these characters appear in a reference token.

Why these specific characters?
- `/` separates tokens, so it must be escaped inside a token
- `~` is the escape character itself, so it must also be escaped

The escape sequences are:
- `~0` represents `~` (tilde)
- `~1` represents `/` (forward slash)

### The Library Handles Escaping Automatically

**Important**: When using `jsont-pointer` programmatically, you rarely need
to think about escaping. The `Mem` variant stores unescaped strings,
and escaping happens automatically during serialization:

```ocaml
# let p = make [`Mem "a/b"];;
val p : t = [`Mem "a/b"]
# to_string p;;
- : string = "/a~1b"
# of_string "/a~1b";;
- : t = [`Mem "a/b"]
```

### Escaping in Action

The `Token` module exposes the escaping functions:

```ocaml
# Token.escape "hello";;
- : string = "hello"
# Token.escape "a/b";;
- : string = "a~1b"
# Token.escape "a~b";;
- : string = "a~0b"
# Token.escape "~/";;
- : string = "~0~1"
```

### Unescaping

And the reverse process:

```ocaml
# Token.unescape "a~1b";;
- : string = "a/b"
# Token.unescape "a~0b";;
- : string = "a~b"
```

### The Order Matters!

RFC 6901, Section 4 is careful to specify the unescaping order:

> Evaluation of each reference token begins by decoding any escaped
> character sequence. This is performed by first transforming any
> occurrence of the sequence '~1' to '/', and then transforming any
> occurrence of the sequence '~0' to '~'. By performing the substitutions
> in this order, an implementation avoids the error of turning '~01' first
> into '~1' and then into '/', which would be incorrect (the string '~01'
> correctly becomes '~1' after transformation).

Let's verify this tricky case:

```ocaml
# Token.unescape "~01";;
- : string = "~1"
```

If we unescaped `~0` first, `~01` would become `~1`, which would then become
`/`. But that's wrong! The sequence `~01` should become the literal string
`~1` (a tilde followed by the digit one).

## URI Fragment Encoding

JSON Pointers can be embedded in URIs. RFC 6901, Section 6 explains:

> A JSON Pointer can be represented in a URI fragment identifier by
> encoding it into octets using UTF-8, while percent-encoding those
> characters not allowed by the fragment rule in RFC 3986.

This adds percent-encoding on top of the `~0`/`~1` escaping:

```ocaml
# to_uri_fragment (of_string "/foo");;
- : string = "/foo"
# to_uri_fragment (of_string "/a~1b");;
- : string = "/a~1b"
# to_uri_fragment (of_string "/c%d");;
- : string = "/c%25d"
# to_uri_fragment (of_string "/ ");;
- : string = "/%20"
```

The `%` character must be percent-encoded as `%25` in URIs, and
spaces become `%20`.

Here's the RFC example showing the URI fragment forms:

| JSON Pointer | URI Fragment | Value |
|-------------|-------------|-------|
| `""`        | `#`         | whole document |
| `"/foo"`    | `#/foo`     | `["bar", "baz"]` |
| `"/foo/0"`  | `#/foo/0`   | `"bar"` |
| `"/"`       | `#/`        | `0` |
| `"/a~1b"`   | `#/a~1b`    | `1` |
| `"/c%d"`    | `#/c%25d`   | `2` |
| `"/ "`      | `#/%20`     | `7` |
| `"/m~0n"`   | `#/m~0n`    | `8` |

## Building Pointers Programmatically

Instead of parsing strings, you can build pointers from indices:

```ocaml
# let port_ptr = make [`Mem "database"; `Mem "port"];;
val port_ptr : t = [`Mem "database"; `Mem "port"]
# to_string port_ptr;;
- : string = "/database/port"
```

For array access, use `Nth`:

```ocaml
# let first_feature_ptr = make [`Mem "features"; `Nth 0];;
val first_feature_ptr : t = [`Mem "features"; `Nth 0]
# to_string first_feature_ptr;;
- : string = "/features/0"
```

### Pointer Navigation

You can build pointers incrementally using `append`:

```ocaml
# let db_ptr = of_string "/database";;
val db_ptr : t = [`Mem "database"]
# let creds_ptr = append db_ptr (`Mem "credentials");;
val creds_ptr : t = [`Mem "database"; `Mem "credentials"]
# let user_ptr = append creds_ptr (`Mem "username");;
val user_ptr : t = [`Mem "database"; `Mem "credentials"; `Mem "username"]
# to_string user_ptr;;
- : string = "/database/credentials/username"
```

Or concatenate two pointers:

```ocaml
# let base = of_string "/api/v1";;
val base : t = [`Mem "api"; `Mem "v1"]
# let endpoint = of_string "/users/0";;
val endpoint : t = [`Mem "users"; `Nth 0]
# to_string (concat base endpoint);;
- : string = "/api/v1/users/0"
```

## Jsont Integration

The library integrates with the `Jsont` codec system, allowing you to
combine JSON Pointer navigation with typed decoding. This is powerful
because you can point to a location in a JSON document and decode it
directly to an OCaml type.

```ocaml
# let config_json = parse_json {|{
    "database": {
      "host": "localhost",
      "port": 5432,
      "credentials": {"username": "admin", "password": "secret"}
    },
    "features": ["auth", "logging", "metrics"]
  }|};;
val config_json : Jsont.json =
  {"database":{"host":"localhost","port":5432,"credentials":{"username":"admin","password":"secret"}},"features":["auth","logging","metrics"]}
```

### Typed Access with `path`

The `path` combinator combines pointer navigation with typed decoding:

```ocaml
# let db_host =
    Jsont.Json.decode
      (path (of_string "/database/host") Jsont.string)
      config_json
    |> Result.get_ok;;
val db_host : string = "localhost"
# let db_port =
    Jsont.Json.decode
      (path (of_string "/database/port") Jsont.int)
      config_json
    |> Result.get_ok;;
val db_port : int = 5432
```

Extract a list of strings:

```ocaml
# let features =
    Jsont.Json.decode
      (path (of_string "/features") Jsont.(list string))
      config_json
    |> Result.get_ok;;
val features : string list = ["auth"; "logging"; "metrics"]
```

### Default Values with `~absent`

Use `~absent` to provide a default when a path doesn't exist:

```ocaml
# let timeout =
    Jsont.Json.decode
      (path ~absent:30 (of_string "/database/timeout") Jsont.int)
      config_json
    |> Result.get_ok;;
val timeout : int = 30
```

### Nested Path Extraction

You can extract values from deeply nested structures:

```ocaml
# let org_json = parse_json {|{
    "organization": {
      "owner": {"name": "Alice", "email": "alice@example.com", "age": 35},
      "members": [{"name": "Bob", "email": "bob@example.com", "age": 28}]
    }
  }|};;
val org_json : Jsont.json =
  {"organization":{"owner":{"name":"Alice","email":"alice@example.com","age":35},"members":[{"name":"Bob","email":"bob@example.com","age":28}]}}
# Jsont.Json.decode
    (path (of_string "/organization/owner/name") Jsont.string)
    org_json
  |> Result.get_ok;;
- : string = "Alice"
# Jsont.Json.decode
    (path (of_string "/organization/members/0/age") Jsont.int)
    org_json
  |> Result.get_ok;;
- : int = 28
```

### Comparison: Raw vs Typed Access

**Raw access** requires pattern matching:

```ocaml
# let raw_port =
    match get (of_string "/database/port") config_json with
    | Jsont.Number (f, _) -> int_of_float f
    | _ -> failwith "expected number";;
val raw_port : int = 5432
```

**Typed access** is cleaner and type-safe:

```ocaml
# let typed_port =
    Jsont.Json.decode
      (path (of_string "/database/port") Jsont.int)
      config_json
    |> Result.get_ok;;
val typed_port : int = 5432
```

The typed approach catches mismatches at decode time with clear errors.

## Summary

JSON Pointer (RFC 6901) provides a simple but powerful way to address
values within JSON documents:

1. **Syntax**: Pointers are strings of `/`-separated reference tokens
2. **Escaping**: Use `~0` for `~` and `~1` for `/` in tokens (handled automatically by the library)
3. **Evaluation**: Tokens navigate through objects (by key) and arrays (by index)
4. **URI Encoding**: Pointers can be percent-encoded for use in URIs
5. **Mutations**: Combined with JSON Patch (RFC 6902), pointers enable structured updates

The `jsont-pointer` library implements all of this with type-safe OCaml
interfaces, integration with the `jsont` codec system, and proper error
handling for malformed pointers and missing values.