PPX extension for Yojson literals and patterns

Based on an original idea by @emillon.

Overview

ppx_yojson lets you write Yojson expressions and patterns using ocaml syntax to make your code
more concise and readable.

It rewrites %yojson extension points based on the content of the payload.

For example you can turn:

let json =
  `List
    [ `Assoc
        [ ("name", `String "Anne")
        ; ("grades", `List [`String "A"; `String "B-"; `String "B+"]
        ]
    ; `Assoc
        [ ("name", `String "Bernard")
        ; ("grades", `List [`String "B+"; `String "A"; `String "B-"]
        ]
    ]

into:

let json =
  [%yojson
    [ {name = "Anne"; grades = ["A"; "B-"; "B+"]}
    ; {name = "Bernard"; grades = ["B+"; "A"; "B-"]}
    ]
  ]

Installation and usage

You can install ppx_yojson using opam:

$ opam install ppx_yojson

If you're building your library or app with dune, add the following field to your library,
executable or test stanza:

(preprocess (pps ppx_yojson))

You can now use the %yojson extension in your code. See the
expressions and
patterns sections for the detailed syntax.

Syntax

Expressions

The expression rewriter supports the following Yojson values:

• Null: [%yojson None]

• Bool of bool: [%yojson true]

• Float of float: [%yojson 1.2e+10]

• Int of int: [%yojson 0xff]. As long as the int literal in the payload fits in an int,
  the 0x, 0o and 0b notations are accepted.

• Intlit of string: [%yojson 100000000000000000000000000000000]. For arbitrary long integers.
  int64, int32 and nativeint literals are also rewritten as Intlit for consistency with
  ppx_deriving_yojson.
  0x, 0o and 0b notations are currently not supported and the rewriter will raise an error.

• String of string: [%yojson "abc"]

• List of json list: [%yojson [1; 2; 3]]. It supports mixed type list as well such as
  ["a"; 2].

• Assoc of (string * json) list: [%yojson {a = 1; b = "b"}]

• Any valid combination of the above

The resulting expression are not constrained, meaning it works with Yojson.Safe or Yojson.Basic
regardless.

Anti-quotation

You can escape regular Yojson expressions within a payload using [%y json_expr]. You can use
this to insert variables in the payload. For example:

let a = `String "a"
let json = [%yojson { a = [%y a]; b = "b"}]

is rewritten as:

let a = `String "a"
let json = `Assoc [("a", a); (b, `String "b")]

Note that the payload in a %y extension should always subtype one of the Yojson types.

Patterns

Note that the pattern extension expects a pattern payload and must thus be invoked as
[%yojson? pattern].

The pattern rewriter supports the following:

• Null, Bool of bool, Float of float, Int of int, Intlit of string, String of string,
  List of json list with the same syntax as for expressions and will be
  rewritten to a pattern matching that json value.

• Assoc of (string * json) list with the same syntax as for expressions but with a few
  restrictions. The record pattern in the payload must be closed (ie no ; _}) and have less than
  4 fields. See details below.

• Var patterns: they are just rewritten as var patterns meaning they will bind to a
  Yojson.Safe.json or whatever Yojson type you're using that's compatible with the above.

• The wildcard pattern: it gets rewritten as, well, a wildcard pattern

• Any valid combination of the above

Record patterns

Json objects fields order doesn't matter so you'd expect the {a = 1; b = true} pattern to match
regardless of the parsed json being {"a": 1, "b": true} or {"b": true, "a": 1}.

Since json objects are represented as lists, the order of the fields in the rewritten pattern does
matter.

To allow you to write such patterns concisely and without having to care for the order of the
fields, the record pattern is expanded to an or-pattern that matches every permutation of the
(string * json) list. This is the reason of the limitations mentioned in the above list.
Also note that there is no limitation on nesting such patterns but you probably want to avoid doing
that too much.

This is provided mostly for convenience. If you want efficient code and/or to handle complex json
objects I recommend that you use
ppx_deriving_yojson or
ppx_yojson_conv instead.

To clarify, the following code:

let f = function
  | [%yojson? {a = 1; b = true}] -> (1, true)

is expanded into:

let f = function
  | ( `Assoc [("a", `Int 1); ("b", `Bool true)]
    | `Assoc [("b", `Bool true); ("a", `Int 1)]
    ) -> (1, true)

Anti-quotation

You can also escape regular Yojson patterns in ppx_yojson pattern extensions' payload
using [%y? json_pat]. You can use it to further deconstruct a Yojson value. For example:

let f = function
  | [%yojson? {a = [%y? `Int i]} -> i + 1

is expanded into:

let f = function
  | `Assoc [("a", `Int i)] -> i + 1