Compositional JSON encode/decode library and PPX for Melange.
Based on @glennsl/bs-json.
The Decode module in particular provides a basic set of decoder functions to be composed into more complex decoders. A
decoder is a function that takes a Js.Json.t
and either returns a value of the desired type if successful or raises a
DecodeError
exception if not. Other functions accept a decoder and produce another decoder. Like array
, which when
given a decoder for type t
will return a decoder that tries to produce a value of type t array
. So to decode an
int array
you combine Json.Decode.int
with Json.Decode.array
into Json.Decode.(array int)
. An array of arrays of
ints? Json.Decode.(array (array int))
. Dict containing arrays of ints? Json.Decode.(dict (array int))
.
type line = {
start: point,
end_: point,
thickness: option(int)
}
and point = {
x: int,
y: int
};
module Decode = {
let point = json =>
Json.Decode.{
x: json |> field("x", int),
y: json |> field("y", int)
};
let line = json =>
Json.Decode.{
start: json |> field("start", point),
end_: json |> field("end", point),
thickness: json |> optional(field("thickness", int))
};
};
let data = {| {
"start": { "x": 1, "y": -4 },
"end": { "x": 5, "y": 8 }
} |};
let line = data |> Json.parseOrRaise
|> Decode.line;
NOTE: Json.Decode.{ ... }
creates an ordinary record, but also opens the Json.Decode
module locally, within the
scope delimited by the curly braces, so we don't have to qualify the functions we use from it, like field
, int
and
optional
here. You can also use Json.Decode.( ... )
to open the module locally within the parentheses, if you're not
creating a record.
See examples for more.
Install opam package manager.
Then:
opam install melange-json
Add melange-json
to the libraries
field in your dune
file:
; ...
(libraries melange-json)
; ...
For the moment, please see the interface files:
If you look at the type signature of Json.Decode.array
, for example, you'll see it takes an 'a decoder
and returns an
'a array decoder
. 'a decoder
is just an alias for Js.Json.t -> 'a
, so if we expand the type signature of array
we'll get (Js.Json.t -> 'a) -> Js.Json.t -> 'a array
. We can now see that it is a function that takes a decoder and
returns a function, itself a decoder. Applying the int
decoder to array
will give us an int array decoder
, a
function Js.Json.t -> int array
.
If you've written a function that takes just Js.Json.t
and returns user-defined types of your own, you've already been
writing composable decoders! Let's look at Decode.point
from the example above:
let point = json => {
open! Json.Decode;
{
x: json |> field("x", int),
y: json |> field("y", int)
};
};
This is a function Js.Json.t -> point
, or a point decoder
. So if we'd like to decode an array of points, we can just
pass it to Json.Decode.array
to get a point array decoder
in return.
To write a decoder builder like Json.Decode.array
we need to take another decoder as an argument, and thanks to
currying we just need to apply it where we'd otherwise use a fixed decoder. Say we want to be able to decode both
int point
s and float point
s. First we'd have to parameterize the type:
type point('a) = {
x: 'a,
y: 'a
}
Then we can change our point
function from above to take and use a decoder argument:
let point = (decodeNumber, json) => {
open! Json.Decode;
{
x: json |> field("x", decodeNumber),
y: json |> field("y", decodeNumber)
};
};
And if we wish we can now create aliases for each variant:
let intPoint = point(Json.Decode.int);
let floatPoint = point(Json.Decode.float);
Encoders work exactly the same way, just in reverse. 'a encoder
is just an alias for 'a -> Js.Json.t
, and this also
transfers to composition: 'a encoder -> 'a array encoder
expands to ('a -> Js.Json.t) -> 'a array -> Js.Json.t
.
A ppx deriver plugin is provided to automatically convert OCaml values to and from JSON.
The PPX is included in the melange-json
package. To use it, just add the
dune
configuration to your project:
(library
(modes melange)
(preprocess (pps melange-json.ppx)))
To generate JSON converters for a type, add the [@@deriving json]
attribute to
a type declaration:
type t = {
a: int;
b: string;
} [@@deriving json]
This will generate the following pair of functions:
val of_json : Js.Json.t -> t
val to_json : t -> Js.Json.t
You can also generate JSON converters for a type expression using the to_json
and of_json
extension points:
let json = [%to_json: int * string] (42, "foo")
Note that variants where all constructors have no arguments are treated as enumeration-like variants:
type t = A | B [@@deriving json]
Such variants are represented as strings in JSON:
let json = to_json A
(* "A" *)
You can specify default values for record fields using the [@json.default E]
attribute:
type t = {
a: int;
b: string [@json.default "-"];
} [@@deriving of_json]
let t = of_json (Json.parseOrRaise {|{"a": 42}|})
(* t = { a = 42; b = "-"; } *)
When a field has type _ option
then you can use the [@json.option]
attribute
to specify that the default value is None
:
type t = {
a: int;
b: string option [@json.option];
} [@@deriving of_json]
let t = of_json (Json.parseOrRaise {|{"a": 42}|})
(* t = { a = 42; b = None; } *)
You can specify custom keys for record fields using the [@json.key E]
attribute:
type t = {
a: int [@json.key "A"];
b: string [@json.key "B"];
} [@@deriving of_json]
let t = of_json (Json.parseOrRaise {|{"A": 42, "B": "foo"}|})
(* t = { a = 42; b = "foo"; } *)
You can specify custom representation for a variant case using the [@json.as E]
attribute:
type t = A | B [@json.as "bbb"] [@@deriving json]
let json = to_json B
(* "bbb" *)
This work is dual-licensed under LGPL 3.0 and MPL 2.0. You can choose between one of them if you use this work.
Please see LICENSE.LGPL-3.0 and LICENSE.MPL-2.0 for the full text of each license.
SPDX-License-Identifier: LGPL-3.0 OR MPL-2.0