Simple Values

The following decoders are provided out-of-the-box for decoding values from JSON. To keep things terse, we'll assume that Decode.ParseError is open in the local scope and we've aliased the decode module as module D = Decode.AsResult.OfParseError.

Note that the output of our decode functions is wrapped in a result because we're using Decode.AsResult..... You could instead use Decode.AsOption, which would decode successes as Some(value) and errors as None.

String

let json = Js.Json.string("foo");
D.string(json); // Ok("foo")

let json = Js.Json.number(3.14);
D.string(json); // Error(Val(`ExpectedString, json))

Int and Float

JSON numbers can be decoded as either floats or ints. These decoders are suffixed with ...FromNumber to allow you to open locally without getting compiler complaints about shadowing float from OCaml's Pervasives.

let json = Js.Json.number(3.);
D.floatFromNumber(json); // Ok(3.0)
D.intFromNumber(json); // Ok(3)

let json = Js.Json.number(3.14);
D.floatFromNumber(json); // Ok(3.14)
D.intFromNumber(json); // Error(`ExpectedInt, json)

Note that intFromNumber will reject numbers with fractional parts (rather than rounding up or down). If you don't want this behavior, you can use floatFromNumber, then map the Result through int_of_float, which will drop the fractional part.

Boolean

let json = Js.Json.boolean(true);
D.boolean(json); // Ok(true)

Lists and Arrays

JSON arrays can be decoded into either arrays or lists. Decoding a JSON array requires you to also pass a decoder for the inner type.

let json = Js.Json.array([| Js.Json.string("a") |]);

D.array(D.string, json); // Ok([| "a" |])
D.list(D.string, json); // Ok([ "a" ])

// Error(Arr(NonEmptyList.pure((0, Val(`ExpectedInt, Js.Json.string(""))))))
D.list(D.intFromNumber, json);

Date

JSON doesn't natively support dates, but bs-decode provides decoders that will try to build a date from a JSON float or from a JSON string that can be understood by new Date in JavaScript.

let json = Js.Json.string("2018-11-17T05:40:35.869Z");
D.date(json); // Ok(Js.Date.fromString("2018-11-17T05:40:35.869Z"))

let json = Js.Json.number(1542433304450.0);
D.date(json); // Ok(Js.Date.fromFloat(1542433304450.0));

While the Js.Date.from* functions in the comments above could return invalid Date objects, bs-decode will reject invalid dates as Result errors.

Dict

JSON objects can also be used to store key/value pairs, where the keys are strings. This can be represented as a Js.Dict.t('a) in ReasonML.

 // Assume `json` is a value that looks like:
 // { "foo": 3, "bar": 2 }

// Ok(Js.Dict.fromList([("foo", 3), ("bar", 2)]))
D.dict(D.intFromNumber, json);