Exact numbers

JSON.parse does not keep the text a number was written with.

10 and 10.0 give the same value, and integers above 2⁵³ are rounded.

Because of that, when using Thoth.Json.JavaScript, some numbers decode differently than on .NET and Python.

The last column is what .NET and Python return.

If you need exact numbers, pass DecodeOptions to Decode.fromStringWithOptions:

open Thoth.Json.Core
open Thoth.Json.JavaScript

let options =
    { DecodeOptions.defaults with
        ExactNumbers = true
    }

Decode.fromStringWithOptions (options, Decode.int16) "25.0"
// Error "Error at: `$`
// Expecting an int16 but instead got: 25.0
// Reason: Value is not an integral value"

Decode.fromStringWithOptions (options, Decode.int64) "9223372036854775806"
// Ok 9223372036854775806L

Decode.unsafeFromStringWithOptions takes the same options and raises on failure. Both accept a Codec<'T> in place of a Decoder<'T>.

This option is only in Thoth.Json.JavaScript. The other packages already tell 10 apart from 10.0.

Options

ExactNumbers

type: bool

default: false

Read the text of number literals. A number written with a fraction, such as 10.0, is not an integral value, and an integer keeps every digit it was written with.

Cost

ExactNumbers installs a JSON.parse reviver, which parses more slowly. Measured on Node 22 against a payload of records:

The share of a decode spent parsing falls as the payload grows, so the overhead is largest on small payloads.

Parsing the JSON yourself

Decode.numberLiteralReviver is the reviver behind the option. Pass it to your own JSON.parse call and decode the result with Decode.fromValue.

open Fable.Core.JsInterop
open Thoth.Json.Core
open Thoth.Json.JavaScript

let json = "25.0"

let parsed: obj =
    emitJsExpr (json, Decode.numberLiteralReviver) "JSON.parse($0, $1)"

Decode.fromValue Decode.int16 parsed
// Error "Error at: `$`
// Expecting an int16 but instead got: 25.0
// Reason: Value is not an integral value"

Decode.fromValue receives an already parsed value, so it reads number literals only when the value was parsed with this reviver.

Runtime support

ExactNumbers needs a runtime that passes the source text to the reviver.

On an older runtime the option has no effect and decoding behaves as it does by default.