Farse 0.2.6

Farse

Simple parsing library for F# using System.Text.Json.

Inspired by Thoth.Json and its composability.

Farse uses a slightly different syntax, includes a computation expression, and a few custom operators that simplify parsing. It also tries to keep a low overhead while still producing acceptable error messages.

Installation

dotnet add package Farse

Benchmarks

There are some initial benchmarks here.

BenchmarkDotNet v0.15.2, macOS Sequoia 15.6 (24G84) [Darwin 24.6.0]
Apple M1 Pro, 1 CPU, 8 logical and 8 physical cores
.NET SDK 9.0.203
  [Host]     : .NET 9.0.4 (9.0.425.16305), Arm64 RyuJIT AdvSIMD DEBUG
  DefaultJob : .NET 9.0.4 (9.0.425.16305), Arm64 RyuJIT AdvSIMD

| Method                 | Mean      | Ratio | Gen0   | Gen1   | Allocated | Alloc Ratio |
|----------------------- |----------:|------:|-------:|-------:|----------:|------------:|
| System.Text.Json       |  3.743 us |  0.77 | 0.1106 |      - |     696 B |        0.22 |
| System.Text.Json*      |  3.771 us |  0.77 | 0.4082 | 0.0076 |    2562 B |        0.80 |
| Farse                  |  4.877 us |  1.00 | 0.5035 |      - |    3200 B |        1.00 |
| Newtonsoft.Json*       |  6.164 us |  1.26 | 1.5182 | 0.0229 |    9544 B |        2.98 |
| Thoth.System.Text.Json |  8.095 us |  1.66 | 1.5717 | 0.0153 |    9944 B |        3.11 |
| Newtonsoft.Json        |  8.536 us |  1.75 | 2.8229 | 0.1373 |   17720 B |        5.54 |
| Thoth.Json.Net         | 10.423 us |  2.14 | 3.3569 | 0.1526 |   21136 B |        6.61 |

* Serialization

Example

Given the following JSON string.

{
    "id": "c8eae96a-025d-4bc9-88f8-f204e95f2883",
    "name": "Alice",
    "age": null,
    "email": "alice@domain.com",
    "profiles": [
        "01458283-b6e3-4ae7-ae54-a68eb587cdc0",
        "bf00d1e2-ee53-4969-9507-86bed7e96432",
        "927eb20f-cd62-470c-aafc-c3ce6b9248b0"
    ],
    "subscription": {
        "plan": "Pro",
        "isCanceled": false,
        "renewsAt": "2026-12-25T10:30:00Z"
    }
}

And the two (optional) operators.

// Parses a required property.
let (&=) = Parse.req

// Parses an optional property.
let (?=) = Parse.opt

We can create a parser.

open Farse
open Farse.Operators

module User =
    open Parse

    let parser =
        parser {
            let! id = "id" &= UserId.parser
            let! name = "name" &= string
            let! age = "age" ?= Age.parser
            let! email = "email" &= Email.parser
            let! profiles = "profiles" &= set ProfileId.parser

            // Inlined parser example.
            let! subscription = "subscription" &= parser {
                let! plan = "plan" &= Plan.parser
                let! isCanceled = "isCanceled" &= bool
                let! renewsAt = "renewsAt" ?= Parse.instant // Custom parser example.
    
                return {
                    Plan = plan
                    IsCanceled = isCanceled
                    RenewsAt = renewsAt
                }
            }

            // Simple "path" example, which can be very useful
            // when we just want to parse a (few) nested value(s).
            let! _isCanceled = "subscription.isCanceled" &= bool
      
            return {
                Id = id
                Name = name
                Age = age
                Email = email
                Profiles = profiles
                Subscription = subscription
            }
        }

With the following types.

open Farse
open NodaTime

type UserId = UserId of Guid

module UserId =

    let asString (UserId x) =
        string x

    let parser =
        Parse.guid
        |> Parser.map UserId

type Age = Age of byte

module Age =

    let asByte (Age x) = x

    let fromByte age =
        match age with
        | age when age >= 12uy -> Ok <| Age age
        | age -> Error $"Invalid age: %u{age}."

    let parser =
        Parse.byte
        |> Parser.validate fromByte

type Email = Email of string

module Email =

    let asString (Email x) = x

    let fromString str =
        // Some validation.
        Ok <| Email str

    let parser =
        Parse.string
        |> Parser.validate fromString

type ProfileId = ProfileId of Guid

module ProfileId =

    let asString (ProfileId x) =
        string x

    let parser =
        Parse.guid
        |> Parser.map ProfileId

type Plan =
    | Pro
    | Standard
    | Free

module Plan =

    let fromString = function
        | "Pro" -> Ok Pro
        | "Standard" -> Ok Standard
        | "Free" -> Ok Free
        | invalid -> Error $"Invalid plan: %s{invalid}."

    let asString = function
        | Pro -> "Pro"
        | Standard -> "Standard"
        | Free -> "Free"

    let parser =
        Parse.string
        |> Parser.validate fromString

type Subscription = {
    Plan: Plan
    IsCanceled: bool
    RenewsAt: Instant option
}

type User = {
    Id: UserId
    Name: string
    Age: Age option
    Email: Email
    Profiles: ProfileId Set
    Subscription: Subscription
}

Then we can just run the parser.

let user =
    User.parser
    |> Parser.parse json
    |> Result.defaultWith failwith

printf "%s" user.Name

Custom parsers

You can use Parse.custom to create custom parsers.

open Farse
open NodaTime
open NodaTime.Text
open System.Text.Json

module Parse =

    let instant =
        Parse.custom (fun (element:JsonElement) ->
            let string = element.GetString()
            match InstantPattern.General.Parse(string) with
            | result when result.Success -> true, result.Value
            | _ -> false, Instant.MinValue
        ) JsonValueKind.String

Creating JSON

We can also create JSON strings with the Json type.

Note: Use JNum<'a> if you want to be explicit.

JObj [
    "id", JStr <| UserId.asString user.Id
    "name", JStr user.Name
    "age",
        user.Age
        |> Option.map (Age.asByte >> JNum)
        |> JNil
    "email", JStr <| Email.asString user.Email
    "profiles",
        user.Profiles
        |> Seq.map (ProfileId.asString >> JStr)
        |> JArr
    "subscription", JObj [
        "plan", JStr <| Plan.asString user.Subscription.Plan
        "isCanceled", JBit user.Subscription.IsCanceled
        "renewsAt",
            user.Subscription.RenewsAt
            |> Option.map (_.ToString() >> JStr)
            |> JNil
    ]
]

This creates an object, but you can create any type and convert it with Json.asString.

Errors

More examples can be found here.

Error: Could not parse property 'prop'.
Expected: Number, actual: String.
Object:
{
  "prop": "1"
}

Farse doesn't throw exceptions and only catches JsonException, that is thrown when parsing invalid JSON.