Simple parsing library for F# using System.Text.Json.
Heavily inspired by Thoth.Json.Net and its composability; Farse uses a slightly different syntax, includes a computational expression, and a few custom operators that simplify parsing. It also tries to keep a low overhead while still providing utility and acceptable error messages.
Things are still changing but a pre-release version is available.
dotnet add pa
8000
ckage Farse --prereleaseThere are some initial benchmarks here.
BenchmarkDotNet v0.15.2, macOS Sequoia 15.5 (24F74) [Darwin 24.5.0]
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 | 3.674 us | 0.72 | 0.1106 | - | 696 B | 0.17 |
| 'System.Text.Json (Auto)' | 3.683 us | 0.72 | 0.4082 | 0.0076 | 2562 B | 0.64 |
| Farse | 5.105 us | 1.00 | 0.6409 | - | 4024 B | 1.00 |
| 'Newtonsoft.Json (Auto)' | 6.240 us | 1.22 | 1.5182 | 0.0229 | 9544 B | 2.37 |
| Thoth.System.Text.Json | 8.239 us | 1.61 | 1.5869 | 0.0305 | 9944 B | 2.47 |
| Newtonsoft.Json | 8.910 us | 1.75 | 2.8229 | 0.1373 | 17720 B | 4.40 |
| Thoth.Json.Net | 10.510 us | 2.06 | 3.3569 | 0.1526 | 21136 B | 5.25 |Given the following JSON string.
{
"id": "c8eae96a-025d-4bc9-88f8-f204e95f2883",
"name": "Alice",
"age": null,
"email": "alice@domain.com",
"groups": [
"01458283-b6e3-4ae7-ae54-a68eb587cdc0",
"bf00d1e2-ee53-4969-9507-86bed7e96432",
"927eb20f-cd62-470c-aafc-c3ce6b9248b0"
],
"subscription": {
"plan": "Pro",
"isCanceled": true,
"renewsAt": null
}
}And the two (optional) operators.
/// <summary>
/// Parses a required property.
/// </summary>
let (&=) = Parse.req
/// <summary>
/// Parses an optional property.
/// </summary>
let (?=) = Parse.optWe can create the following 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" ?= int
let! email = "email" &= Email.parser
let! groups = "groups" &= list GroupId.parser
let! subscription = "subscription" &= parser {
let! plan = "plan" &= Plan.parser
let! isCanceled = "isCanceled" &= bool
let! renewsAt = "renewsAt" ?= dateTime
return {
Plan = plan
IsCanceled = isCanceled
RenewsAt = renewsAt
}
}
// Simple "path" support, 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
Groups = groups
Subscription = subscription
}
}With the following types.
open Farse
type UserId = UserId of Guid
module UserId =
let asString (UserId x) =
string x
let parser =
Parse.guid
|> Parser.map UserId
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 GroupId = GroupId of Guid
module GroupId =
let asString (GroupId x) =
string x
let parser =
Parse.guid
|> Parser.map GroupId
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: DateTime option
}
type User = {
Id: UserId
Name: string
Age: int option
Email: Email
Groups: GroupId list
Subscription: Subscription
}Then we can just run the parser.
let user =
User.parser
|> Parser.parse json
|> Result.defaultWith failwith
printfn "%s" user.NameWe can also construct JSON strings.
let jsonString =
JsonString.create [
"id", JStr (UserId.asString user.Id)
"name", JStr user.Name
"age",
user.Age
|> Option.map (Int >> JNum)
|> JOpt
"email", JStr (Email.asString user.Email)
"groups",
user.Groups
|> List.map (GroupId.asString >> JStr)
|> JArr
"subscription",
JObj [
"plan", JStr (Plan.asString user.Subscription.Plan)
"isCanceled", JBit user.Subscription.IsCanceled
"renewsAt",
user.Subscription.RenewsAt
|> Option.map (DateTime.asString >> JStr)
|> JOpt
]
]
let json =
jsonString
|> JsonString.asString
printfn "%s" json