LanguageExt.Json

A small suite of tools based on LanguageExt V5 to enable parsing and querying json in a functional manner.

Add Nuget Package

dotnet add package LanguageExt.Json --version 0.1.0 or LanguageExt.Json 0.1.0 on nuget

"Module Style" Static Import

This library defines no new concrete types (there's no "json query monad" or anything of that sort), instead you specify any Fallible Applicative when importing the static class. You can start with LanguageExt.Fin for the simplest use case

using static LanguageExt.Json<LanguageExt.Fin>;

However, you could also use IO to better fit into realistic application stacks

using static LanguageExt.Json<LanguageExt.IO>

Or, of course, your own custom application monad

using static LanguageExt.Json<My.Own.Namespace.MyApp>

This approach is inspired by LanguageExt.Megaparsec (which is itself most likely inpsired by OCaml modules) and makes a lot of sense in C# to clean up excessive type parameter specification in application logic, as we'll see in the examples below.

Non-"Module Style" Use

If needed (perhaps you intentionally want do a particular json-based action within a file using a different type than you imported), can always call the static class directly with the specific type (Json<Fin> in example below)

using static LanguageExt.Json<LanguageExt.IO>
// ... other application logic ...
parse("{foo: 'bar'}") >> key("foo") >> cast<string> // returns IO<string> == IO("bar")

// ... other application logic ...

// returns Fin<string> == "bar"
Json<Fin>.parse("{foo: 'bar'}") >> Json<Fin>.key("foo") >> Json<Fin>.cast<string>;

Usage Examples

(see UsageExamples.cs in test project)

Basic serialization and deserialization

public record Product(int id, string title, Seq<string> tags, 
    double price, Option<string> thumbnail);
// ...
var product = new Product(123, "Product 123", ["tag1", "tag2"], 9.99, None);
var productString = serialize(product);
Assert.Equal(product, productString >> deserialize<Product>);

This will potentially be the most common use of this library. Note that default converters for Seq and Option are provided and automatically used under the hood. Seqs are treated as equivalent to json arrays and Option as values that could potentially be null

Querying nested data structures

public record Review(string reviewerEmail, int rating);
public record Product2(int id, Seq<Review> reviews);
//...
var product = new Product2(123, [
    new Review("foo@bar.com", 5), 
    new Review("you@bar.com", 2),
    new Review("me@bar.com", 1)]);
var productString = serialize(product);

var emails = productString >> parse
                           >> key("reviews")
                           >> iterate
                           >> traverse(key("reviewerEmail") >> cast<string>);
                           
Assert.Equal(product.reviews.Map(r => r.reviewerEmail), emails);

Zooming in on the right side of var emails = ..., we see several (monadic) operations chained together using the bind (>> in C# 10) operator

• parse takes a string (in this case) or stream and converts it to a JsonElement
• key looks up the given key from the given (object) JsonElement
• iterate converts a JsonElement that represents an array to a Seq<JsonElement>
• traverse applies the given operation to each element in the given Seq
  • This is a Seq-specific (to aid generic type inference) convenience method that's not json-specific, a sort of "missing Prelude method" for Traverse
• cast converts the given JsonElement to the given type
• index (not demo'ed here) works like key for JsonElements that represent arrays.

As there are obviously lots of potential failure points at every step of this query, each method returns the type specified in the static import, with a hopefully useful JsonError (derived from Error and has its own JsonError.Code) to aid in issue diagnosis.

var shouldThrow = serialize(product) >> parse
                                     >> key("reviews")
                                     >> key("reviewzzz");

var ex = Assert.Throws<JsonErrorException>(() => shouldThrow.ThrowIfFail());
Assert.Contains("reviewzzz", ex.Message);
Assert.Contains(JsonValueKind.Array.ToString(), ex.Message);
Assert.Equal(JsonError.Code, ex.Code);

Since nearly all types used with this library will also be Monads, and all of the above methods are "curried" (using C# overloaded methods) we can use >> to chain these operations in a visually elegant way.

Other Examples

See the example file in LanguageExt.Http for a more-realistic-but-ultimately-still-contrived example of how this might work "in the wild", and with a custom monad type.

Provided and Custom Converters

This library, as mentioned above, provides converters for Seq and Option that it will use automatically for any serialization or deserialization operation.

To provide your own custom converters, use GlobalJsonConfig.AddCustomConverters, like so

// you'll likely call this in Startup.cs or similar area of application
GlobalJsonConfig.AddCustomConverters(new MyCustomConverter(), new MyCustomFactory())

While this is explicitly a global(!) mutable(!!!) variable, the vast majority of applications are only ever going to want to define custom converters once, so this makes the most sense egonomically, especially compared to the main alternative of some kind of configuration threaded through a Readable monad.

TODOs

• Main missing feature is Newtonsoft support
  • Very possible to add by introducing a layer of abstraction between the main methods deserialize, parse, index, key, etc. and any interaction with System.Text.Json types, and then adding another parameter to the "module import" to specify which underlying json library to use.
  • The design is fully sketched out in my head, just need to implement ;-)
• Remove this library's Kleisli composition operator once the equivalent is in LanguageExt.Core

Copyright 2026 Michael Marsh

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