LanguageExt HTTP

A functional wrapper around HttpClient intended to integrate into LanguageExt V5 based workflows.

Provides the expected methods (get, post, delete, etc.) returning either Http<HttpResponseMessage>, an "Http Monad", or a custom type (see "Usage in Larger Applications" below)

Rationale

If you're already convinced of the general preferability of the functional approach, you probably don't need this Rationale section.

If you're not convinced but curious, check out the code cleanup project, noting the differences between the functional and imperative approaches, not just in total lines of code, but also the greater simplicity[^1] of nearly every part of the functional approach.

I may create a more dedicated "literate coding" style writeup of the above in the future (as of 3/3/2026), but for now hopefully the code can speak for iself.

[^1] Simplicity in the Rich Hickey sense of the word, it may not be easy at first if you're not familiar with the concepts!

Usage

dotnet add package LanguageExt.Http or LanguageExt.Http on nuget

// add to GlobalUsings as appropriate
using LanguageExt;// <- you're probbably already using this
using static LanguageExt.Http;

The Http monad implements and thus gives us

• Monad, for basic sequencing and composition, Bind (with LINQ syntax) and Traverse being the bread and butter of most of what you'll do

// `parseUsersResponse` made up for example purposes
from users in get("http://api-one.url/old_users").Bind(parseUsersResponse) 
// `serializeUser` made up as an example
from results in users.Traverse(user => post("http://api-two.url/backfill_users", serializeUser(user)))
select results

• Fallible, for generalizable and modular error handling

get("http://api-one.url/old_users").Bind(parseUsersResponse) 
    // error codes and handlers made up for example purposes
    | @catch<Http, Seq<User>>(ParseErrorCode, HandleParseError)
    | @catch<Http, Seq<User>>(UrlErrorCode, HandleUrlError)

• MonadUnliftIO for not only lifting arbitrary IO operations (such as debugging logs), but also access to Retry, Fork, Repeat and related goodes

var attempts = Atom(0);
var lookup =
    from _1 in attempts.SwapIO(i => i + 1)
    from response in get("http://api-one.url/old_users")
    from _ in IO.lift(() => Console.WriteLine($"Query attempt {attempts.Value}"))
    select response;
lookup.RetryIO(Schedule.linear(1.Seconds()).Take(3));

• Readable, to enable threading of aHttpClient throughout the application
  • If you need to thread CancellationToken as well, you can utilize IO's built-in EnvIO

get("http://example.com")
    .RunIO(new HttpClient()) // Run HTTP Monad
    .Run(EnvIO.New(token: cancellationToken)); // Normal IO Monad run

JSON Requests

Parsing

Parsing JSON responses is outsourced almost entirely to LanguageExt.Json (included as a depenedency to this library)

That library's documentation as well as the json parsing example in this repo are hopefully sufficiently instructive.

Serializing

Use the jsonContent method to serialize any object into a JSON encoded request, defined as

public static HttpContent jsonContent(object? value) => 
    JsonContent.Create(value, options: GlobalJsonConfig.Options);

where GlobalJsonConfig is from LanguageExt.Json, which give you access to any global custom converters defined there.

Usage in Larger Applications

However, since a concrete Http type is an obstacle to composition in large applications, nearly every method in this library has both an Http-based and generalized version, for exmaple

• "The basics" (get, post, delete, etc.), can be generalized to any MonadIO that implements Readable for an Env that implements this library's HasHttpClient interface
• parseUri can be generalized to any Fallible Applicative
• Response parsing methods such as stream can be generalized to any MonadIO

For example, if we have the following hypothetical method

K<M, Stream> getStreamWithDebug<M, Env>(string rawUri)
    where M : Readable<M, Env>, MonadIO<M>, Fallible<M>
    where Env : HasHttpClient
    =>
        from uri in parseUri<M>(rawUri)
        from rawResponse in get<M, Env>(uri)
        from _1 in IO.lift(() => Console.WriteLine($"Successful fetch from {rawUri}"))
        from response in stream<M>(rawResponse)
        from _2 in IO.lift(() => Console.WriteLine($"Successfully read as stream"))
        select response;

We can use it with this libarary's Http

// genericMethod<>().As() is how most of this library 
// is currently implemented under the hood
getStreamWithDebug<Http, HttpEnv>("http://example.com").As();

With LanguageExt's built-inEff

public record MyCustomConfig(HttpClient Client, string ApiKey, int MagicNumber) : HasHttpClient;
getStreamWithDebug<Eff<MyCustomConfig>, MyCustomConfig>("http://example.com");

Or with your application's very own monad(s)

public record MyCustomApp<A>(ReaderT<MyCustomConfig, IO, A> run) : K<MyCustomApp, A>;
// ... full implemetation of above omitted for brevity ...

getStreamWithDebug<MyCustomApp, MyCustomConfig>("http://example.com");

There's also a branch "static-module-import" that experiments with "Module Style" using statements that I may try to merge int master and release if this the above approach with the per-method type parameters ends up being too cumbersome for too many people.

Testing

Mocking HttpClient is much more awkward than it should be, so this library provides a Http.client method that, given a Func<HttpResponseMessage, HttpResponseMessage> ( or other overload ) handles all of the nasty business of dealing with an HttpMessageHandler for you.

var mockHttpClient = Http.client((HttpRequestMessage message) => new HttpResponseMessage(HttpStatusCode.OK));

This combined with the natural structure of the "reader monad pattern" this follows should enable much smoother mocking of http functionality in general. It may even be convenient enough to justify sneaking this library (and by extension LanguageExt) into a "regular" imperative/OO codebase that uses HttpClient!

TODO

https://github.com/micmarsh/LanguageExt.Http/issues

There's a lot of work to be done on "the LanguageExt ecosystem" in general, as V5 itself is technically still in beta. Feel free to open discussions, issues or PRs to communicate how this library can better fit your particular use case

Copyright 2026 Michael Marsh

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