Easy-to-use, type-safe, expandable, high-level HTTP library

Version on this page:0.2.0@rev:3
LTS Haskell 22.29:3.13.2@rev:4
Stackage Nightly 2024-07-13:3.13.3
Latest on Hackage:3.13.3

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BSD-3-Clause licensed and maintained by Mark Karpov
This version can be pinned in stack with:req-0.2.0@sha256:b77f15fa1001785ea46659f8ae987ea9cd4ef394fb9abf212300d77284ba1ad3,5955

Module documentation for 0.2.0


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{-# LANGUAGE OverloadedStrings    #-}
{-# OPTIONS_GHC -fno-warn-orphans #-}

module Main (main) where

import Control.Exception (throwIO)
import Network.HTTP.Req
import Data.Aeson

-- Just make your monad stack an instance of MonadHttp in your application
-- and start making requests, enjoy automatic connection sharing.

instance MonadHttp IO where
  handleHttpException = throwIO

main :: IO ()
main = do
  let payload = object
        [ "foo" .= (10 :: Int)
        , "bar" .= (20 :: Int) ]
  -- One function, full power and flexibility.
  r <- req POST -- method
    (https "" /: "post") -- safe by construction URL
    (ReqBodyJson payload) -- use built-in options or add your own
    jsonResponse -- specify how to interpret response
    mempty       -- query params, headers, explicit port number, etc.
  print (responseBody r :: Value)

This is an easy-to-use, type-safe, expandable, high-level HTTP library that just works without any fooling around.

What does the “easy-to-use” phrase mean? It means that the library is designed to be beginner-friendly, so it’s simple to add it to your monad stack, intuitive to work with, well-documented, and does not get in your way. Doing HTTP requests is a common task and Haskell library for this should be very approachable and clear to beginners, thus certain compromises were made. For example, one cannot currently modify ManagerSettings of default manager because the library always uses the same implicit global manager for simplicity and maximal connection sharing. There is a way to use your own manager with different settings, but it requires a bit more typing.

“Type-safe” means that the library is protective and eliminates certain class of errors. For example, we have correct-by-construction URLs, it’s guaranteed that user does not send request body when using methods like GET or OPTIONS, amount of implicit assumptions is minimized by making user specify his/her intentions in explicit form (for example, it’s not possible to avoid specifying body or method of a request). Authentication methods that assume TLS force user to use TLS on type level. The library carefully hides underlying types from lower-level http-client package because it’s not safe enough (for example Request is an instance of IsString and if it’s malformed, it will blow up at run-time).

“Expandable” refers to the ability of the library to be expanded without ugly hacking. For example, it’s possible to define your own HTTP methods, new ways to construct body of request, new authorization options, new ways to actually perform request and how to represent/parse response. As user extends the library to satisfy his/her special needs, the new solutions work just like built-ins. That said, all common cases are covered by the library out-of-the-box.

“High-level” means that there are less details to worry about. The library is a result of my experiences as a Haskell consultant, working for several clients who have very different projects and so the library adapts easily to any particular style of writing Haskell applications. For example, some people prefer throwing exceptions, while others are concerned with purity: just define handleHttpException accordingly when making your monad instance of MonadHttp and it will play seamlessly. Finally, the library cuts boilerplate considerably and helps write concise, easy to read and maintain code.

The library uses the following mature packages under the hood to guarantee you best experience without bugs or other funny business:

It’s important to note that since we leverage well-known libraries that the whole Haskell ecosystem uses, there is no risk in using Req, as the machinery for performing requests is the same as with http-conduit and Wreq, it’s just the API is different.

Motivation and Req vs other libraries

This section is my opinion and it contains criticisms of other well-known libraries. If you’re user/fan of one of these libraries, please remember not to react aggressively and respect the fact that I may have different views on API design from yours.

I have spent time to write the library because sending HTTP requests is such a common thing and still there is no high-level library for that in Haskell that I could use with pleasure. I’ll explain why.

First of all there is http-client and http-client-tls. They just work. I have no issues with the libraries except that they are too low-level for my taste. Indeed, even the docs say that they are low-level and “intended as a base layer for more user-friendly packages”. This is exactly how I use them in Req, as base level. Req is nothing but a different API to http-client, so it only works because of hard work put into http-client.

http-conduit definitely has its place. For one thing it allows you to stream request and response bodies in constant memory, what other library allows you to do that? On the other hand if you take a look at Network.HTTP.Simple, then although it’s said that it’s a “higher level API”, it’s mostly the same as vanilla http-client in spirit/approach and just adds conduit-powered functions to perform requests and allows to use global implicit Manager (super-cool idea, BTW, Req does the same). If I tried to frame what exactly I don’t like about http-conduit in words, then it would be “the way requests are constructed”. You set, set, set instead of being forced to declare necessary bits and being allowed to declare optional bits in a way that their combination is certainly valid. And you parse request from a string without the protection of TH that otherwise saves the day as in Yesod.

Then there is Wreq. wreq doesn’t see much development lately. wreq is by itself a weird library, IMO. You have functions per method — not very good, as there may be new methods, like PATCH which is not new but still missing (well you have customMethod, but what is the point of having per-method functions if you have a more general way to use any method? you should be able to just insert methods in the “argument slot” of customMethod and end up with a more general solution). Now every method function has a companion that takes Options (like you have get and getWith). Why the duplication? Where is generality and flexibility? This is not all though, because you cannot really use get you see in the main module, because you want to have connection sharing. Wreq’s author does not take the gift of automatic connection re-use Manager from http-client provides, he invents the whole new thing of “sessions”. Only inside a session your connections will be shared and re-used. However with the session stuff you have yet another set of per-method functions like get and getWith — these are different ones, to be used with sessions! Now if you have multi-threaded app, here is a surprise for you: you can’t share connections between threads as connections are shared only inside withSession friend and “session will no longer be valid after that function returns”. Disclaimer: I don’t use Wreq, see below. If something in this paragraph is not correct, please let me know and I’ll remove it. Also there are valid uses for sessions, but the point is that they are too inconvenient for common tasks.

It’s funny that one client I worked for had to have his own little wrapper around http-client just because he could not possibly use wreq and http-client and friends were too low-level. The previous paragraph is extracted from a talk with a Haskell developer who works for that client. I thought to myself “something is wrong with HTTP client libraries in Haskell if they had to make a wrapper”.

What else? I used servant-client a couple of times but amount of boilerplate is too high. If you have several query parameters, and you use just one of them, good luck passing lots of Nothings.

Unsolved problems

AWS request signing is problematic because request body can be in form of action to execute (and all that “popper” stuff for streaming), not just ByteString and so getting its digest (hash) is not trivial without running the action and consuming body in its entirety before the request in made. In Wreq the author chose to just use error when body is not a (strict or lazy) ByteString. Maybe it’s OK for Wreq, but I don’t consider this proper solution for Req as we support full variety of body options. For example what if I want to upload 1 Gb file to S3? I want to stream it in constant memory but at the same time I need to calculate its hash before I start streaming. One solution to the problem seems to be in taking the hash explicitly (as an argument of hypothetical awsAuth) and making it responsibility of library user to calculate the hash correctly. I don’t like this because it’s not user-friendly. So the question stays open, for now there is no AWS signing functionality provided out-of-the-box.

Related packages

The following packages are designed to be used with Req:

  • req-conduit — support for streaming request and response bodies in constant memory.

If you happen to have written a package that adds new features to Req, please submit a PR to include it in this list.


Issues, bugs, and questions may be reported in the GitHub issue tracker for this project.

Pull requests are also welcome and will be reviewed quickly.


Copyright © 2016–2017 Mark Karpov

Distributed under BSD 3 clause license.


Req 0.2.0

  • Added support for multipart form data in form of ReqBodyMultipart body option and reqBodyMultipart helper function. This also required a change in type signature of getRequestContentType, which now takes body, not Proxy body because we need to extract boundary from body and put it into Content-Type header. This change, however, shouldn’t be too dangerous for end-users.

  • Added support for OAuth 1.0 authentication via oAuth1 option.

Req 0.1.0

  • Initial release.