Simple HTTP in Haskell

I ended up playing around a bit with the Network.HTTP module. My goal was to make a one-liner to load test an HTTP service I work on. I wanted to see how far I could towards doing what I normally do in ab, but in Haskell.

Here’s the one-liner for use with GHCi:

import Network.HTTP
import Control.Monad

mapM_ (\x -> replicateM_ 100 (simpleHTTP (getRequest "http://localhost:8000/"){rqBody="waa", rqHeaders=[Header (HdrCustom "x-project-id") (show x)]}) >> print x) [1..150]
--

Let’s break it down:

  mapM_ :: Monad m => (a -> m b) -> [a] -> m ()
  mapM_ (some action) [1..150] {-- apply the action with each
                                   integer as input --}

  (>>) :: Monad m => m a -> m b -> m b
  (\x -> (action x) >> print x) {-- take a value, do something with it,
                                    discard the result, then print it --}

  replicateM_ :: Monad m => Int -> m a -> m ()
  (\x -> (replicateM_ 100 $ (action x)) -- replicate the action 100 times

  simpleHTTP :: HStream ty =>
                Request ty -> IO (Network.Stream.Result (Response ty))
  (\x -> simpleHTTP (... x)) {-- the action is an HTTP request
                                consisting of... --}

  getRequest :: String -> Request_String
  (\x -> getRequest Url{... x}) -- a request on a url

  -- and this is what a Request looks like
  data Request a
    = Request {rqURI :: Network.URI.URI,
               rqMethod :: RequestMethod,
               rqHeaders :: [Header],
               rqBody :: a}
  rqBody :: Request a -> a
  rqHeaders :: Request a -> [Header]

  -- modify the initial request body
  (\x -> {rqBody="waaa", rqHeaders=[... x]} {-- with a particular body and
                                                headers --}

  -- modify the initial request headers: {x-project-id: x}
  HdrCustom :: String -> HeaderName
  (\x -> rqHeaders=[Header (HdrCustom "x-project-id") (show x)])

And now for a decomposed module:

module Main where

import Network.HTTP
import Network.Stream (Result)
import Control.Monad (replicateM_)

request :: Show a => a -> IO (Result (Response String))
request proj = simpleHTTP $ bldRequest (show proj)
    where bldRequest project = (getRequest url){ rqBody="waaa"
                                               , rqHeaders=mkHeaders project
                                               }
          mkHeaders project = [Header (HdrCustom "x-project-id") project]
          url = "http://localhost:8000"

main :: IO ()
main = mapM_ (replicateM_ 100 . request) [1..150]

It was surprisingly fun to tinker and get to this. It took several GHCi experiments. The output from running this in GHCi looks like:

> main
1
2
3
4
5
6
7
8
9
10
11
...
150
it :: ()

It was also fun to realize how easy it was to modify by taking advantage of GHCi’s readline support. I’d hit <up>, and I could modify any of:

• How many projects do I want to range over? [1..150]
• How many times do I want to repeat each request? replicateM_ 100
• What do I want the request body to be? rqBody="waaa"
• Do I want to print to the console as I go? >> print x

This exercise gave me a chance to work with simple HTTP in Haskell and to fiddle with monadic I/O. The experience on a whole was pleasant.

I leave you with this thought: Never underestimate the power of a good REPL when learning a new language/library/module/system.