Hello, World!

First Application

Let us build our first application with WebGear. To achieve this goal, you need to create a new project with three files as shown below.

Main.hshello.cabalstack.yaml

{-# LANGUAGE Arrows #-}
{-# LANGUAGE OverloadedStrings #-}

module Main where

import qualified Network.HTTP.Types as HTTP
import qualified Network.Wai.Handler.Warp as Warp
import WebGear.Server

main :: IO ()
main = Warp.run 3000 (toApplication helloHandler)
  where
    helloHandler =
      method HTTP.GET $
        path "/api/hello" $
          proc request -> do
            unlinkA <<< respondA HTTP.ok200 "text/plain" -< "Hello, World!" :: Text

cabal-version:      2.4
name:               hello
version:            0.1.0.0

executable hello
    main-is:          Main.hs
    build-depends:    base >=4.13.0.0 && <4.17
                    , http-types ==0.12.3
                    , warp ==3.3.*
                    , webgear-core ==1.0.5
                    , webgear-server ==1.0.5
    default-language: Haskell2010

resolver: nightly-2022-06-06
allow-newer: true

You can omit stack.yaml if you are not using stack.

Running the Server

Compile this program with Stack or Cabal and run it. Access the URL http://localhost:3000/api/hello and you should be greeted with a message Hello, World!.

Exploring the code

Here is an explanation of how the above code works:

1. The method HTTP.GET function indicates that this API accepts only GET requests.
2. The path "/api/hello" function matches a specific request path, any request that does not match this path will result in a 404 response.
3. The proc notation is special syntax introduced by arrows. You should read the arrows chapter if you are not familiar with it.
4. Finally, respondA is used to send a text/plain response with a body.
5. The helloHandler is converted to a WAI application with the help of toApplication and a Warp server is launched.

We will see more details about all these in the upcoming chapters.