Web application in Haskell

Haskell can be (of course) used for network communication and also for building various web applications. In this tutorial, we are going to look at basics of network communication in Haskell, some specialized libraries making it simpler, and then at web frameworks.

Network communication

On the way to web applications, it is good to know how you can work with network communication on lower levels than is some web framework.


The most low-level solutions work directly with sockets via Network.Socket module. With that, you have a full control over the communication. Essentially the entire C socket API is exposed through this module, so if you are familiar with sockets from C, then it will be easy for you in Haskell: bind, listen, receive, send, getAddrInfo, etc.

Server-client demo with sockets

There is a demo with echo server and client in the Network.Socket documentation. We will show a bit simpler example without forking.

{-# LANGUAGE OverloadedStrings #-}
module Main (main) where

import qualified Control.Exception as E
import Control.Monad (unless, forever, void)
import qualified Data.ByteString as S
import qualified Data.ByteString.Char8 as C
import Network.Socket hiding (recv)
import Network.Socket.ByteString (recv, sendAll)

main :: IO ()
main = withSocketsDo $ do
    addr <- mkAddr "localhost" "3000"
    E.bracket (open addr) close loop
    mkAddr host port = do
        let hints = defaultHints {
                addrFlags = [AI_PASSIVE]
              , addrSocketType = Stream
        addr:_ <- getAddrInfo (Just hints) (Just host) (Just port)
        return addr
    open addr = do
        sock <- socket (addrFamily addr) (addrSocketType addr) (addrProtocol addr)
        setSocketOption sock ReuseAddr 1
        bind sock (addrAddress addr)
        listen sock 10
        return sock
    loop sock = forever $ do
        (conn, peer) <- accept sock
        putStrLn $ "Connection from " ++ show peer
        msg <- recv conn 1024
        C.putStrLn $ msg
        unless (S.null msg) $ do
          sendAll conn (S.reverse msg)
        close conn
{-# LANGUAGE OverloadedStrings #-}
module Main (main) where

import qualified Control.Exception as E
import qualified Data.ByteString.Char8 as C
import Network.Socket hiding (recv)
import Network.Socket.ByteString (recv, sendAll)

main :: IO ()
main = withSocketsDo $ do
    addr <- mkAddr "localhost" "3000"
    E.bracket (open addr) close talk
    mkAddr host port = do
        let hints = defaultHints { addrSocketType = Stream }
        addr:_ <- getAddrInfo (Just hints) (Just host) (Just port)
        return addr
    open addr = do
        sock <- socket (addrFamily addr) (addrSocketType addr) (addrProtocol addr)
        connect sock $ addrAddress addr
        return sock
    talk sock = do
        putStrLn "What do you want to send?"
        toSend <- getLine
        sendAll sock (C.pack toSend)
        msg <- recv sock 1024
        putStr "You received: "
        C.putStrLn msg

A you can see, most of the work happens in do of IO and it highly resembles classic imperative programming. Indeed, networking and also most of UI is inherently not a show room of beautiful Haskell code, but it gets the job done ;-).

Specialized libraries

Naturally, there are many specialized libraries that provide a higher-level interface for network communication than plain sockets when you want to work with some specific protocol (POP3, SMTP, SSH, or HTTP). Some are listed here but you can find more on Hackage.

The need of REST API client is something very common. For writing a simple one, you may use wreq package. It provides simple but powerful lens-based API and it supports often-used techniques like OAuth, decompression, file upload, etc.

-- GitHub API: list public repositories of user
{-# LANGUAGE OverloadedStrings #-}
import Control.Lens
import Data.Aeson.Lens
import Data.ByteString.Char8 hiding (putStrLn, getLine)
import qualified Data.Text as T (unpack)
import Data.Foldable
import Network.Wreq

mkURI :: String -> String
mkURI username = "https://api.github.com/users/" ++ username ++ "/repos"

getRepos :: String -> IO [String]
getRepos username = do
  r <- get (mkURI username)
  return . fmap T.unpack $ r ^.. responseBody . values . key "full_name" . _String

main :: IO ()
main = do
  putStrLn "Enter GitHub username:"
  username <- getLine
  repos <- getRepos username
  putStrLn $ "## First 25 public repos: "
  traverse_ putStrLn repos

Web Frameworks overview

As with other languages, you usually don’t want to build a web application from the scratch which would bind ports, listen and parse requests and compose responses. For a higher abstraction, you might want to use a web framework.

There are several frameworks in Haskell (see here) and here is our list of the most used ones:

As you can see, there is quite an above-average offer of them. They mostly differ at the level of abstraction and scope: Scotty being relatively low-abstraction routing and middleware and Yesod being a complete solution including templating and persistence, everything on a high abstraction level. The choice depends on your preference and needs. As always, a higher abstraction means the system does a lot of job for yourself, you write fewer code, which means a higher effectiveness and less bugs. On the other hand, you may face a leaky abstraction problem at some point.

We are going to show briefly Snap and Yesod, because they are used often and then we will go deeper with our favourite versatile Scotty. Next time, we will look at a different one to build quickly a REST API for our front-end app(s).

However, before we dive into the topic, there is one more note, which may excite you. In the first lecture, we explained what a referential transparency is and that it brings certain qualities to code – ability to reason about, reusability, testability, parallelism “for free”. In the case of web frameworks, you can experience reusability coming from reference transparency very clearly: web frameworks are typically built of certain independent, shared components like a web server (warp) or a web application interface (wai). These highly-specialised components are developed independently, tested independently and as such, the whole ecosystem exercises an unparalleled separation of concerns and thanks to it, it is easier evolvable, reliable and lively. Moreover, you can use the low-level components independently and integrate them easily in another framework or even an non-web application – you can e.g. use templating from Yesod in Scotty, if you like, or its persistence layer in a simple CLI application.


Snap is a simple web development framework for UNIX systems, written in the Haskell programming language. It consists of:

  • A fast HTTP server library
  • A sensible and clean monad for web programming
  • An HTML-based templating system for generating pages (heist)

More examples are in the documentation.

“Hello World”

import Snap

site :: Snap ()
site =
    ifTop (writeBS "hello world") <|>
    route [ ("foo", writeBS "bar")
          , ("echo/:echoparam", echoHandler)
          ] <|>
    dir "static" (serveDirectory ".")

echoHandler :: Snap ()
echoHandler = do
    param <- getParam "echoparam"
    maybe (writeBS "must specify echo/param in URL")
          writeBS param

main :: IO ()
main = quickHttpServe site


Snap also has a very nice philosophy in form of an optional system for building reusable pieces web functionality called “snaplets”. Snaplets make it easy to share and reuse common code across multiple web apps. The default snaplets let you get a full-featured web application up and running very fast.

If you want to build such application, read this.


Yesod is a Haskell web framework for productive development of type-safe, RESTful, high performance web applications. It builds on Haskell features such as compile-time errors (instead of runtime), seamlessly asynchronous computation, scalability, good performance and light-weight syntax.

Another advantage of Yesod is comprehensive documentation including:

If that is not enough you can ask the community.

Yesod is a “Mercedes” of Haskell web frameworks. It means a lot of comfort is prepared for you to enjoy, however it also means that people needing an agile small car for Italian crooked streets may experience troubles ;-).

“Hello World”

From the following example, you can see that Yesod uses a lot of Template Haskell which makes it a little bit hard to get at the beginning, but it enchants you with a lot of pleasant magic. And yes, every magic has some costs ;-) (see the previous lecture).

{-# LANGUAGE OverloadedStrings     #-}
{-# LANGUAGE QuasiQuotes           #-}
{-# LANGUAGE TemplateHaskell       #-}
{-# LANGUAGE TypeFamilies          #-}
import Yesod

data HelloWorld = HelloWorld

mkYesod "HelloWorld" [parseRoutes|
/ HomeR GET

instance Yesod HelloWorld

getHomeR :: Handler Html
getHomeR = defaultLayout [whamlet|Hello World!|]

main :: IO ()
main = warp 3000 HelloWorld


Scotty is another Haskell web framework inspired by Ruby’s Sinatra, using WAI and Warp (a fast, light-weight web server for WAI applications). You can write your own application just with WAI (Web Application Interface), but Scotty provides you with abstractions from a low-level communication. Sadly, there is not so much documentation about Scotty, everything is just on GitHub. Scotty uses primarily Blaze HTML for HTML “templates”, however, as we explained, you may also integrate it with any templating library you like.

“Hello World”

{-# LANGUAGE OverloadedStrings #-}
import Web.Scotty

main = scotty 3000 $ do
  get "/" $ do
    html "Hello World!"

Surprisingly easy, right?!


Writing HTML fragments as strings and compose them together can be fairly bothersome. For that purpose, there are libraries for templating known from other languages and frameworks as well.

Blaze templates

One of the well-known and widely used solution for HTML templates is Blaze HTML. It is “a blazingly fast HTML combinator library for the Haskell programming language”. A huge advantage of Blaze is that you write HTML via HTML-like lightweight DSL in Haskell with the great type system. Blaze and Haskell won’t allow you to make a non-sense HTML, although it does not check a full conformity, of course.

{-# LANGUAGE OverloadedStrings #-}
import Data.Text as T
import Text.Blaze.Html5 as H hiding (main)
import Text.Blaze.Html5.Attributes as A
import Text.Blaze.Html.Renderer.Pretty (renderHtml)

type User = String

userInfo :: Maybe User -> Html
userInfo u = H.div ! A.id "user-info" $ case u of
    Nothing ->
        a ! href "/login" $ "Please login."
    Just user -> do
        "Logged in as "
        toHtml $ T.pack user

somePage :: Maybe User -> Html
somePage u = html $ do
    H.head $ do
        H.title "Some page."
    H.body $ do
        userInfo u
        "The rest of the page."

main :: IO ()
main = putStr . renderHtml $ somePage (Just "Marek")

An interesting tool, that you might find useful, is blaze-from-html.

You might ask “What about styles?” or “What if want to have some JavaScript there?”. For styles, there is clay - a CSS preprocessor like LESS and Sass, but implemented as an embedded domain specific language (EDSL) in Haskell. Similarly to Blaze, you write CSS in Haskell. For JavaScript, stay tuned for the next tutorial ;-).

Hastache templates

If you are already familiar with some web development, you’ve probably heard about the popular templates. In Haskell, we have Haskell implementation of Mustache templates called hastache.

import Text.Hastache
import Text.Hastache.Context
import qualified Data.Text.Lazy.IO as TL

main = hastacheStr defaultConfig (encodeStr template) (mkStrContext context)
    >>= TL.putStrLn

template = "Hello, world! We know you, !"

context "reverse" = MuLambda (reverse . decodeStr)
context "name" = MuVariable "Haskell"

A useful source of information what can you do with Hastache are examples.


If you are familiar with Python and more specifically with Jinja2 templates, then you might be a fan of Ginger library. It is implementation of Jinja2 templates in Haskell and it have quite nice documentation. Usage is quite simple, just look at the example:

You have some HTML file with Jinja2 tags:

{# There could be some "extends" for layout #}
<!DOCTYPE html>
        <h1>Hello, !</h1>

            <p>Condition is false or undefined</p>

Then you can load template, supply context into it, and render as text:

{-# LANGUAGE OverloadedStrings #-}
module Main where

import Data.HashMap.Strict (fromList, HashMap)
import qualified Data.HashMap.Strict as HashMap
import Data.Hashable
import Data.Text (Text)
import System.Exit (exitFailure)
import System.IO (IOMode(ReadMode), openFile, hGetContents)
import System.IO.Error (tryIOError)
import Text.Ginger
       (makeContextHtml, Template, toGVal, runGinger, parseGingerFile, VarName)
import Text.Ginger.GVal (ToGVal, GVal)
import Text.Ginger.Html (htmlSource)

-- A simple hashmap that we'll use as our template context
sampleContext :: HashMap Text Text
sampleContext = fromList [("name", "Alice")]

-- Given a Template and a HashMap of context, render the template to Text
render :: Template -> HashMap VarName Text -> Text
render template contextMap =
  let contextLookup = flip scopeLookup contextMap
      context = makeContextHtml contextLookup
  in htmlSource $ runGinger context template

-- Wrapper around HashMap.lookup that applies toGVal to the value found.
-- Any value referenced in a template, returned from within a template, or used
-- in a template context, will be a GVal
  :: (Hashable k, Eq k, ToGVal m b)
  => k -> HashMap.HashMap k b -> GVal m
scopeLookup key context = toGVal $ HashMap.lookup key context

loadFileMay :: FilePath -> IO (Maybe String)
loadFileMay fn =
  tryIOError (loadFile fn) >>= \e ->
    case e of
      Right contents -> return (Just contents)
      Left _ -> return Nothing

    loadFile :: FilePath -> IO String
    loadFile fn' = openFile fn' ReadMode >>= hGetContents

main :: IO ()
main = do
  template <- parseGingerFile loadFileMay "base.html"
  case template of
    Left err -> print err >> exitFailure
    Right template' -> print $ render template' sampleContext


Again, several abstraction levels are available. First, you can employ a low-level approach where you incorporate SQL in the code. For that, you can usually use module Database.X where X is type of datase:

A slightly better services are provided by mid-level libraries:

Going higher with the abstraction, you can then use Haskell Database Connectivity (HDBC) for SQL databases. A good introduction to HDBC is in Chapter 21 - Using Databases of the Real World Haskell book.

INSERT INTO test(str) VALUES ('test string 1');
INSERT INTO test(str) VALUES ('test string 2');
{-# LANGUAGE OverloadedStrings #-}
import Control.Applicative
import Database.SQLite.Simple
import Database.SQLite.Simple.FromRow

data TestField = TestField Int String deriving (Show)

instance FromRow TestField where
  fromRow = TestField <$> field <*> field

main :: IO ()
main = do
  conn <- open "test.db"
  execute conn "INSERT INTO test (str) VALUES (?)"
    (Only ("test string 2" :: String))
  r <- query_ conn "SELECT * from test" :: IO [TestField]
  mapM_ print r

Persistence with Persistent

Now skyrocketing the abstraction to the heights of Template Haskell, we get to persistent that comes also with various extensions. There is a nice documentation of this package in the Yesod book, but, as we already explained, you can use it with any framework or even without any framework — just whenever you need to persist some data in a database.

{-# LANGUAGE EmptyDataDecls             #-}
{-# LANGUAGE FlexibleContexts           #-}
{-# LANGUAGE GADTs                      #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE MultiParamTypeClasses      #-}
{-# LANGUAGE OverloadedStrings          #-}
{-# LANGUAGE QuasiQuotes                #-}
{-# LANGUAGE TemplateHaskell            #-}
{-# LANGUAGE TypeFamilies               #-}
import           Control.Monad.IO.Class  (liftIO)
import           Database.Persist
import           Database.Persist.Sqlite
import           Database.Persist.TH

share [mkPersist sqlSettings, mkMigrate "migrateAll"] [persistLowerCase|
    name String
    age Int Maybe
    deriving Show
    title String
    authorId PersonId
    deriving Show

main :: IO ()
main = runSqlite ":memory:" $ do
      runMigration migrateAll :: IO ()

    johnId <- insert $ Person "John Doe" $ Just 35
    janeId <- insert $ Person "Jane Doe" Nothing

    insert $ BlogPost "My fr1st p0st" johnId
    insert $ BlogPost "One more for good measure" johnId

    oneJohnPost <- selectList [BlogPostAuthorId ==. johnId] [LimitTo 1]
    liftIO $ print (oneJohnPost :: [Entity BlogPost])

    john <- get johnId
    liftIO $ print (john :: Maybe Person)

    delete janeId
    deleteWhere [BlogPostAuthorId ==. johnId]

As you noticed, Persistent uses Template Haskell for a declaration of a persistent model.

For other possibilities of persistence libraries, take a look here or search the Hackage.

WAI and testing web apps

There must be some interface between a web application and the web server where the application is running. You may have heard about something like CGI, FastCGI, WSGI, or similar. As is WSGI for Python web applications, we have Web Application Interface (WAI) in Haskell.

Web app with plain WAI

It is possible to write a simple web application with just WAI and without any additional web framework.

{-# LANGUAGE OverloadedStrings #-}
import Network.Wai
import Network.HTTP.Types
import Network.Wai.Handler.Warp (run)

app :: Application
app _ respond = do
    putStrLn "I've done some IO here"
    respond $ responseLBS
        [("Content-Type", "text/plain")]
        "Hello, Web!"

main :: IO ()
main = do
    putStrLn $ "http://localhost:8080/"
    run 8080 app

HSpec & WAI

Web applications in Haskell can be tested via WAI. All applications that conform with WAI can be tested in the same way like a black box - send a request and check the response. In our favourite Hspec, there is an extension hspec-wai that allows you to test web applications in a very easy and readable way, as we are used to with hspec.

{-# LANGUAGE OverloadedStrings, QuasiQuotes #-}
module Main (main) where

import           Test.Hspec
import           Test.Hspec.Wai
import           Test.Hspec.Wai.JSON

import           Network.Wai (Application)
import qualified Web.Scotty as S
import           Data.Aeson (Value(..), object, (.=))

main :: IO ()
main = hspec spec

app :: IO Application
app = S.scottyApp $ do
  S.get "/" $ do
    S.text "hello"

  S.get "/some-json" $ do
    S.json $ object ["foo" .= Number 23, "bar" .= Number 42]

spec :: Spec
spec = with app $ do
  describe "GET /" $ do
    it "responds with 200" $ do
      get "/" `shouldRespondWith` 200

    it "responds with 'hello'" $ do
      get "/" `shouldRespondWith` "hello"

    it "responds with 200 / 'hello'" $ do
      get "/" `shouldRespondWith` "hello" {matchStatus = 200}

    it "has 'Content-Type: text/plain; charset=utf-8'" $ do
      get "/" `shouldRespondWith` 200 {matchHeaders = ["Content-Type" <:> "text/plain; charset=utf-8"]}

  describe "GET /some-json" $ do
    it "responds with some JSON" $ do
      get "/some-json" `shouldRespondWith` [json|{foo: 23, bar: 42}|]

This is just a simple (but often sufficient) example. Of course, you can test much more:

  • https://begriffs.com/posts/2014-10-19-warp-server-controller-test.html
  • https://www.spock.li/tutorials/testing

Example apps:

Here are a few examples of simple and more complex web apps:

Continuation-style web development

We would also like to raise your attention to an interesting approach to web development based on the continuation. A continuation is “something” that enables you to save the state of computation, suspend it (do something else) and later resume it. This “something” may be a first-class language feature (such as in Scheme), or a library feature - in Haskell, surprisingly, we have a continuation monad ;-).

A need for continuation occurs typically in web development (and generally in UI development) when you want a modal dialogue. Today, most of the dialogues are handled on client-side, however if you need to do a modal dialogue on server-side, it is hard - HTTP behaves like a programming language, which does not have subroutines, only GOTOs (URLSs are the ‘line numbers’). Continuation can provide the missing abstraction here, which is embodied in the MFlow library. Sadly, the project seems abandoned for several years.

At the same time, the continuation-style web server programming is typically the first choice in the Smalltalk (OOP) world - Seaside is purely continuation-based, and as such it gives a “desktop programming” experience for the web development resulting in no need of dealing with routing and URLs. As for the FP world, continuation-style web programming is surprisingly not used much in practice, but there are solutions such as the Racket web server or cl-weblocks in Common Lisp.

The next time, we will deal a bit with frontend technologies for Haskell, functional reactive programming and The JavaScript problem. So you will also see how to develop server-side and client-side separately and connect them through a (REST) API.

Further reading

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