A very simple JSON API in Haskell

Posted on January 11, 2021

The simplest Haskell JSON API tutorial

I was following along on this blog post by Felix Mulder and I found it quite challenging to grasp the main concepts as a newcomer to Haskell for a variety of reasons.

This is my attempt to recreate the JSON API from that blog post using a much more simplified approach which doesn’t require as much Haskell knowledge to understand the code. I hope this is useful to others and I’m sure it will be a useful learning experience for myself!

Credit goes to Sean Hess for this blog post which also helped me along the way

Let’s build an JSON API!

In this tutorial we’re going to be building a very simple API with two endpoints:

  • Create a user POST /user, receive the new user ID (String) in the response

  • Delete a user DELETE /user/:userId

Getting started

This tutorial assumes that you will be using stack to build your Haskell project.

A package.yaml file you can copy to ensure you have the right dependencies and language extensions can be found here: https://github.com/cdimitroulas/simple-json-api-haskell/blob/main/package.yaml

All the code we write in this post can also be found at: https://github.com/cdimitroulas/simple-json-api-haskell

Our domain types

Lets begin by defining some simple types for our domain and the necessary FromJSON instances for them.

-- User.hs
module User where

import Data.Aeson
import Data.Text
import GHC.Generics

data User = User
  { userId :: Text,
    userName :: Text

-- Data type which describes the request which
-- will be received to create a user
data CreateUserRequest = CreateUserRequest
  { name :: Text,
    password :: Text
  deriving (Generic)

-- We define a FromJSON instance for CreateUserRequest
-- because we will want to parse it from a HTTP request
-- body (JSON).
instance FromJSON CreateUserRequest

Faking a database

To keep this tutorial simple and avoid us having to set up a database we will be using a mutable variable to store our users. Mutable variables in Haskell I hear you cry?! Is that even a thing?!

Well, it’s probably not exactly what you are imagining if you are coming from another language where mutating variables is easily achievable but in Haskell IORef provides a way to mutate a variable, so we are going to use this! (This approach is obviously just for educational purposes and is in no way recommended for a real application :D)

-- Db.hs
module Db
  ( DbUsr (..),
    UserStore (..),

import Data.IORef
import Data.List (sort)
import Data.Map (Map)
import qualified Data.Map as Map
import Data.Text (Text)

data DbUsr = DbUsr
  { dbUsrName :: Text,
    dbUsrPassword :: Text
  deriving (Show)

newtype UserStore = UserStore {unUsrStore :: IORef (Map Int DbUsr)}

-- Creates our initial empty database
mkDb :: IO UserStore
mkDb = do
  ref <- newIORef (Map.empty :: Map Int DbUsr)
  pure $ UserStore ref

-- Accepts a default value to return in case the list is empty
safeLast :: a -> [a] -> a
safeLast x [] = x
safeLast _ (x : xs) = safeLast x xs

-- Utility to allow us to read the data in our "database"
getUserStore :: UserStore -> IO (Map Int DbUsr)
getUserStore (UserStore store) = readIORef store

-- VERY naive utility to get the next unique user ID
getNextId :: UserStore -> IO Int
getNextId x = (+ 1) . safeLast 0 . sort . Map.keys <$> getUserStore x

-- insertUser uses getNextId to get the next ID and then updates our database using
-- modifyIORef from the Data.IORef library. It returns the new ID as a result.
insertUser :: UserStore -> DbUsr -> IO Int
insertUser userStore usr = do
  nextId <- getNextId userStore
  modifyIORef (unUsrStore userStore) (Map.insert nextId usr)
  pure nextId

-- deleteUser updates our database by deleting the relevant user data 
deleteUser :: UserStore -> Int -> IO ()
deleteUser usrStore uid = modifyIORef' (unUsrStore usrStore) (Map.delete uid)

Putting together our JSON API

Now we have all of the pieces of our application that we need, it’s time to put them together and expose them via a JSON API! In order to do this we will be using the scotty library (very similar to Sinatra from Ruby!).

-- Main.hs
module Main where

import Control.Monad.IO.Class
import qualified Db
import User (CreateUserRequest (..))
import Web.Scotty

main :: IO ()
main = do
  -- Initialize our fake DB
  db <- Db.mkDb

  -- Run the scotty web app on port 8080
  scotty 8080 $ do
    -- Listen for POST requests on the "/users" endpoint
    post "/users" $
        -- parse the request body into our CreateUserRequest type
        createUserReq <- jsonData

        -- Create our new user.
        -- In order for this compile we need to use liftIO here to lift the IO from our
        -- createUser function. This is because the `post` function from scotty expects an
        -- ActionM action instead of an IO action
        newUserId <- liftIO $ createUser db createUserReq

        -- Return the user ID of the new user in the HTTP response
        json newUserId

    -- Listen for DELETE requests on the "/users/:userId" endpoint
    delete "/users/:userId" $ do
      -- Get the value of the userId from the URL
      userId <- param "userId"

      -- Delete the relevant user
      -- Same as with the user creation, we need to use liftIO here.
      liftIO $ Db.deleteUser db userId

-- Our createUser function simply deals with constructing a DbUsr value and passes it
-- to the Db.insertUser function
createUser :: Db.UserStore -> CreateUserRequest -> IO Int
createUser db CreateUserRequest {name = uname, password = pwd} = Db.insertUser db dbusr
    dbusr = Db.DbUsr {Db.dbUsrName = uname, Db.dbUsrPassword = pwd}


We now have our completed JSON API! We can run it using stack run.

You should see this output:

Setting phasers to stun... (port 8080) (ctrl-c to quit)

We can then make some http requests to check it’s all working. I like to use the httpie command line tool for this: Screenshot of the results of running some POST and DELETE requests against our API