A family of Haskell libraries for writing type-safe web APIs. Servant lets you declare your API at the type level and automatically derive servers, clients, and documentation.

type UserAPI =
       "users" :> Get '[JSON] [User]
  :<|> "users" :> Capture "userid" Int
                :> Get '[JSON] User
  :<|> "users" :> ReqBody '[JSON] User
                :> Post '[JSON] User

server :: Server UserAPI
server = getUsers :<|> getUserById :<|> createUser
  where
    getUsers     = liftIO fetchAllUsers
    getUserById  = liftIO . fetchUser
    createUser   = liftIO . insertUser

The universal document converter. Written in Haskell, Pandoc converts between dozens of markup formats, powering academic publishing, documentation pipelines, and static site generators worldwide.

import Text.Pandoc
import Text.Pandoc.Builder

main :: IO ()
main = do
  result <- runIO $ do
    doc <- readMarkdown def "# Hello\n\nWorld!"
    writeHtml5String def doc
  case result of
    Left err  -> print err
    Right html -> putStrLn html

-- Custom filter: capitalize all headers
capHeaders :: Pandoc -> Pandoc
capHeaders = walk capitalizeHeader
  where
    capitalizeHeader (Header n a inlines) =
      Header n a (walk toUpper inlines)
    capitalizeHeader x = x