Like python’s SimpleHTTPServer.

warpspeed.hs

#!/usr/bin/env nix-shell
#!nix-shell -i runhaskell -p "haskellPackages.ghcWithPackages (hps: with hps; [wai-middleware-static warp unix])"
{-# LANGUAGE OverloadedStrings #-}
module Main where

import Network.Wai
import Network.Wai.Middleware.Static
import Network.Wai.Handler.Warp
import Network.HTTP.Types.Status
import System.Posix.Directory
import System.Environment

main = do
  args <- getArgs
  let policy =
        case args of
          [ dir ] -> addBase dir
          _       -> mempty
  runEnv 8000 $ staticPolicy policy $ \req resp -> resp $ responseLBS notFound404 [] "not found"