We can make it a lot more performant, shorter, and also safer by using lazy byte strings:

{- cabal:
build-depends: base, network, network-run, bytestring
-}

{-# LANGUAGE OverloadedStrings #-}

import Network.Run.TCP (runTCPServer)
import qualified Network.Socket.ByteString.Lazy as Net
import qualified Data.ByteString.Lazy.Char8 as Str

main = runTCPServer (Just "127.0.0.1") "9999" $ \s -> do
  request <- Net.getContents s
  case Str.words (Str.takeWhile (/= '\r') request) of
    ["GET", resource, "HTTP/1.1"] -> do
      let path = Str.concat
            [ "htdocs/"
            , Str.dropWhile (== '/') resource
            , if Str.last resource == '/' then "index.html" else ""
            ]
      page <- Str.readFile (Str.unpack path)
      Net.sendAll s ("HTTP/1.1 200 OK\r\n\r\n" <> page)
    _ -> error "todo"