Lazy IO is really useful for commands which stream data over stdio, e.g. this is a really useful template, where 'go' is a pure function for producing a stdout string from a stdin string:

    {-# LANGUAGE OverloadedStrings #-}
    import qualified Data.ByteString.Lazy.Char8 as BS

    main = BS.interact go

    go :: BS.ByteString -> BS.ByteString
    go input = -- Generate output here

If you don't mind Haskell's default String implementation, then it's just:

    main = interact go

    go :: String -> String
    go input = -- Generate output here