Maurí­cio wrote:

I see in the documentation and in many messages in this list that, if you want multithreading in your program, you need to use -threaded in ghc.

Concurrency is supported just fine without -threaded. You need -threaded if you want to: 1) make foreign calls that do not block other threads 2) use multiple CPUs 3) write a multithreaded Haskell library or DLL Some library functions use foreign calls, for example System.Process.waitForProcess, so if you want to use them without blocking other threads you need -threaded. From an implementation perspective, -threaded enables OS-thread support in the runtime. Without -threaded, everything runs in a single OS thread. The features listed above all require multiple OS threads.

Why isn't that option default? Does it imply some kind of overhead?

There may be an overhead, because the runtime has to use synchronisation in various places. Hopefully the overhead is negligible for most things. It's not the default mostly for historical reasons, and because there are people who don't like to get multiple OS threads unless they ask for it. Also there are one or two things that don't work well with -threaded (Gtk2Hs, and System.Posix.forkProcess). Cheers, Simon