I was not aware that it was something a poster was supposed to do, but
now I notice that while the messages I get from the list have
[Haskell-Cafe] on their subject, the ones I post don't.
The list software adds it. You don't see it because the copy from the list has the same message id as the copy gmail saves when you send it, so gmail tosses the list copy from your mailstore as a duplicate. Don't worry about it.