[Haskell-cafe] New STM Data Structures Collection

[Richard A. O'Keefe]

I'm really puzzled by the interface.

class Bag b
  where
    new :: STM (b v)
      -- no problem here
    add :: b v -> v -> STM ()
      -- no problem here
    isEmpty :: b v -> STM Bool
      -- no real problem except the pervasive one that whatever
      -- you learn about the state of a concurrent object must
      -- always be treated as out of date when you use it
    take :: b v -> STM v
      -- what is supposed to happen if the bag is empty?

I was expecting an operation like

    maybeTake :: b v -> STM (Maybe v)