Bruce, All this discussion of 'interact' makes me feel quite nostalgic. In the late 80s(!) I did some work on this, since Miranda's only IO was in the form of interact. Understanding the interleaving behaviour is problematic, and hand-coded solutions proved to be difficult. I came up with a way of analysing these programs, and proved some elementary properties. The paper also contains a set of combinators for writing interactive programs, which prefigure the monadic interaction combinators. The paper is at: http://www.cs.kent.ac.uk/~sjt/Pubs/yoptrace.ps Best regards, Simon Thompson