On Wednesday 10 May 2006 12:30 pm, Brian Hulley wrote: > Bjorn Lisper wrote: > > Nontermination is not > > the precisely the same as _|_. Only certain kinds of nontermination > > can be modeled by _|_ in a non-strict language. > > What kinds of non-termination are *not* modelled by _|_ in Haskell?
Non-termination that is "doing something". For example consider: ] ones = 1 : ones If I try to take its length, I get _|_. So: ] main = print (length ones) Will churn my CPU forever without producing any output. However, if I print each item sequentially: ] main = mapM print ones I'll get a never-ending stream of '1' on my console. This is not the same as bottom because it's "doing something". Now, obviously this definition is pretty imprecise, but maybe it helps you get the idea. Now for the corner cases. What about: ] main = sequence_ repeat (return ()) ? I'd personally say it is _not_ bottom. Even though "return ()" is a completely useless action, I'm inclined to say its "doing something" in some theoretical sense (mostly because I think of _|_ as being a property of the functional part of Haskell). _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
