[Haskell] line-based interactive program

Andrew Pimlott andrew at pimlott.net
Fri Jul 8 12:43:02 EDT 2005

On Fri, Jul 08, 2005 at 02:51:11PM +0100, Colin Runciman wrote:
> >>My interaction depends on the (subtle order of) evaluation of a pure and
> >>total function?
> >>
> Pure, yes; total, no.
> Many important things depend on order of evaluation in lazy programs:
> for example, whether they compute a well-defined value at all!   The
> interleaving of demand in the argument of a function with computational
> progress in its result seems a perfectly natural view of interaction in
> a lazy functional language.  This sort of interaction is what actually
> happens when your function applications are evaluated whether you
> exploit it or not.  I embrace it as part of lazy functional programming;
> you prefer an appeal to something extraneous.

It is one thing to embrace lazy evaluation order, and another to embrace
lazy IO (implemented using unsafeInterleaveIO).  As a relative newcomer
to Haskell, I got the impression that the "interact" style was always a
hack, discarded for good reason in favor of the IO monad.  Is there a
strong case for interact?


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