On Oct 31, 2005, at 9:37 AM, Ben Rudiak-Gould wrote:
I don't know if this helps, but there's a straightforward way to
understand the IO monad in terms of continuation passing.
You can think of a value of type IO a as being a CPS expression
with a hole in it; the hole is to be filled with a continuation
which expects a value of type a. The only way to fill the hole is
by using >>=, whose second argument is a continuation with another
(nested) hole in it. So effectively with >>= you build a CPS
expression from the outside in. The final continuation, which takes
() and aborts the program, is ultimately filled in by the runtime
system.
This viewpoint doesn't work for other monads, since they always
provide some sort of destructuring operations on monadic values,
e.g. runState or the standard list deconstructors. But it works
fine for IO provided you ignore the existence of unsafePerformIO
and friends.
-- Ben
No, that's definitely helpful. The analogy in the case of IO was
obvious, but I had a sense that it was getting in the way of
understanding what a monad is (i.e., by leading me to focus on the
wrong issues).
===
Gregory Woodhouse
[EMAIL PROTECTED]
"One must act on what has not yet happened."
--Lao Tzu
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