Combining monads

[Mark T.B. Carroll]

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Hello! I am trying and failing to use one monad in another. A tiny example
is enclosed, where I try to use IO to help along a State-ful computation.
Is the real mistake in do_next? Is there a minimal rewrite that will get
this working that I can use as a template for other things? Or, is there a
good tutorial somewhere on this type of thing? Any pointers are much
appreciated!

-- Mark

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