2b. You can define brand new flow control constructs *inside* the language itself. (E.g., in Java, a "for" loop is a built-in language construct. In Haskell, "for" is a function in Control.Monad. Just a plain ordinary function that anybody could write.)
Psst, heard about Scheme & call/cc?
But, very importantly, purity allows you to *restrict* the flow constructs that client code has available. If you have continuations + mutable state you can do anything, but the more code can *do*, the less you *know* about it. For example, providing parser combinators as an applicative functor offers more power than a traditional parser generator, but not enough that we can no longer parse efficiently.
Exactly my fear over unsafePerformIO in 3rd party Haskell libraries :-). One can present an interface in Haskell that looks safe, but it can be very unsafe in its implementation. Dave
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