[Haskell-cafe] Implementing the State Monad

[oleg at pobox.com]

apfelmus showed the implementation of the state monad as free term
algebra, using GADT. Here's an implementation that does not use GADT
	http://okmij.org/ftp/Haskell/types.html#state-algebra

All the smarts are in the observation function. This style is _very_
well explained by Ralf Hinze in his Functional Pearl
`Deriving Backtracking Monad Transformers' (ICFP00)
	http://citeseer.ist.psu.edu/hinze00deriving.html

He also showed how to optimize this term algebra (aka, `initial')
implementation using continuations (sec 3.3).

Where this style begins to break down is with complex monads, such as
backtracking *with cut* or some other control. Note that Hinze was not
able to implement his version of backtracking with cut using _free_
term algebra. The situation can be improved however if we use a
special operation to reflect the search (msplit of MonadMinus). We
obtain an efficient implementation then and no longer have to match on
the contexts.