Hello Michael, you are correct. Only
* (a `seq` return a) = evaluate a *right now*, then produce an IO action which, when executed, returns the result of evaluating a. Thus, if a is undefined, throws an exception right now.
is a bit misleading as there is no evaluation "right now". It's better to say that (a `seq` return a) is _|_ ("bottom", i.e. undefined) when a == _|_. The subtle point is the difference between the action of throwing an exception (throw some_error :: IO a) and an undefined value (_|_). (throw ..) will cause your program to crash when "executed", but it's still a well defined value of type (IO a). For a more detailed semantics of exceptions in Haskell, see " Tackling the awkward squad: monadic input/output, concurrency, exceptions, and foreign-language calls in Haskell" http://research.microsoft.com/%7Esimonpj/Papers/marktoberdorf/ I further think (evaluate x) can be implemented as follows: evaluate x = catch (x `seq` return x) throw Regards, apfelmus