On Fri, Mar 08, 2013 at 08:53:15PM -0800, Edward Z. Yang wrote:
Are these equivalent? If not, under what circumstances are they not equivalent? When should you use each?
evaluate a >> return b [...] - Use 'evaluate' when you mean to say, "Evaluate this thunk to HNF before doing any other IO actions, please." Use it as much as possible in IO.
I've never looked at evaluate before but I've just found it's haddock and given it some thought. http://hackage.haskell.org/packages/archive/base/latest/doc/html/Control-Exc... Since it is asserted that evaluate x = (return $! x) >>= return is it right to say (on an informal level at least) that evaluating an IO action to WHNF means evaluating it to the outermost >>= or return?
For non-IO monads, since everything is imprecise anyway, it doesn't matter.
Could you explain what you mean by "imprecise"? Tom