-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Stefan O'Rear wrote:
On Tue, May 08, 2007 at 05:47:27PM -0700, Ashley Yakeley wrote:
If Isaac's definition is accurate, we might as well generalise it to any Monad.
Isaac's definition is equivalent to the standard evaluate as I proved. However, my proof could be using misfeatures of IO.
Monads like Maybe and [] have no definition of evaluate consistent with all the laws, they only have (return) and (return $!) as distinct possibilities. In which case my definition makes evaluate in those monads be equivalent to (return $!). Which might not be that bad. For monads such as Lazy ST, it is clear that (>>=) cannot be strict in its first argument in order for the laziness to work, so evaluate can be defined with all the laws. In other function-based monads - including IO - it is not so documented or necessary. What is evaluate used for? Based on Google codesearch and my own experiences: In a forkIO for the sole purpose of parallel evaluation. Could be summarized by: forkEvaluate :: a -> IO a forkEvaluate a = forkIO (evaluate a >> return ()) Aren't there supposed to be more functional ways like `par` to achieve this? Anyway, looks like that definition only works in the IO monad. To immediately catch exceptions from it (Control.Exception.catch or try). This also only works in the IO monad (or derived monads that can define catch... there is always (liftIO . evaluate) I guess). To simply make things be evaluated sooner. This is sometimes necessary, to counteract the effects of file-reading using unsafeInterleaveIO, for example. I haven't seen this use in non-IO monads (which might violate separation of concerns) but it could be. In practice, even in IO code, (return $!) ends up being used. After all it is not hidden away in a Control.Exception module (that is officially non-portable perhaps). No opinion :) Isaac -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.6 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFGQ4W0HgcxvIWYTTURAiXIAKDWeixSdezmSz2bGCfxZ1Ox3MYMHQCghVKh xe9UYKbp/IVkx3t6thYj/xM= =ZmRd -----END PGP SIGNATURE-----