----- Original Message ----
From: Jeff Polakow <
jeff.polakow@db.com>
One general intuition about monads is that they represent
computations rather than simple (already computed) values:
I still want to re-iterate that they represent /complex/ computations - multiple, conditional results, extra stuff etc.
Hence the need to perform a "run" operation like runIdentity, evalState or runParser (for Parsec) to get something useful to happen. Except for lists we don't seem to do this. I suppose lists are so simple that the operators :, ++ and
the [] constructor do all we ever need with them. Finally there is no runIO because "main" is essentially that function in every real program? - Greg
What the run functions do is unwrap the monad. They take apart the 'm a' and give you back whatever a's might be
inside, and whatever extra stuff too. (Also feeding extra stuff in when m is like that) Doing that will involve actually evaluating
the value, forcing all the data dependencies and making the 'actions' happen.
If the monad type 'm a' is already a type we can take apart directly (list, maybe etc.) theres no need for a run function.
Note that of course unsafePerformIO is runIO. Its just that it doesn't really nest safely, so we like to only
use the top level one from main.