[Haskell-cafe] Monad explanation

Tim Newsham newsham at lava.net
Fri Feb 6 13:34:17 EST 2009

> So if IO represents a program that when executed interacts with the world's
> state, is it safe to say that when I return (State Int Int), that I'm
> returning a "State program"?  That'd make sense as it really does look like
> we force the State to be evaluated with runState, evalState or execState.

Yes, exactly.

> The only difference with IO then is that to get IO programs to run, you have
> to do it inside another IO program.

Almost.  Add to your mental model a "runIO" that is invoked when your 
program runs as:  "runIO main".  Your haskell compiler or interpretter 
arranges this for you as part of its contract with you.

> I hope I'm not making this worse! :-)

I dont think so.

Tim Newsham

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