On Mon, 2007-08-13 at 22:29 +0200, Benjamin Franksen wrote:
Brian Brunswick wrote:
One thing that I keep seeing people say (not you), is that monads /sequence/ side effects. This is wrong, or at least a limited picture.
/All/ of the above structures are about combining compatible things things together in a row. /None/ of them force any particular order of evaluation - that all comes from the particular instance. So its only a particular feature of IO that it sequences the side effects. Others don't - we can have a lazy State monad that just builds up big thunks.
I am a bit astonished.
Let's take the simplest example: Maybe. The effect in question is the premature abortion of a computation (when Nothing is returned). And of course Maybe sequences these effects, that's what you use it for: the _first_ action to be encountered that returns Nothing aborts the computation. Clearly sequencing goes on here.
You are wrong. Proof: Let's take a simpler example: Identity. QED This also disproves David Roundy's statement that do x <- return 2; undefined; return (x*x) will hit bottom. Reader also does not sequence it's "actions". Writer is a kind of funny example. Certainly, any monad instance where (>>=) needs it's first argument to determine whether to continue, e.g. Maybe, Either, IO, Parser, Cont, List, Tree will clearly need to force it's first argument before continuing, but that's just the nature of the situation.