[Haskell-cafe] >> and sequencing [newbie]
David Powers
david at grayskies.net
Mon Apr 16 20:52:31 EDT 2007
Ah... so the secret is in the hidden variables. On some level I am
beginning to fear that Monads resurrect some of the scariest aspects of
method overriding from my OO programming days. Do you (all) ever find that
the ever changing nature of >>= makes code hard to read?
On 4/15/07, jeff p <mutjida at gmail.com> wrote:
>
> Hello,
>
> On 4/15/07, David Powers <david at grayskies.net> wrote:
> > so... this is likely a question based on serious misunderstandings, but
> can
> > anyone help me understand the exact mechanism by which monads enforce
> > sequencing?
> >
> Monads do not enforce sequencing.
>
> In general, data dependencies enforce sequencing (i.e. if expression x
> depends upon expression y then expression y will have to be evaluated
> first). Consider:
>
> let x = case y of Just y' -> f y'
> Nothing -> g
> y = some code
> in more stuff
>
> Here y must be evaluated before x because x needs to look at y in
> order to compute.
>
> > Specifically, I'm confused by the >> operator. If I understand
> > things properly f a >> g expands to something like:
> >
> > f >>= \_ -> g
> >
> > What I'm missing is how the expansion of f is ever forced under lazy
> > evaluation. Since the result is never used, doesn't it just stay as a
> > completely unevaluated thunk?
> >
> (>>=) is an overloaded function. Some instances of it will cause f to
> be evaluated, others won't. Consider the State monad:
>
> instance Monad (State s) where
> return a = State $ \s -> (a, s)
> m >>= k = State $ \s -> case runState m s of
> (a, s') -> runState (k a) s'
>
> Note that (>>=) causes m to be evaluated (up to a pair) before
> evaluating k because (>>=) needs to look at the result of m.
>
> An example of a monad in which (>>=) doesn't force evaluation of the
> first argument before the second is the Identity monad:
>
> instance Monad Identity where
> return a = Identity a
> m >>= k = k (runIdentity m)
>
> Note that (>>=) actually forces the evaluation of k before m.
>
> -Jeff
>
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