Lazy bind...

[Martin Norbäck]

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fre 2002-08-02 klockan 14.57 skrev Lauri Alanko:
> On Tue, Jul 30, 2002 at 01:41:43PM +0100, MR K P SCHUPKE wrote:
> > Is there any way to do a lazy bind operation, something like
> >=20
> >     a <- $ getLine
> >     return (Constructor $ a)
> >=20
> > this works for computations in the IO monad :-
> >=20
> >     a <- unsafeInterleaveIO getLine
> >     return (Constructor $ a)
> >=20
> > but I need to do this for a general monad M.
>=20
> I don't think this is possible. In general, you have to write separate
> strict and lazy binding operations. For instance, there are both ST and
> LazyST for state transformer monads, and the latter's >>=3D operation is
> lazy.
>=20
> Here's a concrete example for the basic state-carrying monad. (Not tested=
)
>=20
> newtype State s a =3D State { unState :: s -> (a, s) }
> getState =3D State (\s -> (s, s))
> setState x =3D State (\s -> ((), x))
> -- Monad.return
> returnState a =3D State (\s -> (a, s))
> -- strict Monad.>>=3D
> State m `strictBindState` f=20
>   =3D State (\s -> case m s of (a, s') -> unState (f a) s')
> -- lazy Monad.>>=3D
> State m `lazyBindState` f=20
>   =3D State (\s -> case m s of ~(a, s) -> unState (f a) s')
>=20
> As you can see, the difference between lazy and strict binding is an
> inherent part of the definition of >>=3D, so there's no way to automatize
> it. This is kind of a shame, for I too sometimes need both lazy and
> strict versions of a monad transformer, but this requires two separate
> types with different monad instances...

Perhaps just some more syntactic sugar would be in order. Say something
like this:

infixl 1 >>=3D$

class Monad m =3D> LazyMonad m where
  (>=3D=3D$) :: m a -> (a -> m b) -> m b

and then make

   a <-$ m

use >=3D=3D$ instead of >>=3D.

The operator names are just meant as an example of course.

Regards,

	Martin

--=20
Martin Norb=E4ck          d95mback@dtek.chalmers.se             =20
Kapplandsgatan 40       +46 (0)708 26 33 60                   =20
S-414 78  G=D6TEBORG      http://www.dtek.chalmers.se/~d95mback/
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