[Haskell-cafe] Re[2]: strict Haskell dialect
Chris Kuklewicz
haskell at list.mightyreason.com
Fri Feb 3 16:06:20 EST 2006
Brian Hulley wrote:
> > ...
>
> [Apologies for replying to a reply of a reply but I don't seem to have
> received the original post]
>
> I've been thinking along these lines too, because it has always seemed
> to me that laziness is just a real nuisance because it hides a lot of
> inefficiency under the carpet as well as making the time/space behaviour
> of programs difficult to understand...
>
> One question is how to get some kind of "do" notation that would work
> well in a strict setting.
> The existing "do" notation makes use of lazyness in so far as the second
> arg of >> is only evaluated when needed. Perhaps a new keyword such as
> "go" could be used to use >>= instead ie:
>
> go {e1;e2;e3} === e1 >>= (\_-> (e2 >>= (\_->e3)))
>
> Of course this doesn't solve the problem of how to translate programs
> that make heavy use of mapM etc.
>
> I wonder: is monadic programming really dependent on lazyness or is
> there a realistic (ie not impossibly complicated) way to use monads in a
> strict setting?
>
> A related question is: could monadic programming ever be as efficient as
> side-effect programming?
>
> Regards, Brian.
What about writing functions in a modified form of Control.Monad.Identity that
ensures the return value that forces the return values:
> module Control.Monad.Strict (Weak,mkWeak,unsafeMkWeak,runWeak,
> Deep,mkDeep,unsafeMkDeep,runDeep) where
Weak uses seq to achieve WHNF for it's argument
> newtype Weak a = WeakCon {runWeak :: a}
> mkWeak x = seq x (WeakCon x)
> unsafeMkWeak x = WeakCon x
>
> instance Functor Weak where
> fmap f w = mkWeak (f (runWeak w))
>
> instance Monad Weak where
> return x = mkWeak x
> w >>= f = f (runWeak w)
>
I can't make the deepSeq version typecheck:
Deep uses deepSeq to evaluate it's argument
> newtype Deep a = DeepCon {runDeep :: a}
> mkDeep x = deepSeq x (DeepCon a)
> unsafeDeep x = DeepCon x
>
> instance Functor Deep where
> fmap f d = mkDeep (f (runDeep d))
>
> instance Monad Deep where
> return d = mkDeep d
> d >>= f = f (runDeep d)
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