[Haskell-cafe] Announcing OneTuple-0.1.0

[Don Stewart]

derek.a.elkins:
> On Fri, 2008-10-03 at 15:38 -0400, David Menendez wrote:
> > On Fri, Oct 3, 2008 at 3:17 AM, Jason Dusek <jason.dusek at gmail.com> wrote:
> > >  Perhaps I am lacking in imagination, but I still can't see the
> > >  value of one tuples.
> > 
> > You can use them to defeat seq.
> > 
> > undefined `seq` x == undefined
> > OneTuple undefined `seq` x == x
> > 
> > That might be useful if a polymorphic function is using seq to force
> > evaluation, and you don't want it to. But I can't imagine that coming
> > up much in practice.
> 
> Think element strict polymorphic containers, e.g.
> 
> data HeadStrictList a = Nil | Cons !a (HeadStrictList a)
> 
> then
> 
> type LazyList a = HeadStrictList (OneTuple a)


Used in practice to prevent strict state components in list fusion
leaking into user's lazy code,

    data    L a = L a  -- lazy / lifted
    newtype S a = S a  -- strict / unlifted

    class Unlifted a where

    instance Unlifted (L a) where
      expose (L _) s = s

    instance Unlifted (S a) where
      expose (S a) s = seq a s

    data Stream a = forall s. Unlifted s =>
                              Stream !(s -> Step a s)  -- ^ a stepper function
                                     !s                -- ^ an initial state

So we can then ensure

    stream :: [a] -> Stream a
    stream xs0 = Stream next (L xs0)
      where
        next (L [])     = Done
        next (L (x:xs)) = Yield x (L xs)

Has the appropriate strictness properties.

-- Don