[Haskell-cafe] Shouldnt this be lazy too?
Lennart Augustsson
lennart at augustsson.net
Mon Sep 24 18:14:20 EDT 2007
PS.
Prelude Data.List Data.Number.Natural> genericLength [1..] > (10 :: Natural)
True
On 9/24/07, Lennart Augustsson <lennart at augustsson.net> wrote:
>
> Since natural numbers are trivial to implement (inefficiently) I took 15
> minutes and added them to my numbers package in Hackage.
> http://hackage.haskell.org/cgi-bin/hackage-scripts/package/numbers-2007.9.24
>
>
> -- Lennart
>
> On 9/24/07, Andrew Coppin <andrewcoppin at btinternet.com> wrote:
> >
> > Neil Mitchell wrote:
> > > Hi
> > >
> > >
> > >>> Pretty much, yes.
> > >>>
> > >>>
> > >> So I just need to write
> > >>
> > >> newtype LazyNatural = LazyNatural [()]
> > >>
> > >
> > > or
> > >
> > > data Nat = Zero | Succ Nat
> > >
> > > it's your choice really.
> > >
> >
> > I'm guessing there's going to be fairly minimal performance difference.
> > (Or maybe there is. My way uses a few additional pointers. But it also
> > allows me to elegantly recycle existing Prelude list functions, so...)
> >
> > >> and then add some suitable instances. ;-)
> > >>
> > >
> > > Yes. Lots of them. Lots of instances and lots of methods.
> > >
> > >
> > >> Hey, the "length" function would then just be
> > >>
> > >> ln_length :: [x] -> LazyNatural
> > >> ln_length = LazyNatural . map (const ())
> > >>
> > >> Ooo, that's hard.
> > >>
> > >
> > > Nope, its really easy. Its just quite a bit of work filling in all the
> > > instances. I bet you can't do it and upload the results to hackage
> > > within 24 hours :-)
> > >
> >
> > *ALL* the instances? No.
> >
> > A small handful of them? Sure. How about this...
> >
> >
> >
> > module LazyNatural (LazyNatural ()) where
> >
> > import Data.List
> >
> > newtype LazyNatural = LN [()]
> >
> > instance Show LazyNatural where
> > show (LN x) = "LN " ++ show (length x)
> >
> > instance Eq LazyNatural where
> > (LN x) == (LN y) = x == y
> >
> >
> > instance Ord LazyNatural where
> > compare (LN x) (LN y) = raw_compare x y
> >
> > raw_compare ([]) (_:_) = LT
> > raw_compare ([]) ([]) = EQ
> > raw_compare (_:_) ([]) = GT
> > raw_compare (_:x) (_:y) = raw_compare x y
> >
> >
> > instance Num LazyNatural where
> > (LN x) + (LN y) = LN (x ++ y)
> > (LN x) - (LN y) = LN (raw_minus x y)
> > (LN x) * (LN y) = LN (concatMap (const x) y)
> > negate _ = error "negate is not defined for LazyNatural"
> > abs = id
> > signum (LN []) = LN []
> > signum (LN _) = LN [()]
> > fromInteger = LN . flip genericReplicate ()
> >
> > raw_minus (_:a) (_:b) = raw_minus a b
> > raw_minus (a) ([]) = a
> > raw_minus _ _ = error "negative result from subtraction"
> >
> > _______________________________________________
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> >
>
>
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