[Haskell-cafe] Odd lack of laziness

[Neil Mitchell]

Hi

I have most of a Data.Nat library done, I should finish it off and release it...

Thanks

Neil

On 6/22/07, Stefan O'Rear <stefanor at cox.net> wrote:
> On Fri, Jun 22, 2007 at 07:14:39PM +0100, Andrew Coppin wrote:
> > Chaddaï Fouché wrote:
> > >You should be using BS.null f rather than BS.length f > 0.
> > >
> > While we're on the subject... anybody know a neat way to check, say,
> > whether a list contains exactly 1 element? (Obviously pattern matching
> > can do it, but that requires big case-expressions...)
>
> data LazyNat = Zero | Succ LazyNat  deriving(Eq,Ord)
>
> instance Enum LazyNat where
>     succ = Succ
>     pred (Succ x) = x
>
>     toEnum 0 = Zero
>     toEnum (x+1) = succ (toEnum x)
>
>     fromEnum Zero = 0
>     fromEnum (Succ x) = fromEnum x + 1
>
> instance Num LazyNat where -- this is a lie, the lifted naturals only
>                            -- form a *semi*ring.  Sigh.
>     fromIntegral = toEnum
>
>     Zero + y = y
>     Succ x + y = Succ (x + y)
>
>     Zero * y = 0
>     Succ x * y = y + x * y
>
>     abs = id
>     signum 0 = 0
>     signum _ = 1
>
>     x - Zero = x
>     Succ x - Succ y = x - y
>
>
> length' [] = Zero
> length' (x:xs) = Succ (length xs)
>
> null x = length' x == 0
>
> one x = length' x == 1
>
> atLeastFive x = length' x >= 5
>
> Stefan
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