Re: [Haskell-cafe] Finite but not fixed length...

...and you can always do hack :: Vec n a -> FixedVec a hack x :: FixedVec undefined Also I'm guessing 1, 2 and 17 are just examples, he really wants arbitrary length finite lists. /J On 13 October 2010 20:47, Daniel Peebles wrote:

One option could be something like:

data Z data S n

data Vec n a where Nil :: Vec Z a Cons :: a -> Vec n a -> Vec (S n) a

data Length n where One :: Length (S Z) Two :: Length (S (S Z)) Seventeen :: Length (S (S (S (S (S (S (S (S (S (S (S (S (S (S (S (S (S Z)))))))))))))))))

data FixedVec a where FixedVec :: Legnth n -> Vec n a -> FixedVec a

But it's obviously rather cumbersome :)

On Wed, Oct 13, 2010 at 7:57 AM, Jason Dusek wrote:

...

Is there a way to write a Haskell data structure that is necessarily only one or two or seventeen items long; but that is nonetheless statically guaranteed to be of finite length?

-- Jason Dusek Linux User #510144 | http://counter.li.org/ _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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