Hi
Catch (www.cs.york.ac.uk/~ndm/catch) can infer that certain uses of
numbers fit into the {Neg, Zero, One, Pos} abstraction - so for
example it can infer that length returns {Zero, One, Pos}, but not
Neg. If you then do:
xs !! length ys
It will detect that length ys is natural, and will be safe. However,
if you pass any arbitrary value as the index to !! it will warn of a
possible pattern match error.
You can of course use type Nat = Int, and write additional
documentation, even if this documentation isn't a static guarantee.
Thanks
Neil
On 8/2/07, brad clawsie
as far as i know, the haskell standard does not define a basic Int type that is limited to positive numbers.
would a type of this kind not potentially allow us to make stronger verification statements about certain functions?
for example, 'length' returns an Int, but in reality it must always return a value 0 or greater. a potential counter-argument would be the need to possibly redefine Ord etc for this more narrow type...
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