[Haskell-cafe] Constructor constraints
Malcolm.Wallace at cs.york.ac.uk
Tue Sep 20 12:22:56 EDT 2005
Rich Neswold <rich.neswold at gmail.com> writes:
> For instance, I want to make a new type that is a 4 element tuple
> where each element is greater than or equal to the previous entry.
> data Category = Membership a a a a
You can make a 'smart' constructor function, and hide the real data
constructor so that it cannot be used:
module Category (Category(),membership) where
data Category = Membership a a a a
membership :: Ord a => a -> a -> a -> a -> Category a
membership a b c d
| d>=c && c>=b && b>=a = Membership a b c d
| otherwise = error "bad values"
However, you probably don't really want your program to die with
"error" when you get badly-constrained data, so it might be better
to look into something like the Maybe type for your smart constructor:
membership :: Ord a => a -> a -> a -> a -> Maybe (Category a)
so you can catch the failing cases and do something sensible with them.
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