[Haskell-cafe] Creating a type for a subset of the integers
brad.larsen at gmail.com
Tue Dec 18 21:07:25 EST 2007
Hi there list,
How would one go about creating a new type for a subset of the integers,
for (contrived) example just the even integers? I was thinking of making
a new type
newtype EvenInt = EvenInt Integer
but the problem with this is that it accepts any integer, even odd ones.
So to prevent this, the module does not export the constructor for
it---rather, the module exports a function `mkEvenInt' that creates an
EvenInt if the given value is acceptable or raises an error otherwise.
What's the right way to do this? Thanks!
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