Is there a standard Haskell trick for checking run-time assignment to data types? I'd like a data type of Probability that ensures its Double argument is between 0 and 1.
Jim
A fairly common technique to achieve this is smart constructors (apparently also called factory functions). http://www.haskell.org/hawiki/FactoryFunction You simply enforce the invariant on construction. For your example, it would look like, newtype Probability = Probability Double probability p | 0.0 <= p && p <= 1.0 = Probability p unProbability (Probability p) = p Simply don't export Probability's constructor and it will be impossible to create a Probability with a value outside of [0,1]. However, you may want to export the constructor, or alternatively, export a function with a name like unsafeProbability (= Probability) for situations where you can statically decide that the invariant will hold if the superfluous checking costs too much, which doesn't seem particularly likely in this case. Providing a function of type Double -> Maybe Probability is also likely a good idea.