[Haskell-cafe] numerical subtyping
Derek Elkins
ddarius at hotpop.com
Tue Dec 7 22:27:09 EST 2004
> 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.
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