You can also do this in a way that statically guarantees the property... First you need to lift the numeric argument to the type level... This is harder for a double than an integer, but not impossible... First convert the double to a pair of Integers (an infintite precision rational number)... Then each Integer can be represented as a uniary number (this is the simplest but you will run out of context reduction stack on any kind of real number, so binary or decimal representation is better). The definitions required for this are in the attached library. class Rat x -- class of type level rationals instance (Number x,Number x) => Rat (x,x) Now the specific class: class Rat0to1 r instance Div x y Zero => Rat0to1 (x,y) -- gives range 0.0000 -> 0.9999 instance (Div x y (Suc Zero),Mod x y Zero) => Rat0to1 (x,y) -- gives 1.0000 -> 1.0000 an exmaple function applying the constraint: rat0to1 :: Rat0to1 => r -> r rat0to1 = id The constraint can be converted from a static constraint to a runtime constraint by using existentials: data Ext0to1 = forall x y . (Number x,Number y,Rat0to1 (x,y)) => Ext0to1 (x,y) Keean. Derek Elkins wrote:
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