Question

I would like to create a new integral type which is bounded to a certain range. I have tried:

data PitchClass = PC Int deriving (Ord, Eq, Show)

instance Bounded PitchClass where
  minBound = PC 0
  maxBound = PC 11

However, what I want is something that will fail if something like

PC 12

or

PC (-1)

is attempted.

Is the general approach for a situation in which you wish to place constraints on creating new types one in which the value constructors are not exported from the module, but rather functions which return instances of the type and which perform constraint checks are exported?

Answer 1

Yes, not exporting the data constructor from the module is the way to go.

Instead, you export a function which does the checking as you said. This is often called a smart constructor.