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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?
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Whenever you want to restrict the type parameter to subtypes of a particular class you can use the bounded type parameter. If you just specify a type (class) as bounded parameter, only sub types of that particular class are accepted by the current generic class.
To declare a bounded type parameter, list the type parameter's name, followed by the extends keyword, followed by its upper bound, which in this example is Number .
A type parameter can have multiple bounds.
Using bounded types, you can make the objects of generic class to have data of specific derived types. For example, If you want a generic class that works only with numbers (like int, double, float, long …..) then declare type parameter of that class as a bounded type to java.
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.
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