[Haskell-beginners] Re: Restricting integers in a type?
Colin Paul Adams
colin at colina.demon.co.uk
Tue Dec 30 04:15:42 EST 2008
>>>>> "Ertugrul" == Ertugrul Soeylemez <es at ertes.de> writes:
Ertugrul> Colin Paul Adams <colin at colina.demon.co.uk> wrote:
>> >>>>> "Ertugrul" == Ertugrul Soeylemez <es at ertes.de> writes:
>> >> That would seem to allow x = -3 and y = 13, for instance.
Ertugrul> Not if you disallow an x = -3 and y = 13 to happen in
Ertugrul> the first place. Haskell's module and type system
Ertugrul> allows you to do that.
>> Can you explain how to do that please?
Ertugrul> module Even (Even) where
Ertugrul> newtype Even a = Even a deriving (Eq, Show)
Ertugrul> instance Integral a => Num (Even a) where Even a +
Ertugrul> Even b = Even (a+b) -- ... fromInteger x = if even x
Ertugrul> then Even (fromInteger x) else undefined
Ertugrul> The only way to introduce incorrect Even values would be
Ertugrul> to access the constructor directly, but in that example,
Ertugrul> it's not exported. The interface to constructing Even
Ertugrul> values is the fromInteger function.
Thanks for all your help.
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