[Haskell-beginners] Re: Restricting integers in a type?

30 Dec 2008

      Colin Paul Adams  wrote:
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"Ertugrul" == Ertugrul Soeylemez  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?
Sure:

  module Even (Even) where

  newtype Even a = Even a
    deriving (Eq, Show)

  instance Integral a => Num (Even a) where
    Even a + Even b = Even (a+b)
    -- ...
    fromInteger x = if even x then Even (fromInteger x) else undefined

The only way to introduce incorrect Even values would be to access the
constructor directly, but in that example, it's not exported.  The
interface to constructing Even values is the fromInteger function.

Greets,
Ertugrul.

-- 
nightmare = unsafePerformIO (getWrongWife >>= sex)
http://blog.ertes.de/