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

Ertugrul Soeylemez es at ertes.de
Tue Dec 30 04:13:15 EST 2008

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?


  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.


nightmare = unsafePerformIO (getWrongWife >>= sex)

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