Hi Iain, The wiki page has quite a nice article: http://www.haskell.org/haskellwiki/Smart_constructors
Thanks Neil > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of Iain Barnett > Sent: 09 October 2008 2:03 pm > To: haskell-cafe@haskell.org > Subject: [Haskell-cafe] Constraints at construction > > If I were to create an object in C#, for instance, I could > add code to the constructor that might limit the type further e.g. > > > public class Car > { > string model; > int wheels; > > public Car ( string model, int no_of_wheels ) > { > if ( no_of_wheels <= 2 ) { > throw new Exception( "a car must have > at least 3 wheels"); > } > > this.model = model; > this.no_of_wheels = no_of_wheels; > } > } > > or I could specify all sorts of things, like the size of an > array could only be a certain length, or whatever. Similarly, > when creating a column with SQL I'd be able to specify > further constraints on the column than just it's type. > > I don't see how to do this with Haskell and the data keyword. > Is there a tutorial someone could point me to, or an > explanation of this? Or is it that I have to wrap the > creation in an accessor function that checks the inputs first? > > type Model = String > type Wheels = Int > > data Car = Car Model Wheels > > car_maker model wheels = if wheels <= 2 then error ... > else Car model wheels > > ? > > Most of my programming career (if you can call it that:) has > been primarily with C# and SQL, and perhaps this heady > mixture of OOP and hacked-functional is confusing me! > > > Iain > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > ============================================================================== Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html ============================================================================== _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
