Thankyou, this is exactly what I was looking for. Iain On 9 Oct 2008, at 2:05 pm, Mitchell, Neil wrote:
Hi Iain,
The wiki page has quite a nice article: http://www.haskell.org/haskellwiki/Smart_constructors
Thanks
Neil
-----Original Message----- From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] 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
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