[Haskell-cafe] Constraints at construction

Iain Barnett iainspeed at gmail.com
Thu Oct 9 09:02:44 EDT 2008

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!


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