Re: [Haskell-cafe] Constraints at construction

[Iain Barnett]

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: [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

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