[Haskell-cafe] OOP'er with (hopefully) trivial questions.....

[Thomas Davie]

On 17 Dec 2007, at 11:14, Nicholls, Mark wrote:

> OK I'll have to digest this and mess about a bit....but can I make an
> observation at this point....
>
> If I define "Shape" like
>
> data Shape = Circle Int
>             | Rectangle Int Int
>             | Square Int
>
> Isn't this now "closed"...i.e. the statement is effectively defining
> that shape is this and only ever this....i.e. can I in another module
> add new "types" of Shape? (sorry about all the quotation marks, but  
> it's
> a minefield of potential confusions over types, classes etc).

That's correct, another module could not add constructors to this  
type.  The idea here is that you tell it all of the possible ways to  
construct Shape, and can then write functions to deal with it elsewhere.

> My other observation is...are the things on the right hand side of the
> the "="'s sign not types?
Correct, they're constructors.  So you could never for example write a  
function that accepts only Rectangles (unless you start getting into  
odd type extensions)

> The lower version makes more sense to me...I'll have to give it a  
> go....

Both versions make sense.  They differ only in how "heavy weight" they  
are.  Defining a type allows you to do pattern matching on the  
constructors, and is a much better way of defining anything you know  
the structure of in the first place.  Using the class system on the  
other hand, gives you more flexibility, but at the cost of a lot of  
readability.  The class system is designed to be able to describe  
things that aren't explicitly the same type, but exhibit similar  
properties.  For example the Eq class describes all things that are  
equatable, it defines the (==) and (/=) operators.  Your Shape class  
describes all types in which it's sane to compute an area.

> A P.S. would be...I tend to write code rather than mess about in the
> GHCi shell.....is there a way in code to output the type of a
> value..i.e. the ":t" operation?

Take a look at the Typable class.  Although, pretty much any code that  
you can compile can be loaded into ghci without modification, and  
that's by far the easier way of finding the types of things.

Bob