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

Nicholls, Mark Nicholls.Mark at mtvne.com
Mon Dec 17 06:14:51 EST 2007

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).

My other observation is...are the things on the right hand side of the
the "="'s sign not types?

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

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? 

-----Original Message-----
From: Thomas Davie [mailto:tom.davie at gmail.com] 
Sent: 17 December 2007 11:04
To: Nicholls, Mark
Cc: haskell-cafe at haskell.org
Subject: Re: [Haskell-cafe] OOP'er with (hopefully) trivial

On 17 Dec 2007, at 10:46, Nicholls, Mark wrote:
> I can obviously at a later date add a new class Triangle, and not  
> have to touch any of the above code....

Yes, and you can indeed do a similar thing in Haskell.  The natural  
thing to do here would be to define a type Shape...

data Shape = Circle Int
             | Rectangle Int Int
             | Square Int

area :: Shape -> Int -- Note, this is an interesting type if you want  
the area of circles
area (Circle r) = pi * r^2
area (Rectangle h w) = h * w
area (Square l) = area (Rectangle l l)

If however, you *really* want to keep your shapes as being seperate  
types, then you'll want to invoke the class system (note, not the same  
as OO classes).

class Shape a where
   area :: a -> Int

newtype Circle = C Int

instance Shape Circle where
   area (C r) = pi * r^2

newtype Rectangle = R Int Int

instance Shape Rectangle where
   area (R h w) = h * w

newtype Square = Sq Int

instance Shape Square where
   area (Sq l) = l * l

-- Now we can do something with our shapes
doubleArea :: Shape a => a -> Int
doubleArea s = (area s) * 2

Hope that helps


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