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@gmail.com] Sent: 17 December 2007 11:04 To: Nicholls, Mark Cc: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] OOP'er with (hopefully) trivial questions..... 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 Bob