On 17 Dec 2007, at 12:22, Nicholls, Mark wrote:
Ok...
Thanks I need to revisit data and newtype to work out what the difference is I think.
Beware in doing so -- type, and newtype are not the same either. type creates a type synonim. That is, if I were to declare type Jam = Int then Jam and Int from that point on become completely interchangable, the only thing this does is make things readable. For example, a parser might be described as a function that takes a list of tokens, and outputs a parse tree, and a list of unparsed tokens: type Parser = [Token] -> (ParseTree, [Token]) if I write some parser combinators, I can now give them clear types like <|> :: Parser -> Parser -> Parser I could however still write this, and it would have *exactly* the same meaning. <|> :: ([Token] -> (ParseTree, [Token])) -> ([Token] -> (ParseTree, [Token])) -> [Token] -> (ParseTree, [Token]) newtype on the other hand introduces a new type to the type system. Because of this, the type system has to be able to tell when you're using your new type, so a tag gets attached. newtype Ham = Ham Int This creates a type that contains only an integer, but is different from Int (and Jam) in the type system's eyes. Thus, I cannot for example write (Ham 5) + (Ham 6) Because Ham is not Int and thus (+) does not work (or actually, more specifically, Ham is not a member of the class Num, the numeric types, and therefore (+) doesn't work). This can of course be fixed thus: newtype Ham = Ham Int deriving Num Hope that helps Tom Davie p.s. Sorry for the slip with the newtype Rectangle.