No that's fine....its all as clear as mud!......but that's not your fault. To recap... "type" introduces a synonym for another type, no new type is created....it's for readabilities sake. "Newtype" introduces an isomorphic copy of an existing type...but doesn't copy it's type class membership...the types are disjoint/distinct but isomorphic (thus only 1 constructor param). "data" introduces a new type, and defines a composition of existing types to create a new one based on "->" and "(". "class" introduces a constraint that any types declaring themselves to be a member of this class...that functions must exist to satisfy the constraint. I'm sure that's wrong, but it's a good as I've got at the moment. And to a degree it's all upside down....what Haskell thinks are types...I think are "singnatures" and what Haskell thinks is a type "class" I think of as a type.....it's not going to be easy. -----Original Message----- From: Thomas Davie [mailto:tom.davie@gmail.com] Sent: 17 December 2007 12:35 To: Nicholls, Mark Cc: Haskell Cafe Subject: Re: [Haskell-cafe] OOP'er with (hopefully) trivial questions..... 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.