Stefan O'Rear wrote: > On Tue, Mar 27, 2007 at 11:32:29AM +0100, Chris Kuklewicz wrote: >> Stefan O'Rear wrote:
>>>> newtype Foo = Foo Int deriving(IsIntC) >>>> > >> Note that (Foo 2) + 2 is an attempt to add a Foo and an Int, which cannot >> possibly compile. So I replaced it with just a 2. > > Why not? They are the same type, and I have Curry-Howard proof of this fact. > > Stefan Foo is isomorphic to Int in structure. But it is not the same type. Foo is a new type that is distinct from Int. That means I get type safety -- you cannot pass an Int to a function that expects a Foo and vice versa. Since (+) is defined as (Num a => a->a->a) it cannot add type different types and thus you *cannot* add a Foo and an Int. This (+) is allowed, since "type" is just shorthand: > type Bar = Int > > x :: Bar > x = 2 > y :: Int > y = 3 > z = x+y This (+) is not allowed, since "data" declares a new type: > data Bar = Bar Int deriving (Num) > > x :: Bar > x = Bar 2 > y :: Int > y = 3 > z = x+y This (+) is not allowed, since "newtype" declares a new type: > newtype Bar = Bar Int deriving (Num) > > x :: Bar > x = Bar 2 > y :: Int > y = 3 > z = x+y _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
