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