[Haskell-cafe] OOP'er with (hopefully) trivial questions.....

Jules Bean jules at jellybean.co.uk
Mon Dec 17 12:15:22 EST 2007

Peter Verswyvelen wrote:
> Very interesting, I did not know that!
> I thought newtype was an optimization of data, and that "newtype" was bad terminology. But if newtype is just a wrapper around a type, then the name is choosen well.
> I'm a bit confused why then one needs a data-constructor-like tag to construct a newtype value then? Is this to avoid having to add a type signature (for type inference)? I find this a bit weird since 
> newtype Foo = Foo Int
> bar = Foo 123
> does not safe a lot of keystrokes ;) compared to
> -- Incorrect Haskell follows
> newtype Foo = Int
> bar = 123::Foo

You've broken the principle type property.

let's stay away from numeric literals (which are fiddly because they're 
overloaded already in typeclass Num) and take a concrete type:

data Foo = A | B
newtype Bar = Bar Foo

Now, the type of "A" is "Foo".  If I was allowed to write "A::Bar" then 
I no longer have a simple principle type for "A": it appears that the 
type of "A" is "Foo" or "Bar" depending how I annotate it.

Of course, I can make this work fine, using some form of constraint. 
Haskell already has constraints, we call them classes, and I could 
imagine giving "A" the principle type "(FooOrNewtypeOfFoo a) => a"

However, that's a bunch of added complexity for such a simple feature :)

Much nicer just to write "A" :: Foo and "Bar A" :: Bar.


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