Re: [Caml-list] Single-case union types as strong typedefs

[Nathaniel Gray <n8gray@gmail.com>]

On 23 Oct 2004 15:00:02 +1000, skaller <skaller@users.sourceforge.net> wrote:
> This:
>        newtype metres = float
> meaning
>        type metres = Metres float
> 
> means you have to define all the operations on 'metres' all
> over again:
> 
>        let (++) (Metres x) (Metres y) = Metres (x .+ y)
> 
> I'd guess in Haskell newtype can benefit from typeclasses?

I would have to try writing some code to be sure it makes sense, but I
envisioned this as more of a contract mechanism.  It would be a way of
making "semi-opaque" values without encapsulation guarantees but also
without performance penalties.  A function that needed to do any
serious work with the underlying values would destruct the wrappers:
  let hypoteneuse (Metres x) (Metres y): metres = sqrt(x*x + y*y)

> Actually for quantities, you'd use some phantom types
> so you could represent products of the dimensions,
> such as square metres too .. so the simplistic
> constructor representation isn't very good.

I'll have to read a bit about phantom types.  Maybe they fit the bill.

Cheers,
-n8

-- 
>>>-- Nathaniel Gray -- Caltech Computer Science ------>
>>>-- Mojave Project -- http://mojave.cs.caltech.edu -->

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