[Haskell-cafe] an idea for modifiyng data/newtype syntax: use `::=` instead of `=`

[Tom Ellis]

On the contrary, it *is* the same thing

    Prelude> data D = D !Int deriving Show
    Prelude> D undefined
    *** Exception: Prelude.undefined
    Prelude> undefined :: D
    *** Exception: Prelude.undefined


On Sun, Aug 09, 2015 at 01:30:01PM +0200, MigMit wrote:
> First, the half that I agree with: f . g = id. No doubt.
> 
> But g . f > id. And the value "d" that you want is "undefined". g (f
> undefined) = D undefined, which is not the same as (undefined :: D).
> 
> Отправлено с iPad
> 
> > 9 авг. 2015 г., в 13:17, Tom Ellis <tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk> написал(а):
> > 
> >> On Sun, Aug 09, 2015 at 01:09:21PM +0200, MigMit wrote:
> >> I disagree.
> > 
> > Ah, good.  A concrete point of disagreement.  What, then, is wrong with the
> > solution
> > 
> >    f :: D -> N
> >    f (D t) = N t
> > 
> >    g :: N -> D
> >    g (N t) = D t
> > 
> > If you disagree that `f . g = id` and `g . f = id` then you must be able to
> > find
> > 
> >    * a type `T`
> > 
> > and either
> > 
> >    * `n :: N` such that  `f (g n)` does not denote the same thing as `n`
> > 
> > or
> > 
> >    * `d :: D` such that `g (f d)` does not denote the same thing as `d`
> > 
> > Can you?
> > 
> > Tom
> > 
> > 
> >>> 9 авг. 2015 г., в 12:37, Tom Ellis <tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk> написал(а):
> >>> On Sun, Aug 09, 2015 at 12:15:47PM +0200, MigMit wrote:
> >>>>> Right, you can distinguish data declarations from newtype declarations this
> >>>>> way, but by using Template Haskell you can also distinguish
> >>>>> 
> >>>>>  * data A = A Int
> >>>>>  * data A = A { a :: Int }
> >>>>>  * data A = A' Int
> >>>>>  * data A = A Int !(), and
> >>>>>  * newtype B = B A (where A has one of the above definitions)
> >>>> 
> >>>> Sure, because they are different.
> >>>> 
> >>>>> from each other.  My claim is that
> >>>>> 
> >>>>>  * data B = B !A
> >>>>> 
> >>>>> is as indistinguishable from the above four as they are from each other.
> >>>> 
> >>>> Can you please NOT say that some thing can be distinguished AND that they
> >>>> are indistinguishable in the same post?
> >>> 
> >>> I think we are perhaps talking at cross purposes.
> >>> 
> >>> To clarify, here is an explicit statement (somewhat weaker than the full
> >>> generality of my claim):
> >>> 
> >>>   `data D = D !T` and `newtype N = N T` are isomorphic in the sense that
> >>>   there exist `f :: D -> N` and `g :: N -> D` such that `f . g = id` and
> >>>   `g . f = id`.
> >>> 
> >>> Do you agree or disagree with this statement?  Then we may proceed.