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

[Tom Ellis]

On Sun, Aug 09, 2015 at 07:49:10PM +0200, MigMit wrote:
> You know, you've kinda conviced me.

I hope I'm correct then!

> The difference between strict and non-strict parameters is in how
> constructors work.  "data D = D Int" is still the same as "data D = D
> !Int", but it's constructor — as a function — is more restricted.  It's
> somewhat like defining "d n = D $!  n", and then not exporting D, but only
> d.

Right.

> That said, it might be true that semantics differ depending on what is
> exported.  So, it might be true that your D has the same semantics as N. 
> We still can distinguish between those using various unsafe* hacks — but
> those are what they are: hacks.
> 
> Отправлено с iPad
> 
> > 9 авг. 2015 г., в 13:35, Tom Ellis <tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk> написал(а):
> > 
> > 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.
> > _______________________________________________
> > Haskell-Cafe mailing list
> > Haskell-Cafe at haskell.org
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe