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

[Tom Ellis]

On Sat, Aug 08, 2015 at 11:32:59PM +0200, MigMit wrote:
> >>>> Well, it's certainly unsupported by evidence. Because third-party code CAN
> >>>> distinguish between those.
> >>> 
> >>> OK, show me the code!
> >> 
> >> I did. You removed it when quoting.
> > 
> > Reinstating:
> > 
> >>> Prelude> newtype A = A Int deriving Show                                                                                           
> >>> Prelude> data B = B !Int deriving Show                                                                                             
> >>> Prelude> let x = case x of A n -> A 1 in x                                                                                         
> >>> A 1                                                                                                                                
> >>> Prelude> let y = case y of B n -> B 1 in y                                                                                         
> >>> *** Exception: <<loop>>                                                                                                            
> > 
> > This isn't third party code.  It knows about the constructors of A and B.
> 
> 1) This might be the code written by someone using your library/framework.
> In which case it would know about A and B.

Then it's up to me to define and document whatever strictness properties I
want for my constructors.

To reiterate: I'm *not* suggesting replacing

    newtype A = A a

with

    data A = A!

in existing code.  Of course that won't work.  But for *new* datatypes
choosing one rather than the other gives no difference in terms of
denotational semantics.

> 2) It might be generated by the Template Haskell — which is free to use
> whatever constructor is fed into it.

OK, so show me what goes wrong!