Tuple-like constructors

[Malcolm Wallace]

Robert Dockins <robdockins at fastmail.fm> writes:

> > i would argue against treating tuples as pure syntactic sugar for  
> > nested pairs; since the nesting carries hierarchical information, i  
> > would expect (x,y,z) used in place of (x,(y,z)) to cause an error.

Indeed, quite apart from anything else, transforming tuples into
nested tuples changes the performance of access from constant-time
into linear-time (using the right-nested transformation specified
by Robert), or at best, log-time (using a balanced tree schema).
This is highly undesirable.

> The copout (in my opinion) is the current system where tuples are  
> defined up to some set n (64 in GHC I think -- the report only  
> mandates 15), and if you want bigger tuples you're SOL.

To address this problem, I think we should permit user-definition of
tuple constructors with the expected syntax e.g.  (,,,,,), which is
already accepted by at least some compilers (nhc98,yhc).

> More disturbing is the complete inability to write general functions  
> over tuples.
>               The only fully general solution is to use something  
> like template haskell which 1) is overkill 2) has ugly syntax (sorry  
> but it's true) 3) GHC only and 4) hard to learn.

I don't believe you need to invoke the full awesome majesty of
Template Haskell in this case.  Surely plain -fgenerics will suffice?
I am hoping that some simple form of data-type generics will make it
into the new standard.

Regards,
    Malcolm