Unit unboxed tuples

[Tyson Whitehead]

On December 23, 2011 09:37:04 Ganesh Sittampalam wrote:
> On 23/12/2011 13:46, Ian Lynagh wrote:
> > On Fri, Dec 23, 2011 at 01:34:49PM +0000, Simon Peyton-Jones wrote:
> >> Arguments   Boxed      Unboxed
> >> 3           ( , , )    (# , , #)
> >> 2           ( , )      (# , #)
> >> 1
> >> 0           ()         (# #)
> 
> It's worth mentioning that if you want to write code that's generic over
> tuples in some way, the absence of a case for singletons is actually a
> bit annoying - you end up adding something like a One constructor to
> paper over the gap. But I can't think of any nice syntax for that case
> either.

I believe python uses (expr,) (i.e., nothing following the ,) to distinguish a 
singelton tupple from a braced term.  Not great, but possibly not that bad.

The other option you could do is introduce another unambiguous brace symbol 
for tupples.  The new symbol would be optional except for the singelton.

(- expr, expr, expr -)  =  (expr, expr, expr)
(- expr, expr -)  =  (expr, expr)
(- expr -)  =  <unable to express>
(- -)  =  ()

Nothing has to be done for (# #) as it doesn't have the ambiguity.

Cheers!  -Tyson