# List syntax (was: Re: help from the community?)

Atze Dijkstra atze at xs4all.nl
Fri Feb 2 11:47:23 EST 2007

```On  2 Feb, 2007, at 16:55 , Malcolm Wallace wrote:

> Douglas Philips <dgou at mac.com> wrote:
>
>> What would be the proper way to propose that:
>> 	| ( exp1 , ... , expk )		(tuple, k>=2)
>> 	| [ exp1 , ... , expk ]		(list, k>=1)
>> be amended to:
>> 	| ( exp1 , ... , expk [ , ] )		(tuple, k>=2)
>> 	| [ exp1 , ... , expk [ , ] ]		(list, k>=1)

>
> I believe that ghc at one time did accept any sequence of white-space
> separated commas as if they were a single comma, leading to code
> such as
>     [ 1 , , 2 , 3 , ]
>
> Whilst programmers might be able to see the "obvious" meaning when it
> occurs in list exprs, do stmts, imports, exports, and the like, they
> might run into difficulties in the tuple case.  For instance,
>    (1,2,)
> has been proposed as syntactic sugar for a tuple-section, meaning
>    \x-> (1,2,x)
> and this would be a perfectly reasonable and intuitive interpretation
> IMO.  The number of commas is a powerful visual indication of the
> arity
> of the tuple.

This would be an instance of an arbitrary expression where a
subexpression has been omitted, forming a hole. So

[ 1 , , 2 , 3 , ]		means	\x y -> [ 1 , x , 2 , 3 , y ]
5 + * 6				means	\x -> 5 + x * 6
(3 + *)				means	\x y -> (3 + x * y)			(operator section)

Thus the following interpretations could be chosen from:
1 - an operator/comma without operand is an error (as a safety net
against typos)
2 - a comma without operand is redundant, means the same as an
expression without
3 - an operator/comma without operand has meaning as described above

I do prefer option 1, I do not mind writing \x -> 5 + x * 6
explicitly instead of implicitly (and more cryptic) 5 + * 6. I do not
feel that stretching the syntax in this way warrants the loss in
possibilities to check for errors.

regards,

- Atze -

Atze Dijkstra, Department of Information and Computing Sciences. /|\
Utrecht University, PO Box 80089, 3508 TB Utrecht, Netherlands. / | \
Tel.: +31-30-2534093/1454 | WWW  : http://www.cs.uu.nl/~atze . /--|  \
Fax : +31-30-2513971 .... | Email: atze at cs.uu.nl ............ /   |___\

```