On 2 Feb, 2007, at 16:55 , Malcolm Wallace wrote:
Douglas Philips <[EMAIL PROTECTED]> 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: [EMAIL PROTECTED] ............ / |___\
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