Allen Wirfs-Brock wrote:
The argument could validly be made for IfStatement which we cannot
change, for compatibility reasons. To me, if is probably mostly an
issue of whether we want consistency between IfStatement and
comprehension if clauses. consistency is good but we've already
discussed that we probably don't want to have Expression for if
clauses because of comma confusion. I think this is a discussion that
could go either way, it's just good to consider the full range of
principles we are factoring into the decision
The one distinguishing factor in the comprehension case is the
paren-free 'if' head. IfStatement has less of an issue with comma and
assignment at top level of the condition, although assignment is a
constant worry for all C-family languages that allow it. GNU C has a
warning for this case, you avoid it when you intend to assign at top
level of the condition by over-parenthesizing:
if ((yes = i_am_sure()) {...}
C++ has nice binding variants:
if (int yes = i_am_sure()) { /* yes in scope only here */ }
and this gets '=' back in the door, but as an declaration-initializing
operator, not the assignment operator that could be confused or typo'ed
from ==.
To loop back to your earlier point, in a comprehension:
a = [e for e in set if log(e), e & 1];
to log all elements in the set, but comprehend only the odd ones, looks
a bit odd. Is that ',' separating parts of the if condition, or element
initializers in an array literal?
But there's no ambiguity here, AFAICT. See the toy grammar below.
Paren-free heads are still bracketed in comprehensions, by
'if'/'for'/'let'/']' in full. Note that this would not be the case of
the comprehension expression moved to the right.
I don't believe the paren-free syntax increases the odds of typo'ing or
otherwise mistakenly writing = for ==, so on reflection, I'm not too
worried about comma and assignment expressions as paren-free if
conditions in comprehensions. But I don't think it would be a big deal
to forbid them either.
/be
%token FOR
%token ID
%token IF
%token OF
%%
ArrLit:
'[' AssExpList ']'
| '[' AssExp FOR ID OF AssExp IF Exp ']'
;
AssExpList:
AssExp
| AssExpList ',' AssExp
;
Exp:
AssExp
| Exp ',' AssExp
;
AssExp:
ID
| ID '=' AssExp
;
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