Nannying (was: Array comprehension syntax)

Brendan Eich brendan at
Mon Sep 24 21:00:28 PDT 2012

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.


%token FOR
%token ID
%token IF
%token OF


         '[' AssExpList ']'
|       '[' AssExp FOR ID OF AssExp IF Exp ']'

|       AssExpList ',' AssExp

|       Exp ',' AssExp

|       ID '=' AssExp

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