What are the downsides of optional trailing commas?

Question

Many languages have several rules like the following in their grammars, which forbid trailing commas:

call-expr: ident '(' expr-list? ')';
expr-list: expr | expr-list ',' expr;

However, the following formulation allows (but does not require) trailing commas, which has well-known advantages:

call-expr: ident '(' expr-list? expr? ')';
expr-list: expr ',' | expr-list expr ',';

Is there any downside to always using this, at least internally?

It's not something you'd likely know if you don't know that language. — Laurel, Apr 17 '16 at 20:33
The downside is the possiblity that an amateur, entering a list of data, might leave a bit of data out without it being detected. I think optional trailing commas is an assumption the language users are expert, and your are trying to make their life easier. So whether it is a downside depends on who you think the typical user is. — Ira Baxter, Apr 17 '16 at 21:11

Stefan Haustein · Accepted Answer · 2016-04-17T21:55:52.227

1

The downside is ambiguity where empty entries are permitted. Consider the following arrays in Javascript:

[1,,3]
[1,,3,]

Are they identical? Is their length identical? Should it be? (Answer: yes, but not in IE<=7)

edited Apr 17 '16 at 21:55

answered Apr 17 '16 at 21:49

Stefan Haustein

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score 1 · Answer 2 · answered Apr 17 '16 at 22:26

Languages which allow procedure parameters to be left off (I'm particularly thinking of the old HP systems language SPL; there may be others) would fail on this because the compiler would assume that there was a skipped parameter after the trailing comma which the procedure does not call for. I'll grant that this is a minor disadvantage - on the other hand, IMO the stated benefits are equally minor.

What are the downsides of optional trailing commas?

2 Answers2