Richard A. O'Keefe <ok(at)cs(dot)otago(dot)ac(dot)nz>
Standards Track

EEP 21: Optional trailing commas for lists and tuples #

Abstract #

Allow an extra comma at the end of a list or tuple. Darren New proposed this change; Richard O’Keefe, who doesn’t like it very much, wrote it up as an EEP.

Specification #

A list that would have ended with “,X]” for some term X may instead end with “,X,]”. A tuple that would have ended with “,X}” for some term X may instead end with “,X,}”. The rule for tuples also applies to records and -record declarations.

Motivation #

About 5,000 lines of the Erlang/OTP sources begin with a right square bracket or right curly brace. For example, -record declarations are commonly laid out as

-record(foo, {
    field_1 = default_1,
    field_n = default_n


-record(hostent, {
     h_aliases = [],
     h_addr_list = []

and record creation expressions are often laid out similarly, e.g.,

make_hostent(Name, Addrs, Aliases, ?S_A) ->
    #hostent {
          h_name      = Name,
          h_addrtype  = inet,
          h_length    = 4,
          h_addr_list = Addrs,
          h_aliases   = Aliases

Adding entries to such lists (in the informal sense of “list”), removing entries, and reordering entries would be simpler if they were all punctuated the same way. Lists (in the Erlang sense of “list”) of options are also often laid out like this.

C, C++, Java, and Javascript allow a trailing comma in initial value lists. Python allows trailing commas in lists and dictionaries. Python in particular is evidence that a programming language can support this feature without charges of “C envy” or of extreme ugliness.

Rationale #

I don’t actually feel any need for this proposal; I believe that the answer is better tool support. However, many people are wedded to their tools, even more than their programming languages. Darren New is not the only one to have asked for it, and with about 1 SLOC in 110 of the Erlang/OTP sources reflecting a list or tuple where this feature could have been used, it’s very much a low cost high public appreciation feature.

I wrote that last sentence before working on the parser to make it accept this “feature”. There are 115 lines of plain diffs. I could have made this change to the Prolog parser in 10 minutes, but then the Prolog parser has the enormous advantage of NOT being written using an LR parser generator like Yecc. Still, now that I have hacked on the parser, the cost to everyone else is low.

The specification was carefully worded. Commas are NOT allowed in empty lists or tuples, nor in list or tuple comprehensions. They are only allowed after a final element, so [1|L,] is also not allowed. Nor are trailing commas allowed inside argument lists, only in [] and {}. They are, however, allowed in tuple and record types.

This is very similar to the “optional semicolons” EEP (which was FAR simpler to implement). The heart and soul of that EEP is the desire to make semicolons and commas look DIFFERENT; for this reason it is important NOT to allow optional trailing semicolons. If semicolons may trail, commas must not. If commas may trail, semicolons must not. It is also important NOT to approach the “consistent punctuation for list elements” problem by allowing optional leading commas. If semicolons may lead, commas must not. If commas may lead, semicolons must not. Since trailing commas are established practice in C, C++, Java, ECMAScript, Python, &c, commas trail, semicolons lead.

I repeat that this is not my idea. I’ve just written up the EEP and figured out how to implement it. With nearly 1% of the SLOC in the Erlang/OTP system being cases where people might well have had reason to add a trailing comma, had it been legal, it seemed worth while finding out whether it would be practical.

Backwards Compatibility #

All existing Erlang code remains acceptable with unchanged semantics. The commas are dealt with entirely in the parser; other language manipulation tools never know that they were there, so work perfectly with code using them.

Reference Implementation #

The auxiliary file eep-0021-1.diff is a patch file to be applied to erl_parse.yrl.

You would think that all we’d need to do would be to change

... ']'                    ... '}'


... ',' ']'                ... ',' '}'

in several places. You would be wrong. With a different grammar, maybe. With the current grammar, this was an uncommonly tricky change requiring surgery in all sorts of places. The result gets through Yecc with no complaints other than the two shift/reduce complaints that are expected (and have nothing to do with this change).

Copyright #

This document has been placed in the public domain.