From: Peter Hickman <peterhickman386@...>
Date: 2010-07-07T22:59:17+09:00
Subject: Re: Hashes don't allow preceding commas by design?

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On 7 July 2010 13:43, Brian Candler <b.candler@pobox.com> wrote:

> But I think the question is why the following isn't allowed:
>
> {
> :first => 1
> ,
> :second => 2
> }
>
>
Look at this in irb might help us understand it

irb(main):001:0> a = {
irb(main):002:1* :first => 1
irb(main):003:1> ,
irb(main):004:1* :second => 2
irb(main):005:1> }
SyntaxError: compile error
(irb):3: syntax error, unexpected ',', expecting '}'
from (irb):4
from :0

The irb prompt ends with a > if it is looking for a new line. When it ends
with a *, as in line 2, it means that the parser thinks that the pervious
line is incomplete and that line 2 is a continuation of line 1.

However after entering line 2 the prompt returns to a >. This means that the
parser thinks that line 2 was complete and there is nothing more to be read.
Compare it to this

irb(main):001:0> a = {
irb(main):002:1* :first => 1,
irb(main):003:1* :second => 2
irb(main):004:1> }

The comma is on the end line 2 and the prompt for line 3 ends with a *. The
parser shows us that it does not consider line 2 to be complete and that
line 3 will be treated as a continuation of line 2.

A line ending with a comma tells the parser that (in the cases where a comma
is used such as hashes, lists or functions) that the statement is incomplete
and to treat the next line as a continuation of this line.

I'm not saying that the parser could not be written to handle the case you
present it is just that with the present design that uses the end of line as
an implicit end of statement marker the behaviour you are seeing is the
expected behaviour.

The same thing happens with the if statement

if some_test ||
another_test

will parse the same as

if some_test || another_test

but

if some_test
|| another_test

is something else entirely.

"if some_test" on a line by itself is a complete statement so the parser is
expecting the statement block (or a "then") but "if some_test ||" is
incomplete as || needs something on the right hand side so it treats the
next line as a continuation of the current one.

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