From: Peter Hickman <peterhickman386@...>
Date: 2010-07-07T19:54:49+09:00
Subject: Re: Hashes don't allow preceding commas by design?

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On 7 July 2010 11:02, Josh Cheek <josh.cheek@gmail.com> wrote:

> > So line 4 is taken to be a continuation of lines 1, 2 and 3 and finally
> > completes the statement that started on line 1.
> >
>
> Your explanation implies to me that the OP's desired use case is valid. If
> "line 4 is taken to be a continuation of lines 1, 2 and 3" then how come
> line 4 can't be ",:jim => 43"
>

I can understand the confusion but it can be explained like this

"A hash is a '{' followed by a comma separated list of key / value pairs
finished off with a '}'"

So if we read through a couple of examples:

1: {
2: :first => 1,
3: :second => 2
4 }

Line 1 starts the hash declaration, the parser is looking for a key value
pair or a '}'
Line 2 is a key value pair and a comma. The comma tells the parser than it
should expect another key value pair
Line 3 is the next key value pair and does not have a comma. So the parser
has come to the end of the list of key value pairs that makes up a hash and
is therefore expecting a '}'
Line 4 is the expected '}' and the hash declaration is complete

Now try this:

1: {
2: :first => 1
3: :second => 2
4 }

Again line 1 starts the hash declaration, the parser is looking for a key
value pair or a '}'
Line 2 is a key value pair and does not have a comma. So the parser believes
that it has come to the end of the list of key value pairs and is therefore
expecting a '}'
Line 3 is a key value pair but the parser is expecting a '}' so this is an
error. The compiler spits out the error.
We never get to line 4.

This goes back to what I said about 'incomplete' lines ":first => 1," is
incomplete because the comma makes the parser think that another key value
pair should be expected because it is parsing a hash declaration. So line 3
is treated as a continuation of line 2 and all is well with the world.

However ":first => 1" is a complete line, the absence of a comma does not
set the parser up to expect another key value pair. So when it encounters
":second => 2" it goes "what is going on here, according to my rules the
next thing I should find is an '}'".

It is not to say that the parser couldn't handle this situation or at least
handle a line like ", :second => 2" (with a leading comma) but it does not
as currently implemented.

Some languages use things like a ';' as an explicit end of statement marker
to help the parser. Ruby does not do this and treats the end of a line to be
the implicit end of statement marker unless the statement appears to be
incomplete. What constitutes incomplete depends on what it is parsing, a
hash, a list, an if statement etc.

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