list literal gotcha and suggestion

[Joshua Bloch]

Nick,
Hi! Thanks for the suggestion.  Your point is entirely valid.  That said, I
first wrote the proposal the way you want it, and this ensued:

---------- Forwarded message ----------
From: Tim Peierls <tim at peierls.net>
Date: Fri, Mar 27, 2009 at 4:45 AM
Subject: Re: Collection Literals Proposal
To: Joshua Bloch <jjb at google.com>
Cc: Neal Gafter <neal at gafter.com>, Java-council <java-council at google.com>,
Doug Lea <dl at cs.oswego.edu>, "Joseph D. Darcy" <joe.darcy at sun.com>


On Fri, Mar 27, 2009 at 2:01 AM, Joshua Bloch <jjb at google.com> wrote:

> Incidentally, I picked curly braces for List because the language already
> uses them for array initializers, but the more I look at what I proposed the
> more I think I got it backwards.  It's the opposite of Python, and it just
> feels wrong.
>

Yes, you got it backwards. Years of Perl hacking have burned it into me: []
is empty list/array, {} is empty map/hash/associative array. Also Python,
Javascript, Ruby, ... Not something you want to fight.

--tim

***********************************************************************************

So I suppose it's a question of who gets confused:(  Perhaps I could keep
the syntax as it stands, but encourage IDE maintainers to generate a warning
if programmers fall into the trap that you describe?

              Josh

On Sat, Sep 26, 2009 at 1:37 PM, Nick Parlante <
nick.parlante at cs.stanford.edu> wrote:

> Hi there -- up until recently I taught Stanford's Tons Of Java course,
> CS108, for many years, so I've got a good feel for how new engineers get
> started with the language and where they have problems. I also run
> http://javabat.com where you really see what beginner programmers write.
>
> Looking at the Project Coin stuff, one corner of the syntax for
> collection literals jumped out at me as a potential source of problems.
> Here's the proposed syntax, as lifted from Mark Reinhold's slides.
>
> List<Integer> piDigits = [ 3, 1, 4, 1, 5, 9, 2, 6, 5 ];
> Set<Integer> primes = { 2, 7, 31, 127, 8191, 131071 };
>
> My concern is that students will write code like this:
>
> List<Integer> piDigits = new ArrayList<>({3, 1, 4, 1});
>
> This code looks so reasonable but sadly does something unexpected, since
> the set literal eliminates the second 1.
>
> The students will write it this way, with the { }, since they are
> accustomed to array initializers that way in Java, and also C/C++.
>
> So here's my suggestion:
>
> 1. Use { } for list literals, not [ ]. It's most consistent with array
> literals and avoids the above list/set gotcha.
>
> 2. Use [ ] for set literals, or if that causes problems, just don't have
> a literal for sets. Lists and maps are the most common, so having a
> syntax for those cases is the most important. Even without a special
> syntax, a set is easy to express based on a list literal, like
>   new HashSet<Integer>({1, 2, 3, 1})  //  { } meaning List here
> It's nice that there's no gotcha with the slight []/{} mis-match going
> this direction.
>
> Cheers,
>
> Nick
>
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