Re: why doesnt a[1] = 1 work?

[Don Hopkins <dhopkins@...>]

Peter Odding wrote:
> Markus Heukelom wrote:
> > However, I agree this might have some other (ugly) repercussions not
> > immediately obvious at this moment.
> > (In fact, to implement this behavior, I would have to change the 
> > __newindex
> > methods of the nil, number, string, etc matetable (can only in c), 
> > right?
> > Not that I will do that - just for the idea.)
>
> Apart from whether it's possible to set a metatable for nil (never 
> tried), note that you'd only be able to do this for scalars, i.e. not 
> for tables and userdata, because there's no `global' metatable for 
> these types.
>
> About this concept in general, I don't think it would fit Lua very 
> well. PHP allows it, and that's caused me a few hard to find bugs.
>
> - Peter Odding

It's a VERY GOOD THING that Lua is nothing like PHP!

One of the many horrible design decisions in PHP, which encourages 
people to write sloppy bug-ridden code, was to automatically create an 
associative array when you go "$a[1] = 1".
Programming languages should never try to guess what you mean, nor 
attempt to figure out what you would have wanted them to do when your 
program has errors and ambiguities.

Unfortunately PHP has conditioned many programmers to expect and even 
rely on that kind of misbehavior, many of whom actually run with warning 
and error reporting turned off, so they don't even realize (or care) 
that their programs contain many errors and warnings and ambiguities, 
which is why so many PHP programs have such horrible bugs and security 
holes.

Warren Teitelman's "DWIM" system for Interlisp is widely considered a 
terrible failure.
People should learn from history, like Lua's designers did, instead of 
repeating the mistakes of the past, like PHP's designers did!

-Don

http://en.wikipedia.org/wiki/DWIM
http://www.catb.org/~esr/jargon/html/D/DWIM.html

DWIM: /dwim/

[acronym, ‘Do What I Mean’]

1. adj. Able to guess, sometimes even correctly, the result intended 
when bogus input was provided.

2. n. obs. The BBNLISP/INTERLISP function that attempted to accomplish 
this feat by correcting many of the more common errors. See hairy.

3. Occasionally, an interjection hurled at a balky computer, esp. when 
one senses one might be tripping over legalisms (see legalese).

4. Of a person, someone whose directions are incomprehensible and vague, 
but who nevertheless has the expectation that you will solve the problem 
using the specific method he/she has in mind.

Warren Teitelman originally wrote DWIM to fix his typos and spelling 
errors, so it was somewhat idiosyncratic to his style, and would often 
make hash of anyone else's typos if they were stylistically different. 
Some victims of DWIM thus claimed that the acronym stood for ‘Damn 
Warren’s Infernal Machine!'.

In one notorious incident, Warren added a DWIM feature to the command 
interpreter used at Xerox PARC. One day another hacker there typed 
delete *$ to free up some disk space. (The editor there named backup 
files by appending $ to the original file name, so he was trying to 
delete any backup files left over from old editing sessions.) It 
happened that there weren't any editor backup files, so DWIM helpfully 
reported *$ not found, assuming you meant 'delete *'. It then started to 
delete all the files on the disk! The hacker managed to stop it with a 
Vulcan nerve pinch after only a half dozen or so files were lost.

The disgruntled victim later said he had been sorely tempted to go to 
Warren's office, tie Warren down in his chair in front of his 
workstation, and then type delete *$ twice.

DWIM is often suggested in jest as a desired feature for a complex 
program; it is also occasionally described as the single instruction the 
ideal computer would have. Back when proofs of program correctness were 
in vogue, there were also jokes about DWIMC (Do What I Mean, Correctly). 
A related term, more often seen as a verb, is DTRT (Do The Right Thing); 
see Right Thing.