[rst]

And it's the one thing I really dislike about Ruby. The problem with this is that the interpreter can't tell the difference between declaring a new variable, and misspelling the name of an existing one --- which means that there's a very common class of simple errors which it can't automatically check. Even Perl (with -w) gets this right, and it doesn't really make the language more verbose; declarations are preceded with a 'my ' which is useful punctuation. (Humans also find it useful to see when a new variable is being introduced.)

And you could come up with ways to preserve the distinction while making it even less obtrusive --- ':=' instead of '=' in a declaration, for example.

[klibertp]

It's the same in Python, and it's mostly solved by external static code analysis tools. The trick here is the assumption that misspelled variable will be used only once in a file - exactly where it got misspelled - and tools like pylint or pyflakes warn about such names. If you're running these tools sufficiently often and have them integrated with your editor (my Emacs runs pyflakes once in 3 seconds, after enter is pressed and on save, whichever comes first) this kind of error is essentially eliminated.

OTOH, having a language which doesn't need external tooling to eliminate such errors is an obvious win. Smalltalk, with it's code browsers, refactoring tools and similar does this really well, for example - if you use a new name inside a method definition you are asked if you want to declare new instance or block variable, or turn it into selector, or if you want to rename it (and then it gets renamed throughout the method) and so on. And that's at compile time!