Misspelled Variables

Care to guess what happens when you execute the following PHP?

define('FOO', 'Hi');
print(FO);

It prints 'FO'.

I do believe PHP got this from Perl:

perl
print FOO . "FOO";  # Prints FOOFOO

It works even if you're strict:

perl -w
use strict;
print FOO . "FOO";  # Prints FOOFOO

Ruby behaves differently depending on whether you try to print an undefined variable/method or an undefined attribute:

irb
>> print a
NameError: undefined local variable or method `a' for main:Object
 from (irb):1
>> print @a
nil=> nil

Python raises an exception:

python
>>> print a
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'a' is not defined

In a compiled language, these sorts of errors would be caught at compile time. However, a compiled language would never let me do something like:

python
>>> var_name = 'a'
>>> locals()[var_name] = 'yep'
>>> print a
yep

This example is a bit contrived, but I've definitely done things like it.

Personally, I like the flexibility of a scripting language. I like it even more when there's a tool like pychecker that can catch these sorts of errors. However, just because a scripting language doesn't have a compilation step that can catch stupid spelling mistakes doesn't mean it should accept them at runtime. I'd much rather deal with an exception than spend half an hour fighting a bug caused by a spelling error!

As a general rule, I think software should fail fast rather than glossing over bugs that will surely cause trouble later. I can handle the exception if I need to, but what I can't handle is a silent bug.