From: Ben Giddings <bg-rubytalk@...>
Date: 2005-05-05T03:49:30+09:00
Subject: Re: Typo-checking instead of static typing

On Wednesday 04 May 2005 12:21, Joe Van Dyk wrote:
> On 5/4/05, Jon A. Lambert <jlsysinc@alltel.net> wrote:
> > Ben Giddings wrote:
> > >
> > > Static typing tends to find typos and brainos in code before it's
> > > executed.
> > >
> > 
> > My most frequent Ruby error:
> > "NoMethodError: undefined method `foo' for nil:NilClass"
> > 
> > Not a typo, and arguably anything is a braino.
> > 
> > Fixed by asking oneself two questions
> > 1) How in sam hill did that get set to nil?
> > 2) Can method bar can return nil, if blah blah foozle wimble?
> > 
> 
> That's about my most common error too (it ties with a parse error due
> to a missing 'end' statement).

Python to the rescue!  See, when your language doesn't have end statements 
you can't *possibly* have any of these types of problems, right?  *grin*

Ok, but seriously.  We have "ruby -w" and "ruby -c" and "unit tests".

So let's have an analogy.  Someone writs the following bit of C code:

int do_something(int foo) {
  int i;

  if (foo > 500) {
    printf("Whoa, %d is too many times!  I can't do that!", fo);
  }
  else {
    for (i=0; i<foo; i++) {
      printf("%d\n", i);
    }
  }
}

int main(int argc, char **argv) {
  do_something(argv[1]);
}

The typical approach of a C programmer would be to try to compile that, and 
if they noticed no errors, they'd run it.

The compiler would immediately catch:
* do_something() is being called with an character pointer as an arg, but 
is declared as accepting an integer
* The variable used in the printf line "fo" was never declared.

Once the programmer fixed those two issues, the code would do what the 
author intended.

The equivalent Ruby code would be:

def do_something(foo)
  if foo > 500
    puts "Whoa, #{fo} is too many times!  I can't do that!"
  else
    foo.times { |i| puts i }
  end
end

do_something(ARGV[0])

"ruby -c" would say this is perfect syntax, which it is.  So that's kinda 
useless for typos and brainos

"ruby -w" would run it, and it would barf with this message:

foo.rb:4:in `>': undefined method `>' for false:FalseClass (NoMethodError)
        from tmp/foo.rb:4:in `do_something'
        from tmp/foo.rb:11

The line number is useful, but the message sure isn't.  It turns out that 
string > number complains about FalseClass.  I dunno why.  Anyhow, it's 
obvious there is *some* error, so presumably the programmer would remember 
to 'to_i' the arg to do_something().

Then there's the typo in the "puts" line.  Until the person happens to try 
an arg greater than 500, they will have no indication the code has a flaw.

Now, would a unit test catch this?  If the programmer was careful, maybe.  
But maybe not.

So, what advice would you guys give to a C programmer who wants a Ruby tool 
to look over lines of code he may rarely execute?

Ben