Spooky variable declaration

[danmichaelson]

Weird things can happen when you declare a variable within a conditional that you intend to use outside of it. Example:

if false
  x = 1
end
puts x # nil, even though the code declaring it was never executed
puts y # crash

See http://stackoverflow.com/questions/12928050/why-does-ruby-seem-to-hoist-variable-declarations-from-inside-a-case-statement-e .

I propose variables should never be declared for the first time within a conditional if you intend to use them outside.

Bad (works, but looks so scary):

if false
  x = 1
end
puts x # nil

Bad (works, but if you made a mistake in your logic you get the previous scenario, so it still looks alarming / requires too much inspection to figure out why it won't blow up):

if false
  x = 1
else
  x = nil
end
puts x # nil

Good (self-evidently safe in a way that doesn't rely on weird parser quirk):

x = nil # Declare the default value for the variable outside the conditional
if false
  x = 1
end
puts x # nil

[abotalov]

It can be just classified as yet another feature of Ruby that makes code more concise.

Why should one write an additional line of code if it works without it?

[freemanoid]

Why should one write an additional line of code if it works without it?

Because sometimes it looks really confusing though your words make sense.

[danmichaelson]

Doesn't it make you do a double take every time you see that a variable may not be declared; knowing that at times undeclared variables will crash Ruby?

Scary feeling == bad style. There are plenty of recommendations in this style guide that add a few more characters of verbosity.

This for example does crash:

puts x
x = 1

So it seems to me the parser's behavior in spookily declaring variables that are out of the execution flow, is rather arbitrary/ill-defined, even though it does handle the common gotcha of declaring inside one fork of a conditional.

Or maybe it's just me, I'm also curious about the discussion / what others think.

[jdickey]

Agreed (with @danmichaelson and @freemanoid). @abotalov is perfectly correct when he notes that it's a "concise" feature of the language; if we hadn't all seen cases where Ruby isn't so consistent about avoiding crashes, we'd be less spooked by this.

[LavirtheWhiolet]

x = if false
  1
end
puts x

or

x =
  if false
    1
  end
puts x

[bbatsov]

I agree with @danmichaelson that this looks a bit weird, especially to people coming from other languages. I'd certainly encourage people to declare variables in a more obvious fashion. :-)

[jdickey]

@bbatsov

I'd certainly encourage people to declare variables in a more obvious fashion.

Fervently agreed. Perhaps a gentle reminder in the Style Guide that inspires a warning-level diagnostic in RuboCop and similar? E.g.,

Care is recommended when declaring and assigning to variables. If their only assignment is within a block, consider explicitly assigning nil to the variable immediately before the block, so the reader understands that s/he isn’t missing a previous assignment.

Possibly leading to a tool diagnostic such as

Style/AssignmentReadability: Variable ‘x’ appears to be declared within a block and used outside it. Assigning nil before the block will produce clearer code and silence this warning.

Thoughts?

[bbatsov]

No objections from me. It'd be nice to add some lint cop in RuboCop flagging such issues.