if nil guard doesn't work for instance variable

[stefanwille]

According to the docs (http://crystal-lang.org/docs/syntax_and_semantics/if_var.html) the compiler will assume that a variable 'var' can not be nil after a 'if var':

  if var
    # var is guaranteed not to nil:
    var.some_method ...
  end

This works for local variables, but fails for instance variables:

class Stuff
  def foo
    @instance_variable = [] of String
  end

  def bar
    if @instance_variable
      # Should be ok, but compiler complains:
      @instance_variable << "bar"
    end
  end
end

stuff = Stuff.new
stuff.foo
stuff.bar

For this example, I get a compile time error:

Error in ./instance-variable-nil.cr:17: instantiating 'Stuff#bar()'

stuff.bar
      ^~~

in ./instance-variable-nil.cr:10: undefined method '<<' for Nil

      @instance_variable << "bar"

I would expect this to work the same as for local variables.

[stefanwille]

A temporary workaround is to use a local variable.

[JacobUb]

asterite explained this here #398 (the first reply)

[jhass]

I thought it was mentioned somewhere but here's why it doesn't:

An instance variable is shared data, in a multi-threaded context it's possible that the condition is checked and matches, then control is given to another thread who sets the variable to nil and then control is given back to the thread that checked the variable and that is assuming now that it's not nil while it is. Also given that we do have parallelism these days, that scenario becomes even more likely.

A             |    B
if @foo 
                   @foo = nil
  @foo.bar

[stefanwille]

Thanks again for the quick reply.

I take it that this is the explanation (copied from asterite):

Basically, in Crystal the type flow only applies to local variables. So you must first assign the instance variable to a local variable. Then if you do is_a?(...) the compiler will know that inside the if the variable will be of that type. Anything more complex than that and the compiler won't understand it.

It would be cool to mention in the docs that the "if var" guard only works for local variables. Or to allow instance variables too, as this seems fairly easy for "if var".

[asterite]

@jhass Bingo!

@stefanwille It's explained in the docs: http://crystal-lang.org/docs/syntax_and_semantics/if_var.html (scroll to the bottom). It says "The above logic doesn’t work with instance variables, class variables or global variables" and explains why.

[stefanwille]

@asterite Alright, found it, thanks!

[ysbaddaden]

That being said, assigning to a local variable is a bit boring, and unexpected by newcomers, especially when there are no other methods setting @var to nil (or another type).

I'm not sure, but, does Crystal complains when accessing @var through a getter? Because the same scenario by @jhass would apply.

[jhass]

Pretty sure it does.

[ysbaddaden]

Yes it does, because getter is merely a def foo; @foo; end. There must have been another case that made me think otherwise. Sorry for the noise.

[JacobUb]

@ysbaddaden maybe you were thinking about this https://github.com/manastech/crystal/blob/master/src/object.cr#L213 ?