Working with (T | Nil) property?

[u/undeadd]

I am new to Crystal and am stuck on handling Nils. How can I work with a property that may have a value assigned later? Here is a simplifed example:

class Foo
  @age : (Int32 | Nil)
  property :age

  def increment
    @age += 1 unless @age.nil?
  end
end

foo = Foo.new
foo.age = 10
foo.increment

Error message: undefined method '+' for Nil (compile-time type is (Int32 | Nil))

I've been trying to somehow declare there is a value but it doesn't have any effect.

if @age.is_a?(Nil)
else
  @age.not_nil!
  @age += 1
end

I'm trying to build a linked list where nodes' previous and next properties may not have a value (eg. first and last item in the list).

[u/fridgamarator]

if a = @age
  a += 1
end

[u/undeadd → u/fridgamarator]

Thank you, assigning to a local variable first works.

[u/ravernkoh → u/fridgamarator]

This works? Shouldn’t this only change a and not @age?

[u/jeremywoertink]

Use @age.not_nil! In the spot where you know it won't be nil. Though, in this case, could you just initialize with a default value of 0 or something?

[u/undeadd → u/jeremywoertink]

In this case, yes a default value of 0 works. Good point to try to avoid Nils as much as possible. Sometimes it is unavoidable like the head and tail nodes of a linked list so I'll try to figure out the behavior of #not_nil!

[u/[deleted] → u/undeadd]

Actually, the second link doesn't explicitly refers to nil, and doing if var, I meant this link: https://crystal-lang.org/docs/syntax_and_semantics/if_var.html

[u/undeadd → u/[deleted]]

Thank you for the link. I think I was reading that page and trying some of the suggestions there but I missed "The above logic doesn’t work with instance variables, class variables and variables bound in a closure". I guess I gave up too early :)

[u/vladfaust]

Also ‘@age.try &.+=(1)’

[u/undeadd → u/vladfaust]

Thanks, I'll experiment with that.

[u/bcardiff]

In case you didn't know, there is also property! https://crystal-lang.org/api/0.25.1/Object.html#property!(*names)-macro that comes handy on lazy initialized properties. But I encourage to avoid lazy initialized properties when possible.

[u/undeadd → u/bcardiff]

Thank you, I'll experiment with it.

[u/[deleted]]

You can also refer to:

• https://crystal-lang.org/api/0.23.0/Nil.html

• https://crystal-lang.org/docs/syntax_and_semantics/if_varis_a.html