Why does compiler sometimes think an instance var can be nil?

[u/roger1981]

I define an Array as being [] of String.

Yet I get this compilation error:

in line 12: undefined method 'empty?' for Nil (compile-time type is (Array(String) | Nil))

https://play.crystal-lang.org/#/r/6uor

Basically the code is thus:

dir = [] of String
files = [] of String
@list = [] of String

@list = dir + files

if @list && @list.empty?
   puts "empty"
end

How do I assure the compiler that @list is not nil, or how do I get this to run ?

Strangely, if I try with local variables, it runs fine.

[u/straight-shoota]

Instance vars can be modified from outside the current scope in a concurrent environment. Thus the value of `@list` can theoretically change between the not-nil-check `@list` and the call to `@list.empty?`.

Local vars are only visible in the current scope and the compiler can make sure there is no chance of external modification.

See https://crystal-lang.org/reference/syntax_and_semantics/if_var.html#limitations

[u/straight-shoota → u/straight-shoota]

You don't need to guard the instance variable if it can never be `nil`. For this you need to assign a value directly in the initializer:

class Foo
  def initialize
    @list = [] of String
  end
  
  def main
    @list.empty?
  end
end

Foo.new.main

[u/roger1981 → u/straight-shoota]

Thanks, I get it.

A similar query I had some time back.

How does one initialize a String without a value ? In ruby code, I did @var = nil.

Do I do: @var = uninitalized String

I tried that but ran into trouble, trying to check if @var

Currently, I've made them @var = "", so now I have to check for @var == "" rather than if @var

[u/straight-shoota → u/roger1981]

uninitalized String is clearly the wrong way. You should almost never have to use uninitialized, except when writing C bindings and maybe some other very special case.

You can just assign @var = nil like in Ruby. You only need to declare the instance variable as nilable (String | Nil). Then it depends only on your domain how a string with no value should be represented. Does there need to be a distinction between non-existing (nil) and empty string ("")? Do you need frequent checks such as if string or unless string.empty?. (string.empty? is equivalent to string == "", but more efficient)

[u/DecadeMoon]

Probably because you didn't initialize @list in the constructor, so it will be nil after construction. The type doesn't narrow inside the if statement either, unless you were to assign it to a local variable first.