crystal-lang - 如何在 "if"语句中使用 union [Crystal]

Question

以下代码运行良好并打印“5.0”

$x : Float64
$y : Float64
$x = 3.0_f64
$y = 2.0_f64
puts $x + $y

现在，我更改代码以支持“nil”。

$x : Float64?
$y : Float64?
$x = 3.0_f64
$y = 2.0_f64
puts $x + $y if !$x.nil? && !$y.nil?

但是此代码报告以下错误消息。

no overload matches 'Float64#+' with type (Float64 | Nil)Overloads are: - Float64#+(other : Int8) - Float64#+(other : Int16) - Float64#+(other : Int32) - Float64#+(other : Int64) - Float64#+(other : UInt8) - Float64#+(other : UInt16) - Float64#+(other : UInt32) - Float64#+(other : UInt64) - Float64#+(other : Float32) - Float64#+(other : Float64) - Number#+()Couldn't find overloads for these types: - Float64#+(Nil)    puts $x + $y if !$x.nil? && !$y.nil?

I would like to stop the call of method "#+()" if $x or $y is niland print the calculated result if both are Float64.

What is the best practice for this situation?

---

In above code, I simplified the code for this question.In the result, the meaning of question was changed involuntarily..I wanted to ask following code actually.

class Xyz
  property a, b
  @a : Float64?
  @b : Float64?

  def initialize
    @a = nil
    @b = nil
  end

  def do_calc
    if !@a.nil? && !@b.nil?
      puts @a + @b
    else
      puts "We can't calculate because '@a or @b has nil."
    end
  end
end

x = Xyz.new
x.a = 3.0_f64
x.b = 2.0_f64
x.do_calc

此代码，报告以下错误。

instantiating 'Xyz#do_calc()'x.do_calc  ^~~~~~~in ./a.cr:15: no overload matches 'Float64#+' with type (Float64 | Nil)Overloads are: - Float64#+(other : Int8) - Float64#+(other : Int16) - Float64#+(other : Int32) - Float64#+(other : Int64) - Float64#+(other : UInt8) - Float64#+(other : UInt16) - Float64#+(other : UInt32) - Float64#+(other : UInt64) - Float64#+(other : Float32) - Float64#+(other : Float64) - Number#+()Couldn't find overloads for these types: - Float64#+(Nil)      puts @a + @b

我怎样才能避免这个错误？

Answer 1

请务必阅读有关 if 的文档，并检查是否为 nil:https://crystal-lang.org/docs/syntax_and_semantics/if_var.html和 https://crystal-lang.org/docs/syntax_and_semantics/if_var_nil.html

这仅适用于局部变量，因此您需要先将值分配给局部变量。

作为旁注，从 Crystal 0.19.0 开始，全局变量不再存在于语言中。