if-statement - 为什么 if(not nil) 给我一个 ArgumentError？

Question

defmodule My do
  def go do
    x = nil

    if(not x) do
      IO.puts "hello"
    else
      IO.puts "goodbye"
    end
  end
end

在 iex 中:

/elixir_programs$ iex c.exs
Erlang/OTP 20 [erts-9.3] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:10] [hipe] [kernel-poll:false]

Interactive Elixir (1.6.6) - press Ctrl+C to exit (type h() ENTER for help)

iex(1)> My.go
** (ArgumentError) argument error
    c.exs:5: My.go/0

iex(1)> 

根据 Programming Elixir >= 1.6，第 35 页:

Elixir has three special values related to Boolean operations: true, false, and nil. nil is treated as false in Boolean contexts.

这似乎不是真的:

defmodule My do
  def go do
    x = false

    if (not x) do
      IO.puts "hello"
    else
      IO.puts "goodbye"
    end
  end
end

在 iex 中:

~/elixir_programs$ iex c.exs
Erlang/OTP 20 [erts-9.3] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:10] [hipe] [kernel-poll:false]

Interactive Elixir (1.6.6) - press Ctrl+C to exit (type h() ENTER for help)

iex(1)> My.go       
hello
:ok

iex(2)> 

Answer 1

  @spec not true :: false
  @spec not false :: true
  def not value do
    :erlang.not(value)
  end

Elixir 对not 函数的最新定义表明它只接收false 和true。

但是，nil不属于他们，所以显示argument error。

Elixir has three special values related to Boolean operations: true, false, and nil. nil is treated as false in Boolean contexts.

nil 只是一个atom，即nil === :nil。

可以考虑使用!操作符，其实就是Kernel.!宏。

Receives any argument (not just booleans) and returns true if the argument is false or nil; returns false otherwise.

!nil 将返回 true。