Why does if(not nil) give me an ArgumentError? - if-statement

Question

defmodule My do
def go do
x = nil
if(not x) do
IO.puts "hello"
else
IO.puts "goodbye"
end
end
end
In iex:
/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)>
According to Programming Elixir >= 1.6, p.35:
Elixir has three special values related to Boolean operations: true,
false, and nil. nil is treated as false in Boolean contexts.
It doesn't seem to be true:
defmodule My do
def go do
x = false
if (not x) do
IO.puts "hello"
else
IO.puts "goodbye"
end
end
end
In iex:
~/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 's latest definition of not function shows that it only receive false and true.
However, nil is not belongs to them, so it shows argument error.
Elixir has three special values related to Boolean operations: true, false, and nil. nil is treated as false in Boolean contexts.
nil is just an atom, that is nil === :nil.
You can consider using ! operator, which in fact is Kernel.! macro.
Receives any argument (not just booleans) and returns true if the
argument is false or nil; returns false otherwise.
!nil will return true.

Answer 2

"Kernel.not/1" or not/1 expects a Boolean value
note: Other values different of nil and false are true
try this example:
x = nil
if (x) do true else false end
false
Examples with short-if condition and true, false, nil values
iex> if nil , do: true, else: false
false
iex> if !nil , do: true, else: false
true
iex> if false , do: true, else: false
false
iex> if !false , do: true, else: false
true
iex> if not false , do: true, else: false
true
iex> if true , do: true, else: false
true
iex> if !true , do: true, else: false
false
iex> if not true , do: true, else: false
false