elixir - 在 Elixir 中，为什么我不能在创建 map 时使用不同的符号？

Question

定义 map 有两种不同的语法:

map = %{:a => 1, :b => 2}
#=> %{a: 1, b: 2}
map = %{a: 1, b: 2}   
#=> %{a: 1, b: '2}

在定义 map 时按如下方式使用两者:

map = %{:a => 1, b: 2}
#=> %{a: 1, b: 2}

但是以其他顺序使用会引发错误:

map = %{a: 1, :b => 2}
#=> ** (SyntaxError) iex:37: syntax error before: b

为什么？
编辑
操作系统: Ubuntu 15.4
药剂:1.1.1

Answer 1

根据 my issue on Github (我实际上不应该打开)，这不是错误。

第一个答案(我没有真正得到):

It's not a bug, it's the same syntax sugar that is used for keywords on the last argument of a function.

foo(bar, baz: 0, boz: 1) #=> foo(bar, [baz: 0, boz: 1])

The map syntax is represented as function call in the AST:

iex(1)> quote do: foo(bar, baz: 0, boz: 1)
{:foo, [], [{:bar, [], Elixir}, [baz: 0, boz: 1]]}
iex(2)> quote do: %{baz: 0, boz: 1}
{:%{}, [], [baz: 0, boz: 1]}

That's why the map keyword syntax only works for the last (or only) argument.

第二个答案，从某种意义上来说听起来不错，我想我明白了:

Simple answer: b: 2 is syntax sugar for [b: 2], but the sugar only works when it is at the end of a function call or "construct" such as %{}.