Using of bracket syntax in Elixir record

Question

I'm learning the Elixir programming with elixir lang getting started , and I'm stacked of the record brace syntax.

This is the sample:

defrecord FileInfo, atime: nil, accesses: 0
defmodule FileAccess do
  def was_accessed?(FileInfo[accesses: 0]), do: false
  def was_accessed?(FileInfo[]),            do: true
end

While the author consider Elixir expands the record to a tuple at compilation time. so

def was_accessed?(FileInfo[accesses: 0]), do: false

is the same as:

def was_accessed?({ FileInfo, _, 0 }), do: false

But when I type in the Elixir shell:

iex(13)> FileInfo[access: 0] == {FileInfo, nil, 0}
true
iex(14)> FileInfo[access: 0] == {FileInfo, 0, 2}
false

The result turned out FileInfo[access: 0] only equals to {FileInfo, nil, 0},

not { FileInfo, _, 0 }.

What's the difference of these two scenes?

Answer 1

Very good question!

The difference is the context. In the first example, Elixir knows FileInfo[] is being invoked inside a function signature (in particular, inside a match context), and therefore, instead of using the default values, it expands all non-given values to _.

In the other example, we are not in a match context, it is just the regular context. In this context, _ is not even valid:

iex> _
** (CompileError) iex:1: unbound variable _

In this case, Elixir expands the non-given values to the default ones.