How do these guards work in elixir_quote.erl?

gusaiani · October 7, 2020, 1:20pm

Hi all,

I like to study Elixir’s source and I don’t know a lot of Erlang.

I got curious about these function declarations as I could not understand how a call to this function would match some guards.

We have in line 137:

dot(Meta, Left, Right, nil) when is_atom(Right) ->

then the next declaration in line 145:

dot(Meta, Left, {Right, _, Context}, nil) when is_atom(Right), is_atom(Context) ->

how does it work that the one in line 145 is ever reached? Same for the one in line 148.

Cheers!

Source here:

github.com

elixir-lang/elixir/blob/06f086a21e170a233acf4aaae02f39717d28020d/lib/elixir/src/elixir_quote.erl#L137-L149


dot(Meta, Left, Right, nil) when is_atom(Right) ->
  case atom_to_list(Right) of
    "Elixir." ++ _ ->
      {'__aliases__', Meta, [Left, Right]};
    _ ->
      {{'.', Meta, [Left, Right]}, [{no_parens, true} | Meta], []}
  end;

dot(Meta, Left, {Right, _, Context}, nil) when is_atom(Right), is_atom(Context) ->
  {{'.', Meta, [Left, Right]}, [{no_parens, true} | Meta], []};

dot(Meta, Left, {Right, _, Args}, nil) when is_atom(Right) ->
  {{'.', Meta, [Left, Right]}, Meta, Args};

NobbZ · October 7, 2020, 1:28pm

{Right, _, Context} is not an atom but a tuple, therefore is_atom/1 returns false.

gusaiani · October 8, 2020, 4:22pm

Ah ok! Thanks NobbZ!