(["a", "b"] -- ["a"]) ++ ["a"] vs ["a", "b"] -- ["a"] ++ ["a"]

iex(71)> (["a", "b"] -- ["a"]) ++ ["a"]
["b", "a"]
iex(72)> ["a", "b"] -- ["a"] ++ ["a"]

Hi team,

Why having the () makes the difference?
Is there a term for this behavior?


This is called operator precedence and associativity. You can take a look at the precedence table in the documentation.

The precedence of both operators (-- and ++) is the same, so they wil be applied in order of their associativity which is “right to left”.

So a -- b ++ c is equivalent to a -- (b ++ c).


Thanks! That makes sense now! Do you know why – and ++ are being applied from right to left instead of left to right?

Operator precedence is defined at: https://hexdocs.pm/elixir/master/operators.html

Thanks fmcgeough. I do see the table. I guess my question is what’s the reason behind having the associativity for ++ and – to be right to left instead of left to right.

ah. gotcha. Its a good question. I know the Erlang precedence is the same. But as for why… sorry. I’m not sure why that choice was made.

I see. It makes sense that elixir gets it from Erlang. Thanks! :slight_smile: