Ranges do not equal when it seems they should in a guard

[ollien]

Elixir and Erlang/OTP versions

Erlang/OTP 24 [erts-12.3.2.1] [source] [64-bit] [smp:12:12] [ds:12:12:10] [async-threads:1] [jit]

Elixir 1.13.4 (compiled with Erlang/OTP 24)

Operating system

Fedora 36

Current behavior

I wanted to write a function that would, given a range and a value, n, check if the range spans exactly n..n. My first attempt looked something like this, but does not have the expected behavior (note the first return value is false)

defmodule Range do
  def equal?(n, range) when range == n..n, do: true
  def equal?(n, range), do: false
end

IO.puts(Range.equal?(1, 1..1)) # false
IO.puts(Range.equal?(1, 1..5)) # false

While this function can be written by using pattern matching on the range argument (replacing the first equal? declaration with def equal?(n, n..n), do: true), I was surprised this didn't work.

Some on StackOverflow have speculated this is due to the step evaluating to nil in a guard. Indeed, it does work if when range == n..n is replaced with when range == n..n//1

Expected behavior

I would have expected when range == n..n to correctly guard the call for when the range was exactly n..n, especially given this works

iex(1)> range = 1..1
1..1
iex(2)> n = 1
1
iex(3)> range == 1..1
true
iex(4)> match?({n, n..n}, {1, 1..1})
true

[wojtekmach]

Some on StackOverflow have speculated this is due to the step evaluating to nil in a guard

yup:

Mix.install([
  {:decompilerl, github: "aerosol/decompilerl"}
])

defmodule Range do
  def equal?(n, range) when range == n..n, do: true
  def equal?(n, range), do: false
end
|> Decompilerl.decompile(skip_info: true)

-module('Elixir.Range').

-compile([no_auto_import]).

-export(['equal?'/2]).

'equal?'(_n@1, _range@1)
    when _range@1 ==
             #{'__struct__' => 'Elixir.Range', first => _n@1,
               last => _n@1, step => nil} ->
    true;
'equal?'(_n@1, _range@1) -> false.

[josevalim]

It is fixed, tks for the report! However, be careful with the concept of equality with ranges. For example, those ranges are not structurally equal but they do emit the same elements: 1..1/1, 1..1/-1, 1..1/2, etc.