Mildly confusing `with` and `else` clause error

[lawik]

Elixir and Erlang/OTP versions

Elixir 1.18 and main (around 1.19)
OTP 27

Operating system

Linux

Current behavior

This code:

with [] <- System.ls!() do
  :ok
else
    :error
end

Causes this error:

    error: expected -> clauses for :else in "with"
    │
  1 │ with [] <- System.ls!() do
    │ ^
    │
    └─ bla.exs:1:1

** (CompileError) bla.exs: cannot compile file (errors have been logged)

I was quite tripped up for a moment looking at where the arrow is pointing at the start of with which is not the problem.

I took a brief look at it but fell down a dark hole and couldn't crawl out before my discretionary time ran out. So I figured I'd report it :)

Expected behavior

Ideally it would point at the else clause or at the bad value inside of it.

[josevalim]

Unfortunately we cannot provide precise location here because of the AST structure but we can provide a better error message. Something like:

you have specified "else" block in "with" but instead you must specify "pattern -> expr" clauses

[lawik]

Ah, that figures then. The information is good but it didn't make me snap to the solution. I think your suggested change makes sense.

Is the expected -> clauses for %Key in %Kind used for more structures than with? It seemed fairly generic in how it was written but I don't know if the error is actually only for with. I could PR the error if I get a chance.

[josevalim]

I will push something soon, no worries.