Improve error messages for invalid expression in match

[grych]

Environment

• Elixir & Erlang versions (elixir -v):
  Erlang/OTP 19 [erts-8.2] [source] [64-bit] [smp:8:8] [async-threads:10] [hipe] [kernel-poll:false] [dtrace]
  Elixir 1.4.0
• Operating system:
  OS X El Capitan

Current behavior

iex(1)> f = &List.first/1
&List.first/1
iex(3)> case f do
...(3)>   &List.first/1 -> 11
...(3)> end
** (CompileError) iex:4: invalid expression in match
    (stdlib) lists.erl:1354: :lists.mapfoldl/3

however the other way works as expected:

iex(3)> case &List.first/1 do
...(3)> f -> 22
...(3)> end
22

Expected behavior

As a first-class citizen, function should work in case as it works in other pattern matching cases.

iex(3)> case f do
...(3)>   &List.first/1 -> 11
...(3)> end
11

[josevalim]

It is not about being first-class, the issue is that &List.first/1 is not a valid pattern. There are many other data types that are first-class, such as a PID, that are also not valid patterns.

For example, imagine you want to match exactly on the pid 0.10.0. There is no way you can write such as a pattern. You would first need to build the pid, store it in a variable, and then match on it. That's exactly what you can do with functions:

x = &List.first/1
case &List.first/1 do
  ^x -> true
end

There are also cases like maps, where patterns and values are different. %{} means the empty map as a value but means all maps in a pattern.

I believe there is very little interest in making functions valid patterns in Elixir. So I would recommend going the route of ^x.

[grych]

I see. Actually I found this when I tried to use such syntax, but I agree the case is very rare. Maybe I did not understand what invalid expression in match means.

Thanks for explanation!

[josevalim]

Ok, I will reopen this because we can definitely use a better error message. Plus using the term "expression" in there is not quite correct.