Weird errors in non-matching clause in anonymous function

Questions / Help
1

If you have the following code:

defmodule Fn do
  def f1(g, x) do
    fun = fn
      # We'll mutate the function patterns
      value when is_integer(value) -> g.(value)
    end

    fun.(x)
  end

  def f2(x) do
    fun = fn
      # We'll mutate the function patterns
      value when is_integer(value) -> value
    end

    fun.(x)
  end
end

then you would have:

iex(49)> Fn.f1(fn x -> x end, :invalid) 
** (CaseClauseError) no case clause matching: {:invalid}
    (darwin 0.1.0) lib/darwin.ex:28: Fn.f1/2
iex(49)> Fn.f2(:invalid)               
** (FunctionClauseError) no function clause matching in Fn.f2/1

    The following arguments were given to Fn.f2/1:

        # 1
        :invalid

    Attempted function clauses (showing 1 out of 1):

        def f2(x)

    (darwin 0.1.0) lib/darwin.ex:37: Fn.f2/1

In one case you get a FunctionClauseError and in another case you get a CaseClauseError. Is this supposed behaviour? I came up with this when testing my mutation testing framework, so it’s definitely a weird corner case, but I wonder if the Elixir core team would like to look into it.

2 Likes
2

Which Elixir version do you use?
I tried to run the code using 1.12 and in both cases I have the error that is similar to Fn.f2(:invalid)

1 Like
3 
$ elixir --version

Erlang/OTP 24 [erts-12.0.2] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [jit]
Elixir 1.12.0 (compiled with Erlang/OTP 24)
4

I think this is because the fun inside f1 is immediately invoked with arguments that are in scope so it is transformed into a case expression by the compiler. I know such optimization exists but I cannot find the article or changelog where it is described.

See Immediately-called funs optimization (looking for docs) .

5

Well, I tested with OTP 23 for the first time. Seems like this was introduced in OTP 24

1 Like
6

@fuelen it seems like it has never worked
Test: Weird errors in non-matching clause in anonymous function · eksperimental/debug_me@2ce3e3b · GitHub?

Sorry, i misread your comment @fuelen
here are the tests that verify it is indeed only in OTP 24
Test exceptions · eksperimental/debug_me@422239a · GitHub

1 Like
7

@tmbb I think you could report your findings to OTP.

Here’s a similar issue that has been reported a few days ago
Inconsistent error messages when errors are optimized away by the compiler · Issue #5440 · erlang/otp · GitHub

The Erlang code:

-module(foo).
-export([f1/2, f2/1]).

f1(G, X) ->
    Fun =
        fun(Value) when is_integer(Value) ->
               G(Value)
        end,
    Fun(X).

f2(X) ->
    Fun =
        fun(Value) when is_integer(Value) ->
               Value
        end,
    Fun(X).

The console outputs:

Erlang/OTP 24 [erts-12.1.2] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [jit]

Eshell V12.1.2  (abort with ^G)
1> c('foo').                        
{ok,foo}

2> foo:f1(invalid, fun(X) -> X end).
** exception error: no case clause matching {#Fun<erl_eval.44.65746770>}
     in function  foo:f1/2 (foo.erl, line 31)

3> foo:f2(invalid).                 
** exception error: no function clause matching foo:f2(invalid) (foo.erl, line 38)


Erlang/OTP 23 [erts-11.2.2.8] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [hipe]
1> c('foo').

2> foo:f1(invalid, fun(X) -> X end).
** exception error: no function clause matching foo:'-f1/2-fun-0-'(#Fun<erl_eval.44.79398840>) (foo.erl, line 31)

3> foo:f2(invalid).                 
** exception error: no function clause matching foo:'-f2/1-fun-0-'(invalid) (foo.erl, line 38)
8

Sorry to ask you here @lud but I cannot see to reply to the linked post. May I know what is decompiler you say you use?

9

It has less features than the popular one as it only decompiles to erlang module. But it writes to stdout instead of creating files :slight_smile:

1 Like
10

Thanks, I’ll translate it into Erlang and report it

1 Like
11

I already translated it

1 Like
12

Hi @tmbb
Did you get the chance to report it?
If not I can do it.