String.slice(str, 0, -1) raises FunctionClauseError

[beadsland]

This snippet of Elixir:

String.slice(str, 0, -1)

Started with slice/2 form of this call, but swapped to slice/3 to confirm the error wasn't being generated by the Elixir range.

Raises the following exception

** (EXIT from #PID<0.95.0>) an exception was raised:
** (FunctionClauseError) no function clause matching in ElixirScript.Translate.Form.compile/2
lib/elixir_script/passes/translate/form.ex:28: ElixirScript.Translate.Form.compile(&Inspect.inspect/2, %{anonymous_fn: false, function: {:struct, nil}, in_guard: false, module: Inspect.Opts, pid: #PID<0.200.0>, vars: %{}})
lib/elixir_script/passes/translate/form.ex:16: ElixirScript.Translate.Form.compile!/2
lib/elixir_script/passes/translate/forms/map.ex:37: anonymous fn/2 in ElixirScript.Translate.Forms.Map.compile/2
(elixir 1.10.4) lib/enum.ex:1396: Enum."-map/2-lists^map/1-0-"/2
(elixir 1.10.4) lib/enum.ex:1396: Enum."-map/2-lists^map/1-0-"/2
lib/elixir_script/passes/translate/forms/map.ex:32: ElixirScript.Translate.Forms.Map.compile/2
lib/elixir_script/passes/translate/form.ex:16: ElixirScript.Translate.Form.compile!/2
lib/elixir_script/passes/translate/function.ex:170: ElixirScript.Translate.Function.compile_block/2

Modified Transcript.Form.compile! as follows:

  @spec compile!(any, map) :: ESTree.Node.t()
  def compile!(ast, state) do
    {js_ast, _} = compile(ast, state)
    js_ast
  rescue
    e -> Logger.debug inspect({:compile!, ast, state})
         raise RuntimeError, message: inspect({"compile!/2", e})
  end

Which revealed the following debugging info:

03:45:21.580 [debug] {:compile!, &Inspect.inspect/2, %{anonymous_fn: false, function: {:struct, nil}, in_guard: false, module: Inspect.Opts, pid: #PID<0.200.0>, vars: %{}}}

03:45:21.696 [debug] {:compile!, {:%{}, [line: 74], [struct: Inspect.Opts, base: :decimal, binaries: :infer, char_lists: :infer, charlists: :infer, custom_options: [], inspect_fun: &Inspect.inspect/2, limit: 50, pretty: false, printable_limit: 4096, safe: true, structs: true, syntax_colors: [], width: 80]}, %{anonymous_fn: false, function: {:struct, nil}, in_guard: false, module: Inspect.Opts, pid: #PID<0.200.0>, vars: %{}}}

Thoughts

Not too familiar with this codebase, but I'm working from the premise that inspect_fun: &Inspect.inspect/2 might be malformed, for it to be promoted into the ast parameter of compile!.

[beadsland]

Trying to build out same functionality without library function String.slice/2:

path = to_charlist str

Hit same error condition:

** (EXIT from #PID<0.95.0>) an exception was raised:
** (RuntimeError) {"compile!/2", %RuntimeError{message: "{"compile!/2", %FunctionClauseError{args: nil, arity: 2, clauses: nil, function: :compile, kind: nil, module: ElixirScript.Translate.Form}}"}}
lib/elixir_script/passes/translate/form.ex:19: ElixirScript.Translate.Form.compile!/2
lib/elixir_script/passes/translate/function.ex:170: ElixirScript.Translate.Function.compile_block/2
lib/elixir_script/passes/translate/function.ex:138: ElixirScript.Translate.Function.compile_clause/2
(elixir 1.10.4) lib/enum.ex:1396: Enum."-map/2-lists^map/1-0-"/2
lib/elixir_script/passes/translate/function.ex:100: ElixirScript.Translate.Function.compile_clauses/2
lib/elixir_script/passes/translate/function.ex:60: ElixirScript.Translate.Function.compile/2
(elixir 1.10.4) lib/enum.ex:1520: Enum."-map_reduce/3-lists^mapfoldl/2-0-"/3
lib/elixir_script/passes/translate/module.ex:98: ElixirScript.Translate.Module.compile/3

With similar debug values:

06:15:24.282 [debug] {:compile!, &Inspect.inspect/2, %{anonymous_fn: false, function: {:struct, nil}, in_guard: false, module: Inspect.Opts, pid: #PID<0.166.0>, vars: %{}}}

06:15:24.336 [debug] {:compile!, {:%{}, [line: 74], [struct: Inspect.Opts, base: :decimal, binaries: :infer, char_lists: :infer, charlists: :infer, custom_options: [], inspect_fun: &Inspect.inspect/2, limit: 50, pretty: false, printable_limit: 4096, safe: true, structs: true, syntax_colors: [], width: 80]}, %{anonymous_fn: false, function: {:struct, nil}, in_guard: false, module: Inspect.Opts, pid: #PID<0.166.0>, vars: %{}}}

Breaking it down:

So, reducing it to bare necessary expression to break ElixirScript:

to_charlist "abc"