Improve error when trying to compile a compiled regex

[skwerlman]

Environment

• Elixir & Erlang/OTP versions (elixir --version):

Erlang/OTP 21 [erts-10.2] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [hipe]

Elixir 1.7.4 (compiled with Erlang/OTP 21)

• Operating system: Arch Linux, kernel 4.19.9.a-1-hardened

Current behavior

iex(1)> Regex.compile!(~r//)
** (FunctionClauseError) no function clause matching in Regex.compile/4    
    
    The following arguments were given to Regex.compile/4:
    
        # 1
        ~r//
    
        # 2
        []
    
        # 3
        ""
    
        # 4
        {"8.42 2018-03-20", :little}
    
    Attempted function clauses (showing 1 out of 1):
    
        defp compile(source, opts, doc_opts, version) when is_binary(source)
     
    (elixir) lib/regex.ex:155: Regex.compile/4

An error is also raised when calling the non-bang version.

I expect others who are relatively new to elixir will run into this as well, since it's not immediately obvious that ~r// compiles the regex on its own.

Expected behavior

A Regex.CompileError is raised instead when calling the bang version, or an error tuple is returned from the non-bang version.

[josevalim]

Thanks @skwerlman!

We can improve the error message for sure but note we should not return an error tuple for the non-bang version. The error tuple is about semantic errors, not mechanical ones, such as bad arguments. For example, File.read(1) raises an error instead of an error tuple, because it is a bad argument. :)

[josevalim]

Btw, my suggestion in this case would be simply to move the is_binary/1 guard up. If we do so, the error message would be better without requiring us to manually check for bad args. It would be this:

iex(1)> Regex.compile!(~r//)
** (FunctionClauseError) no function clause matching in Regex.compile!/1    
    
    The following arguments were given to Regex.compile!/1:
    
        # 1
        ~r//
    
    Attempted function clauses (showing 1 out of 1):
    
        def compile!(source) when is_binary(source)