Friday, June 19, 2015

Tilting At Windmills, Accessing Erlang man pages from Iex Part: 1

In the process of learning how to get a cryptographic hash of a file in Elixir, I found myself switching back and forth from iex to erl -man to better understand how
to use the available Erlang functions in Elixir. After all there are over 500 modules in the standard Erlang libraries that are available in Elixir. Wouldn't it be nice I thought to just be able to type

iex> h :crypto.hash 

and get some basic information about the functions. Elixir already supports listing all the functions available in both Elixir and Erlang modules, using the module.TAB sequence

iex> h :crypto.TAB 
If we have Erlang installed, the man pages must be there somewhere. And of course one of the tenants of Iex is that any functionality should be available in the Windows version if at all possible. While creating a command to shell out to erl -man would be simple, it wouldn't be portable.

Erlang installs it's man pages in a separate path to avoid conflicts with the standard unix man pages but puts them in the standard man/man3 locations. The convention is that the man page for an Erlang module is the Erlang module name followed by section number. The standard Erlang functions are documented in the erlang.3 man page.

So my next step was to reimplement erl -man in purely Elixir code. The first part went really quickly, finding the man page for a given module was relatively straight forward. We just find where the erl executable is and work backwards from that to find where the erlang man pages are and then search for the module name.

 def manpath do
    start = System.find_executable("erl")
    case start do
      nil -> nil
      _   -> find_manpath(start) 

  defp find_manpath(erl_path) do
    mpath = erl_path |>
            Path.split |>
            Stream.scan(&Path.join(&2,&1)) |>
            Enum.filter( fn(p)  -> File.exists?(Path.join([p,"man","man3","ets.3"])) end ) |>
    if mpath , do: Path.join(mpath,"man"), else: nil

After that I implemented a very simple-minded man nroff macro to markdown translator and was able to display an Erlang man page from the iex prompt. During this process I noticed that the Erlang man pages use a very consistant, small subset of the man nroff macro package. This lead me to believe that it would be possible to extract the specific function documentation from the man pages.

It turns out that Erlang documentation is maintained in XML and translated by an xmerl based program into both man pages and html documentation. Unfortunately, this XML source is not included in the default binary distributions of Erlang. Unix versions get the man pages ( and html usually ), but the Windows distributions only get the html. Parsing the html is a lot more complicated that dealing with the nroff and there are no HTML parsers in the standard Elixir libs.

At this point, I decided to dig a bit deeper and figure out just how

iex> h Atom.to_char 

really works.
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