NLOOVM

Just a sort of random thought; I think it’s interesting to see the number of efforts to build new languages on old vm’s (NLOOVM).  For example:

JVM:

  • Clojure
  • Scala
  • Kotlin

BEAM:

  • Elixir
  • Lisp Flavor Erlang (LFE)
  • Alpaca

These new languages are getting lots of traction.  Likewise it’s an interesting (and somewhat puzzling) situation with the CLR.  Pretty much anyone working on the CLR is working on C#.  Oh there are other languages on the CLR but they have nowhere near the level of interest or penetration of either Clojure or Elixir respectively.

Some Updates On “LDAP Authentication With Phoenix”

There’s an excellent write-up on authenticating users against an LDAP database with Phoenix which was created by Richard Nyström.  For some reason even though it’s barely a year old, there seem to be a few points that are either out of date or simply incorrect. I worked through his example and I wanted to share a few amendments to his original code in the interests of other developers who may want to use LDAP with Phoenix.

First the set up of Phoenix


mix phx.new ldap_example # was mix phoenix.new ldap_example
...
Fetch and install dependencies? [Yn] Y

cd ldap_example

mix ecto.create (configure your db in config/dev.exs if needed)

mix phx.gen.schema User users username:string name:string email:string
# was
# mix phoenix.gen.model User users username:string name:string email:string

mix ecto.migrate

Next change is in the session_controller.ex file:


#was LdapExample.SessionController

defmodule LdapExampleWeb.SessionController do

#was LdapExample.Web, :controller

use LdapExampleWeb, :controller
alias LdapExample.{User, Repo, Ldap}

def new(conn, _params) do
render conn, "new.html", changeset: User.login_changeset
end

def create(conn, %{"user" => params}) do
username = params["username"]
password = params["password"]
case Ldap.authenticate(username, password) do
:ok -> handle_sign_in(conn, username)
_ -> handle_error(conn)
end
end

defp handle_sign_in(conn, username) do
{:ok, user} = insert_or_update_user(username)
conn
|> put_flash(:info, "Logged in.")
|> Guardian.Plug.sign_in(user)
|> redirect(to: page_path(conn, :index))
end

defp insert_or_update_user(username) do
{:ok, ldap_entry} = Ldap.get_by_uid(username)
user_attributes = Ldap.to_map(ldap_entry)
user = Repo.get_by(User, username: username)
changeset =
case user do
nil -> User.changeset(%User{}, user_attributes)
_ -> User.changeset(user, user_attributes)
end
Repo.insert_or_update changeset
end

defp handle_error(conn) do
conn
|> put_flash(:error, "Wrong username or password")
|> redirect(to: page_path(conn, :new))
end

def delete(conn, _params) do
Guardian.Plug.sign_out(conn)
|> put_flash(:info, "Logged out successfully.")
|> redirect(to: "/")
end
end

The changes to user.ex are not made in the web/model/user.ex; rather they’re made in the existing user.ex file.

And finally a change to session_view.ex

#was LdapExample.SessionView

defmodule LdapExampleWeb.SessionView do

use LdapExampleWeb, :view

end

Quick Elixir Debugging Tip

HT: Dmitri Skliarov for sharing this tip with me.

You can quickly see the call stack being invoked when you call a given Elixir function via a quick and simple call out to the Erlang DBG module. The following tip is a very simple example of how to use this; there’s quite a bit more that can be done with DBG but you need to look at the module docs to see them.

First a little code that we want to trace:


defmodule Trace do

def add(n, m), do: n + m

end

Next you will want to initialize the debugger:


:dbg.tracer()

# => {:ok, #PID<0.66.0>}

NB: you are extremely unlikely to get the same PID I’ve shown here. Next we need to tell the debugger which item or items we want to trace:


:dbg.p(:all,:c)

# => {:ok, [{:matched, :nonode@nohost, 45}]}

Next we tell it precisely what we want to track:


:dbg.tpl(Trace,:add,:x)

# => {:ok, [{:matched, :nonode@nohost, 1}, {:saved, :x}]}

NB: the first parameter to tpl is actually an atom but since a Module name is automatically an atom it’s fine to pass the module name without a colon in front of it. Also note that you can pass the special atom :_ as the second parameter to trace all calls to all functions.

And finally we actually trace our code:


Trace.add(1,1)

# => (<0.57.0>) call 'Elixir.Trace':add(1,1)
# => (<0.57.0>) returned from 'Elixir.Trace':add/2 -> 2
# => 2

And for reference when we use :dbg.tpl(Trace,:_,:x):


Trace.add(1,1)
(<0.57.0>) call 'Elixir.Trace':'__info__'(macros)
(<0.57.0>) returned from 'Elixir.Trace':'__info__'/1 -> []
(<0.57.0>) call 'Elixir.Trace':add(1,1)
(<0.57.0>) returned from 'Elixir.Trace':add/2 -> 2
2

 

And finally when you’re done tracing:


:dbg.stop_clear()

There’s a lot of power using this technique so I’d advise that you take some time to look at the DBG man page before you start using this extensively.  Also, it’s my understanding that you should always be careful to use this technique only in development because if you trace the wrong thing you can cause deadlocks.

 

 

 

 

Quick Elixir Tip

I use Samuel Tonini’s Alchemist (and Elixir mode) to make myself much more productive with Elixir and Emacs.  But one thing that I’ve found to be a minor annoyance is that whenever I start up Emacs I have to set the scratch buffer to be in Elixir mode and I then need to change the comment characters.

Thanks to this excellent tip I no longer have to do this.  I’ve modified my ~/.emacs to add these lines to the bottom:


(setq initial-major-mode 'elixir-mode)

(setq initial-scratch-message "\
# This buffer is for notes you don't want to save, and for Elixir code.
# If you want to create a file, visit that file with C-x C-f,
# then enter the text in that file's own buffer.")

And when I get a scratch buffer I’m all good to go with Elixir!

Making Exrm Work On Windows

So as many of you will know I’m very interested in Elixir.  I also want to see it work better on Windows.  There are a few of us that have been working on it.

I’ve also been very interested in how one can deploy an Elixir app on a machine which doesn’t have Erlang and Elixir already installed.  While most of us don’t mind having to have Erlang and Elixir around because we are doing development work, it would be much more secure to only have to deploy the binaries that need to be deployed for an app.

Since I started on Elixir one of the nicest folks in the community (and that’s saying something) has been Paul Schoenfelder. When I was first struggling to learn Elixir, Paul very patiently helped me to get started.  So I became aware of one of the utilities that Paul built: EXRM.  Exrm is a tool to automate the production of a package of the needed Elixir and Erlang binaries to deploy an app. It allows a developer to deploy his or her Elixir app with only the needed runtime files so that the target machine doesn’t have to have Elixir or Erlang installed.

So I asked Paul what I could do to sort of return the favor for the help he gave me when I started and he mentioned that he wasn’t as pleased as he might be with the support for Windows in EXRM.  So I started digging into EXRM to see what I might do to get it working on Windows.

A few instructive things I spotted right away. I noticed that the way the Windows batch files were constructed they will not work correctly if they’re not run from the correct directory. I mention this for others who may try to use EXRM on Windows.

Spaces In Directory Names

I noted also that there were several places where directories containing spaces and too many characters were also a problem. The original developers of the batch files wrapped double quotes around all the directory names. If you decide to build Windows batch files, please note that wrapping double quotes around directory names is a bit error prone and tough to get right.  Hence I opted to modify all the various places where they had double quoted directory names to using 8.3 versions of the names.  That fixed up all the references to directories so that everything was found correctly.

Getting Services Working

Next there was the question of getting Windows Services working for Erlang. While digging into the batch files I discovered that basically the issue hinged on the installation of the service.  Once the service was installed correctly, it looked as if the rest of the operations on the services (starting, stopping, restarting etc.) would work correctly.

I had to hunt around for a while to figure out what I needed to do to get the service to install correctly.  I finally figured out that what was happening was the wrong parameter was getting passed as the binary to be started when the service gets started.  And so I repaired it.

A Few More Details

You’ll also need to unblock the erl.exe and epmd.exe files for the Windows Firewall.  You can wait for Windows to prompt you or you can proactively unblock them via this escript.

You may also need to deploy msvcr100.dll to your deployment machine as well.  If it’s a Windows 10 machine you may need to do this.

So we’re almost ready to roll the changes into EXRM so we can get people to be able to deploy Elixir apps on Windows.

I will continue to work with Paul on improving support for EXRM on Windows.  Keep watching this space; we’ve got some interesting ideas we’ll be trying to roll out in the next few months or so.

 

 

 

Railway Oriented Programming In Elixir

A while back Scott Wlaschin posted a great blog post on what he called “Railway Oriented Programming.”  It’s a great treatment of a common problem in adopting functional programming; how to handle a chain of functions when one of the functions in the chain may return an error.  Of course stopping the chain when the first error is returned isn’t a difficult issue; the difficulty arises in structuring the code such that the code can continue running all the functions until and unless an error is encountered.  It’s a great read and a powerful idea.

So today someone asked a question on StackOverflow about how to accomplish basically that same idea in Elixir.  A little tricky to translate Scott’s ideas to Elixir because in his article his code is specified in F# and he created a special type to capture the success or failure of a function. F# is statically typed; elixir is dynamically typed which is, of course, a substantial difference.

At any rate, I saw the question and didn’t see that Saša Jurić had already posted a great answer.  So I posted my answer and then noticed his.  I mention this because I want to be clear; I wasn’t trying to steal anyone’s thunder or trying to enhance my rep or anything like that.  I just got excited; “hey, I know the answer to that question!” so I just rushed off and hacked together some demonstration code without bothering to look any further.  I am pleased to see that the answer I came up with was pretty much the same as Saša’s; I consider him an extremely smart developer and coming up with an answer pretty similar to his  independently makes me feel a bit more confident of my own skills.

The TL;DR version is that you need to construct the functions to be applied in such a way that you can carry forward an indication of either success or failure along with the value to be checked.

defmodule Password do

  defp password_long_enough?({:ok = _atom, p}) do
    if(String.length(p) > 6) do
      {:ok, p}
    else
      {:error,p}
    end
  end

  defp starts_with_letter?({:ok = _atom, p}) do
   if(String.printable?(String.first(p))) do
     {:ok, p}
   else
     {:error,p}
   end      
  end


  def password_valid?(p) do
    {:ok, _} = password_long_enough?({:ok,p}) 
    |> starts_with_letter?
  end

end

And one would use it like so:

iex(7)> Password.password_valid?("ties")
** (FunctionClauseError) no function clause matching in Password.starts_with_letter?/1
    so_test.exs:11: Password.starts_with_letter?({:error, "ties"})
    so_test.exs:21: Password.password_valid?/1
iex(7)> Password.password_valid?("tiesandsixletters")
{:ok, "tiesandsixletters"}
iex(8)> Password.password_valid?("\x{0000}tiesandsixletters")
** (MatchError) no match of right hand side value: {:error, <<0, 116, 105, 101, 115, 97, 110, 100, 115, 105, 120, 108, 101, 116, 116, 101, 114, 115>>}
 so_test.exs:21: Password.password_valid?/1
iex(8)>

A couple of comments on the code.  Someone might ask why password_long_enough?  gets a parameter of a tuple of atom and string. Since it’s the first function in the chain it’s not really needed; all it needs is the string.  I constructed in this way for a few reasons:

  1. It’s more parallel to the other function
  2. The functions are really intended to be rules which should be checked so there should not be an order dependency. That is, they should be interchangeable.

Basically by adding any predicate function that we wish given that it takes a tuple of an atom and a string and the atom is matched against :ok, we can add any arbitrary set of tests we wish to add.

As with most things on this blog, I write this down as much for my own reference as for the edification of others who might read it.  I’m pretty sure at some point in the future, I will need to validate a value through a set of predicate functions and I’ll look at this blog post to see how I can do it.


EDIT: I was reminded of a great blog post written up by Zohaib Rauf on precisely this same idea.  Well worth reading.