I have been ploughing through Chris McCords’ Metaprogramming Elixir book. In one of the chapters, he talks about how Macros are key in extending Elixir. An example he gave was that of a parallel comprehension. Also, if you have no idea what Macros are, you should do either or both of this things:

Buy Metaprogramming Elixir. Read Saša Jurić’s outstanding series on Macros. Here’s Part 1. Throw caution to the wind and try out the example anyway.

Let’s jump straight in! Here’s a normal comprehension:

for i <- 1..10, do: i * i

And here’s how a parallel one could look like:

para(for i <- 1..10, do: i * i)

In this case, para is a Macro that runs the body of the comprehension ( i * i ) in ten proceses, and returns the result.

I don’t recall the book containing the actual implementation, but that’s great! This post will cover how I implemented this from scratch. Also, I have one week of Macro experience at the time of this post. What could go wrong?

Look Ma, No Macros!

Before we go metaprogramming crazy, let’s consider how a non-Macro solution would look like. Here’s a sketch:

me = self pids = for i <- 1 .. 10 do spawn ( fn -> send ( me , { self , i * i }) end ) end pids |> Enum . map ( fn pid -> receive do { ^ pid , result } -> result end end )

If this reminds you of parallel map, you’re absolutely right! The only difference here is that the comprehension allows you to have generators and filters so that you can tailor the generated values to your needs. For those who have not seen the code before, let’s unpack the code, because I think it’s pretty cool.

First, we save a reference to the pid of the current process in me .

me = self

Why? Because the value of self changes, like when you are inside a spawn for example. We are saving the pid of the current process so that we can send messages to it. In fact, this is the next thing we do:

pids = for i <- 1 .. 10 do spawn ( fn -> send ( me , { self , i * i }) end ) end

We execute the comprehension here, but instead of merely executing i * i , we wrap the computation in a function and pass it to spawn . We are effectively running the computation (again, i * i ) in a separate process. Of course, we are not simply running the computation in a separate process. If you look closely, we are sending a message of {self, i * i} to me . Therefore, once all the computation is done, me will receive ten messages in its mailbox.

It’s important to realize that the messages can come out of order. Imagine that the first process takes 10 seconds to complete while the rest takes less than a second. This means that the first process would finish its task last, which also means that the message would be seen last. We definitely want to preserve the order of the results coming back. That’s the purpose of the final piece of code:

pids |> Enum . map ( fn pid -> receive do { ^ pid , result } -> result end end )

We know that the pids contain an ordered list of pids created. When map ped with a receive block and matching on the pid , we can be sure that we are matching the pid with the return value. Notice that the ^ in ^pid is super important here, otherwise the pid in fn pid -> ... end and {pid, result} wouldn’t be matched.

So now we have a plan of attack. We have a basic idea of how to implement parallel comprehension using processes. Time to metaprogram this thing.

Macros, Macros Everywhere…

Let’s start with a new module and Macro definition:

defmodule MacroPlayground do defmacro para ( ast ) do me = self end end

I’ve named the input argument ast , short of Abstract Syntax Tree. This is a good reminder that Macros take in ASTs and spit out ASTs. What we do inside of the Macro is the fun stuff.

Here’s the plan. Given a comprehension such as for i <- 1..10, do: i * i , extract the do part ( i * i ) and turn it into this:

spawn ( fn -> send ( me , { self , i * i }) end )

Then, we will add the rest of the pieces such as the code for map ping pids against the receive block.

When working with ASTs, it’s important to understand how the AST is being represented. For example, you can wrap an expression around a quote block:

quote do for ( i <- 1 .. 10 , do : i * i ) end

This evaluates to:

{ :for , [], [{ :< - , [], [{ :i , [], Elixir }, {: .. , [ context: Elixir , import : Kernel ], [ 1 , 10 ]}]}, [ do : {: * , [ context: Elixir , import : Kernel ], [{ :i , [], Elixir }, { :i , [], Elixir }]}]]}

We need to replace the do block with our “spawn” version. How do we do that? Macro.prewalk/2 to the rescue! There’s Macro.postwalk/2 too. So what’s the difference? Observe:

First we’ll create the AST:

iex > ast = quote do ...> for ( i <- 1 .. 10 , do : i * i ) ...> end

Prewalk:

iex > Macro . prewalk ( ast , fn node -> IO . puts "-> #{ inspect node } " end )

This returns:

-> { :for , [], [{ :< - , [], [{ :i , [], Elixir }, {: .. , [ context: Elixir , import : Kernel ], [ 1 , 10 ]}]}, [ do : {: * , [ context: Elixir , import : Kernel ], [{ :i , [], Elixir }, { :i , [], Elixir }]}]]}

What about Postwalk?

iex ( 20 ) > Macro . postwalk ( ast , fn node -> IO . puts "-> #{ inspect node } " end ) Macro . postwalk ( ast , fn node -> IO . puts "-> #{ inspect node } " end )

This time, the result is different:

-> { :i , [], Elixir } -> 1 -> 10 -> {: .. , [ context: Elixir , import : Kernel ], [ :ok , :ok ]} -> { :< - , [], [ :ok , :ok ]} -> :do -> { :i , [], Elixir } -> { :i , [], Elixir } -> {: * , [ context: Elixir , import : Kernel ], [ :ok , :ok ]} -> { :ok , :ok } -> [ :ok ] -> { :for , [], [ :ok , :ok ]}

What Macro.prewalk/2 / Macro.postwalk/2 does is it takes an AST and traverse it. At each node, it applies the given funcion. That is exactly what we need. We will traverse the AST, pick out the node that matches a comprehension, then replace the do block with the “ spawn ” version.

Here’s the full implementation. Scroll down further for the step by step explanations.

defmodule MacroPlayground do defmacro para ( ast ) do # 0. Store the pid of the current process me = self # 6. Save the resulting pids pids = Macro . prewalk ( ast , fn # 1. Pattern matching can be done with function arguments! { :for , meta , args } -> # 2. Extract the do block of the comprehension [ do : do_block ] = args |> List . last # 3. Wrap the do block around a spawn. Send the result to the # current process. spawn_do_block = quote do spawn ( fn -> send ( unquote ( me ), { self , unquote ( do_block )}) end ) end # 4. Swap out the `do_block` (the last element of `args`) # with the `spawn_do_block` { :for , meta , List . replace_at ( args , - 1 , [ do : spawn_do_block ])} # 5. We just output the same node for any other node node -> node end ) # 7. Collect the results quote do unquote ( pids ) |> Enum . map ( fn pid -> receive do { ^ pid , result } -> result end end ) end end end

The function we are passing to Macro.prewalk/2 matches on two cases. The first case is when we encounter a node that is a comprehension, and that happens when there’s a match for {:for, meta, args} (Step 1). Otherwise, we simply emit the same node, leaving it unchanged (Step 5).

Take a look at the AST again:

{ :for , [], [{ :< - , [], [{ :i , [], Elixir }, {: .. , [ context: Elixir , import : Kernel ], [ 1 , 10 ]}]}, [ do : {: * , [ context: Elixir , import : Kernel ], [{ :i , [], Elixir }, { :i , [], Elixir }]}]]}

Here, meta is [] while args is this chunk:

[{ :< - , [], [{ :i , [], Elixir }, {: .. , [ context: Elixir , import : Kernel ], [ 1 , 10 ]}]}, [ do : {: * , [ context: Elixir , import : Kernel ], [{ :i , [], Elixir }, { :i , [], Elixir }]}]]

Our goal is to get to the do bit. Thankfully, args is just a List . And the do bit is the last element (Step 2):

[ do : do_block ] = args |> List . last

Now that we have the AST of the do_block , we can inject this into the spawn (Step 3). We create a new AST with the quote and in it we have the spawn expression:

spawn ( fn -> send ( unquote ( me ), { self , unquote ( do_block )}) end )

Remember, having an expression inside a quote block turns it into an AST. We need to unquote(me) and unquote(do_block) in order to use its value inside the quoted expression. Finally, we savse the resulting AST into spawn_do_block :

spawn_do_block = quote do spawn ( fn -> send ( unquote ( me ), { self , unquote ( do_block )}) end ) end

Now, we can perform the sleight-of-hand. We simply replace the last element of args with [do: spawn_do_block] , effectively transforming the resulting AST of the orignal comprehension (Step 4):

{ :for , meta , List . replace_at ( args , - 1 , [ do : spawn_do_block ])}

Recall that when the comprehension executes with spawn s, the result is going to be a list of pids. As with the non-Macro version, the result of the comprehension is saved in the pids (Step 6).

Finally, we need to collect the results by matching pids.

quote do unquote ( pids ) |> Enum . map ( fn pid -> receive do { ^ pid , result } -> result end end ) end

Again, since we need to be dealing with ASTs, we need to wrap

pids |> Enum . map ( fn pid -> receive do { ^ pid , result } -> result end end

in a quote block. However, as with me and do_block , pids has to be unquoted in order for it to be used in a quoted expression.

Taking it for a spin

defmodule Foo do import MacroPlayground def foo do x = para ( for a <- 1 .. 10 , b <- 1 .. 10 , c <- 1 .. 10 , a + b + c <= 10000 , do : { a , b , c }) IO . inspect x end end Foo . foo

This will give:

[{1, 1, 1}, {1, 1, 2}, {1, 1, 3}, {1, 2, 1}, {1, 2, 2}, {1, 3, 1}, {2, 1, 1}, {2, 1, 2}, {2, 2, 1}, {3, 1, 1}]

For good measure, you can inspect the pid when the results are received:

receive do { ^ pid , result } -> IO . inspect pid # <-- add this line result end

You should be able to see this:

#PID<0.2170.0> #PID<0.2171.0> #PID<0.2172.0> #PID<0.2173.0> #PID<0.2174.0> #PID<0.2175.0> #PID<0.2176.0> #PID<0.2177.0> #PID<0.2178.0> #PID<0.2179.0> [{ 1 , 1 , 1 }, { 1 , 1 , 2 }, { 1 , 1 , 3 }, { 1 , 2 , 1 }, { 1 , 2 , 2 }, { 1 , 3 , 1 }, { 2 , 1 , 1 }, { 2 , 1 , 2 }, { 2 , 2 , 1 }, { 3 , 1 , 1 }]

Great Success! Hopefully this was educational. I took a while to figure this out, so don’t feel bad if you don’t get it the first time round. If you spot any mistakes or have suggestions for improvements, please do so in the comments. Thanks for reading!