sodapopcan

sodapopcan

Can someone explain `do` syntax in a non hand-holding way?

I’m wondering exactly what a do block is.

Is it a closure? Is it a procedure? Is it an anonymous function? Is it a lambda? Is it something else?

Is it more useful to think of an Elixir do block as a Ruby block or as Haskell’s do notation or maybe a combination of the two? Or neither!!? Maybe even a JS closure?

Is it some mutant variation of all of that?? Or maybe none of that!???

I do (haha, get it?) have some intuition about this and have read the meta-programming part of the Elixir guides (which calls it an expression but, like, it’s really a series of expressions… no?) and I’m just looking for some insightful—or even just fun—responses from people in the community, if anyone is willing.

What I already think I know:

  • do is syntactic sugar for calling the, uh, “block of code” provided to the :do key of a keyword list arg provided to a macro
  • That’s it!

Thank you!!

Most Liked

josevalim

josevalim

Creator of Elixir

In a nutshell, do-end blocks is a syntax sugar for a :do keyword list.

if CONDITION do
  EXPR
end

is the same as:

if CONDITION, do: (EXPR)

You can see this by calling quote as shown by @ityonemo. Since keyword lists are just data structures, do-blocks end-up being just data structures too. It doesn’t need to be given only to macros, regular functions can also use and match on it, some helpers in Phoenix.HTML do so, but their usage is indeed more common with macros.

As a syntax sugar, they also allow some specific syntactical constructs, such as left -> right and other keywords (else, rescue, etc), but all of them can be written in the literal keyword syntax format too.

ityonemo

ityonemo

haha maybe I think I should break this down more carefully. A macro in elixir is a function that emits AST + “take the AST generated, right here in the code” (a bare function will drop its value, in that location, not the AST). So macros in the first are a function. Let’s consider the “def” macro.

We’ll look at it three ways:

quote do
  def foo, do: :bar 
end

which yields:

{:def, [context: Elixir, import: Kernel],
 [{:foo, [context: Elixir], Elixir}, [do: :bar]]}

and

quote do
  def foo do
     :bar
  end
end

which yields

{:def, [context: Elixir, import: Kernel],
 [{:foo, [context: Elixir], Elixir}, [do: :bar]]}

and

quote do
  def foo do
     :throwaway_statement
     :bar
  end
end 

which yields

{:def, [context: Elixir, import: Kernel],
 [
   {:foo, [context: Elixir], Elixir},
   [do: {:__block__, [], [:throwaway_statement, :bar]}]
 ]}

Note a) forms 1 and 2 are identical
and b) form 3 groups the two lines of the function into a block.

Why is that necessary? Because according to Kernel.def/2 documentation (Kernel — Elixir v1.20.2), the function half of the def macro must take TWO AST parameters. The first parameter is the name of the function (and also a list of arguments) and the second parameter is the contents of the function. So the only arity-consistent way of representing def is by grouping the AST for the body of the function into a block and labeling it as “do”. So the do “keyword” is just elixir’s cute way of saying "hey everything until the “end” keyword winds up in the block. And if you don’t want a block, you can just pass it as do:, which builds the keyword list manually and passes it into the def function without syntactic sugar.

One more important thing to note is that in elixir, lists, atoms, and two-tuples don’t need escaping, because they are identical to their AST, so that’s how when you do a one-liner the do: part of the code makes it into the macro without being converted into these strange tuple forms.

as for why it’s a keyword list, if you look further down the documentation, you can have other parts to the function code, for example catch or rescue blocks; if you do that, for example this code which is nonsensical:

quote do
  def foo do
     :bar
   catch
     :baz
   end
end

yields:

{:def, [context: Elixir, import: Kernel],
 [{:foo, [context: Elixir], Elixir}, [do: :bar, catch: :baz]]}

so other “segments” of the function can nicely wind up stuffed into an arity-2 statement. Likewise you can arbitrarily mix-and-match catches and rescues, because a keyword-list is what you are supposed to use when you have a maybe-ordered list of arbitrary things that can’t be a map because you can maybe have duplicates.

ityonemo

ityonemo

Just tried it. Mind blown. I mean, of course, what was I thinking, why wouldn’t that work??!. I promise not to use this power for evil.

Where Next?

Popular in Questions Top

beno
I will often find my self writing things similar to: case some_value do nil -> something() "" -> something() _ -> somethi...
New
lessless
I believe there are people here who are dealing with CSV files import on the daily basis, and since Excel is a really popular tool there ...
New
minhajuddin
I have seen a lot of code which picks the first element from a list using Enum.at(0) instead of List.first. Is there a reason why people ...
New
shahryarjb
Hello, I get Persian date from my client and convert it to normal calendar like this: def jalali_string_to_miladi_english_number(persi...
New
johnnyicon
Hi all, I’ve just started learning Elixir and Phoenix Framework, so please pardon my n00bness at this stage. I’m trying to use Postgres...
New
vrod
I am using the Starship cross-shell prompt – it seems pretty nice, but I get some errors: [WARN] - (starship::utils): Executing command ...
New
SoCreat
i’m a new one to elixir which editor can i use vs code? or atom? Thanks! :smiley:
New
vonH
When I run the Plug and I recompile I wind up having to use Ctrl C to quit iex and start again. Witht the help of rlwrap I can use the cu...
New
dblack
I’ve got an issue with an app and I’ve no idea of how to troubleshoot it. I’m hoping someone here might have seen something similar. I p...
New
hariharasudhan94
Lets say I have map like this fetching from my database %{"_id" => #BSON.ObjectId<58eb1a7a9ad169198c3dXXXX>, "email" => ...
New

Other popular topics Top

WestKeys
Currently suffering from paralysis by [HTTP client] analysis. This is rather unusual in Elixirland as there tends to be consensus on the ...
New
mcarvalho
What is the difference between System.get_env and Application.get_env? For example, what are best practices to use one versus another.
New
electic
Hi, I am new to Elixir. I am trying to use the DateTime component to insert a date into MySQL however the there seems to be no way to fo...
New
pmjoe
I have a relationship of love and hate with Elixir. Lots of things are just absolutely right, but there are some things that are kind of ...
New
ashish173
I am using Ecto timestamps with postgres, I can see the timestamps() use the :naive_dateime but for my use case I wanted to store the ti...
New
dblack
I’ve got an issue with an app and I’ve no idea of how to troubleshoot it. I’m hoping someone here might have seen something similar. I p...
New
openscript
Hello! Sorry for this astonishing simple question, but I’m really stuck. I try to set up the intellij-elixir plugin, but I don’t know ho...
New
JorisKok
I have a server on AWS, and was running a load test using artillery. When looking at the Phoenix dashboard I see the Ports going to 100% ...
New
vegabook
I’m brand new to Phoenix and I have stripped one of the demo applications to the bone. I just want to get an svg up on the screen. Here i...
New
vonH
In asking this question I am more interested about the expressiveness of the language itself and less concerned about the availability of...
New

Latest on Elixir Forum

Elixir Forum

We're in Beta

About us Mission Statement