Elixir macros, quoting bitstring patternmatch types

Question

I am working on a project in which I am likely to be writing a lot of code of the form:

defmodule Kind
  defstruct [name1, name2, name3]
  @type t :: %Kind{name1: integer(), name2: integer(), name3: binary()}
  def unpack(input) do
    with <<name1::integer-8>> <- Enum.take(input, 1),
      <<name2::integer-little-32>> <- Enum.take(input, 4),
      <<name3::binary-10>> <- Enum.take(input, 10),
    do: %Kind{name1: name1, name2: name2, name3: name3>>
  end
end

(for arbitrary sets of input names and types, input being a binary stream producing one byte at a time)

It would be very useful to be able to handle this in a macro, so that I could simply write (for example) use Unpack quote([{name1, integer-8}, {name2, integer-little-32}, {name3, binary-10}]) and automatically generate the necessary struct, typedef, and unpacking function, for arbitrary named fields of fixed sizes. Could even expand it, add a third field in the tuples to pass a function to handle variably-sized types. Unfortunately, when I try to do a simpler version of that (only taking one sized field, and only matching 1 to it):

defmodule Unpack do
  defmacro testmacro({name, kind}) do
    quote do
      <<unquote(name)::unqote(kind)>> = 1
    end
  end
end

The system tells me it's got invalid arguments for quote/1. I assume this is because the "types" used in bitstring pattern-matching are a special form, as are bitstring literals in general, and those particular items are not used anywhere else.

So, how do I get around that? I've got more than a dozen kinds of packed struct to unpack, each with between five and twenty different fields. If I don't do this I'll probably resort to Vim macros to at least save my hands... but that won't really help with having large amounts of extremely repetitive code to maintain.

Can you post the complete code that gives that error? The second snippet works for me if I fix the typo (unqote) and use a binary on the RHS (you can't match that pattern with an integer). — Dogbert, Dec 13 '17 at 02:47
...No, that code was copypasted directly from the iex session I tested it in. Apparently, I missed a typo. Well, that's rather embarrassing. Deleting. — Vivian, Dec 13 '17 at 03:06
@Dogbert It occurs to me that even if the issue was small, you still answered it. Go ahead, put it in as a response and I'll approve. — Vivian, Dec 13 '17 at 15:10

score 1 · Accepted Answer · answered Dec 15 '17 at 11:09

Two things: you have a typo in unquote and the RHS must be a binary so that the pattern matches. With those changes, your code works for me:

defmodule Unpack do
  defmacro unpack({name, kind}) do
    quote do
      <<unquote(name)::unquote(kind)>> = "a"
    end
  end
end

defmodule Main do
  import Unpack

  def main do
    unpack({foo, integer-8})
    IO.inspect foo
  end
end

Main.main

Output:

Elixir macros, quoting bitstring patternmatch types

1 Answers1