How to dynamically create modules with functions - macros

Question

On compilation stage I can easily produce functions with:
defmodule A1 do
defmodule A2 do
Enum.each %{m: 42}, fn {k, v} ->
def unquote(k)(), do: unquote(v)
end
end
end
IO.puts A1.A2.m
#⇒ 42
Also, I can produce modules with functions from within a function call:
defmodule B1 do
def b2! do
defmodule B2 do
# enum is for the sake of future example
Enum.each %{m1: 42}, fn {_k, v} ->
# def b2(), do: unquote(v) WON’T WORK (WHY?), BUT
#v v
def b2(), do: #v
end
end
end
end
B1.b2! # produce a nested module
IO.puts B1.B2.b2 # call a method
#⇒ 42
Now my question is: how can I dynamically produce a module with dynamically created function names, e. g.:
defmodule B1 do
def b2! do
defmodule B2 do
Enum.each %{m1: 42, m2: 3.14}, fn {k, v} ->
#k k
#v v
def unquote(#k)(), do: #v # THIS DOESN’T WORK
end
end
end
end
NB I was able to achieve what I wanted with
defmodule B1 do
def b2! do
defmodule B2 do
Enum.each %{m1: 42, m2: 3.14}, fn {k, v} ->
ast = quote do: def unquote(k)(), do: unquote(v)
Code.eval_quoted(ast, [k: k, v: v], __ENV__)
end
end
end
end
but it seems to be quite hacky.

Answer 1

I believe this happens due to nested macro invocations (def and defmodule are both macros). If you place an unquote there, it unquotes from the top level def:
defmodule B1 do
k = :foo
v = :bar
def b2! do
defmodule B2 do
def unquote(k)(), do: unquote(v)
end
end
end
B1.b2!
IO.inspect B1.B2.foo
prints
:bar
The Module.create/3 recommends using that function to dynamically create modules when the body is an AST. With that, the code becomes much more elegant than the hacky solution using Code.eval_quoted/3:
defmodule B1 do
def b2! do
ast = for {k, v} <- %{m1: 42, m2: 3.14} do
quote do
def unquote(k)(), do: unquote(v)
end
end
Module.create(B1.B2, ast, Macro.Env.location(__ENV__))
end
end
B1.b2!
IO.inspect B1.B2.m1
IO.inspect B1.B2.m2
Output:
42
3.14