Elixir - Tips

14 Jul 2017

(how to) convert struct <-> map
(how to) update struct
(how to) remove newline inside or at the end of heredoc
(how to) get current environment at runtime
(how to) convert module name to string without Elixir prefix
(how to) measure function execution time
don’t use require when importing macros

https://github.com/blackode/elixir-tips

(how to) convert struct <-> map

Map.from_struct(%User{id: 1, name: "Alice"})

map to struct:

# ignores keys that don't belong to struct
struct(User, %{id: 1, name: "Alice"})
# raises if key doesn't belong to struct
struct!(User, %{id: 1, name: "Alice", foo: 123})
# ignores keys that don't belong to struct
# (using ExConstructor package)
User.new(%{id: 1, name: "Alice", foo: 123})

(how to) update struct

user = %User{id: 1, name: "Alice"}

# raises if key doesn't belong to struct
user = %{user | foo: "Kate"}
# raises if key doesn't belong to struct
user = struct!(user, %{foo: "Kate"})
# ignores keys that don't belong to struct
user = struct(user, %{foo: "Kate"})
# adds keys that don't belong to struct
user = Map.put(user, :foo, "Kate")
# adds keys that don't belong to struct
user = Map.merge(user, %{foo: "Kate"})

after adding unknown keys to struct using Map module functions struct is no longer recognized as original struct - now it’s just a map with a __struct__ field.

(how to) remove newline inside or at the end of heredoc

https://elixirforum.com/t/line-break-at-the-end-of-a-multiline-string/12694/4

iex> """
...> foo \
...> bar\
...> """
"foo bar"

(how to) get current environment at runtime

https://stackoverflow.com/a/44747870

Mix.env doesn’t work in production or other environments where you use compiled releases (built using Exrm/Distillery) or when Mix just isn’t available.

=> store environment in application configuration since Mix is not available in compiled application:

# config/config.exs

config :my_app,
  env: Mix.env(),
  ecto_repos: [MyApp.Repo]

defmodule MyApp.Foo do
  @env Application.get_env(:my_app, :env)

  def call do
    if @env == :prod do
      # production code
    else
      # non-production code
    end
  end
end

(how to) convert module name to string without `Elixir` prefix

Macro.to_string(MyApp.Foo)
# => "MyApp.Foo"

MyApp.Foo
|> Module.split()
|> Enum.join(".")
# => "MyApp.Foo"

(how to) measure function execution time

defmodule Timer do
  defmacro time_ms(do: body) do
    quote do
      # or surround with calls to System.monotonic_time/1
      {time, result} = :timer.tc(fn -> unquote(body) end)
      {round(time / 1_000), result}
    end
  end
end

usage:

require Timer

{time, result} =
  Timer.time_ms do
    Operation.call()
  end

don’t use bind_quoted option of Kernel.SpecialForms.quote/2 macro to pass body binding to macro - execution time will be always 0 then (it looks like execution time of unquoting body is measured - not execution time of body itself though IDK for sure, just a wild guess):

defmodule Timer do
  defmacro time_ms(do: body) do
    quote bind_quoted: [body: body] do
      # time is always 0 in this case
      # (the same when using System.monotonic_time/1)
      {time, result} = :timer.tc(fn -> body end)
      {round(time / 1_000), result}
    end
  end
end

don’t use `require` when importing macros

if macro is imported from another module, it’s not necessary to require that module:

# when macro is not imported
require Foo
Foo.my_macro()

# when macro is imported
import Foo, only: [my_macro: 0]
my_macro()

tap349