Import with :only option causes undefined function error due to the default argument mechanism

Questions / Help
, ,
1

Here is my .iex.ex file:

alias MyApp.Repo
alias MyApp.CMS
alias MyApp.CMS.Page

import Ecto.Query, only: [from: 2]
# import Ecto.Query
# import Ecto.Query, only: [from: 1]

Then, iex -S mix

Repo.all(from(Page))
** (CompileError) iex:1: undefined function from/1

but it works with

iex(1)> Repo.all(from(Page, []))
[debug] QUERY OK source="pages" db=5.0ms decode=5.1ms
SELECT p0."id", p0."body", p0."published_at", p0."title", p0."slug", p0."views", p0."author_id", p0."category_id", p0."inserted_at", p0."updated_at" FROM "pages" AS p0 []
[
  %MyApp.CMS.Page{ ... },
  %MyApp.CMS.Page{ ... }
]

With the full import of the Ecto.Query module in the .iex.ex file, Repo.all(from(Page)) works.

Here comes the FIX

So finally I did import Ecto.Query, only: [from: 1] and Repo.all(from(Page)) works.
However, when you look at the doc of Ecto.Query, there is no function from/1.

Is it on purpose?
Is there a better way to use import only on functions with default argument ?

Tks ~

1 Like
2

There is a from/2 with a default argument. Default arguments are implemented as recursive function calls. eg:

def foo(opts \\ []), do: IO.inspect opts

will get compiled to code equivalent to this:

def foo(), do: foo([])

def foo(opts), ddo: IO.inspect opts

This is why you need to import from/1.

6 Likes
3

Hi, where could I check this on the elixir code? It would be awesome to be able to see how default params were implemented. (I tried looking at the Kernel and Kernel.SpecialForms without success)

4

This is mostly urban knowledge :smiley:

And I think it is even in the elixir guides described like that, though how it might be implemented is probably well distributed between the implementation of def(p) and defmodule.

1 Like
5

This code from elixir_def.erl seems highly relevant:

1 Like