Support delayed evaluation of code in .iex.exs

[alco]

It is not currently possible to define a module in .iex.exs and import it at once.

defmodule MyIExHelpers do
  def s do :init.stop end
end

import MyIExHelpers


λ iex
...
Error while evaluating: /Users/alco/tmp/.iex.exs
** (CompileError) .iex.exs:5: module MyIExHelpers is not loaded but was defined. This happens because you are trying to use a module in the same context it is defined. Try defining the module outside the context that requires it.
    (elixir) src/elixir_exp.erl:123: :elixir_exp.expand/2
    (stdlib) lists.erl:1352: :lists.mapfoldl/3
    (stdlib) lists.erl:1353: :lists.mapfoldl/3
    (elixir) src/elixir_exp.erl:49: :elixir_exp.expand/2

Even if the module definition is extracted into a separate file, it doesn't work:

Code.require_file "my_iex_helpers.exs"
import MyIExHelpers

λ iex
...
Error while evaluating: /Users/alco/tmp/.iex.exs
** (CompileError) .iex.exs:2: module MyIExHelpers is not loaded and could not be found
    (elixir) src/elixir_exp.erl:123: :elixir_exp.expand/2
    (stdlib) lists.erl:1352: :lists.mapfoldl/3
    (stdlib) lists.erl:1353: :lists.mapfoldl/3
    (elixir) src/elixir_exp.erl:49: :elixir_exp.expand/2

A suggested workaround for this would be the ability to return a quoted expression from .iex.exs which would then be evaluated in the IEx process and therefore in a different context from the one the module has been defined in.

[whatyouhide]

@alco I'd like to help on this as I feel IEx "custom helpers" (trivial to implement if we fix this) are a very desired feature :). So, let me make sure I got things straight, your suggestion is to delay the evaluation of the (quoted) contents of .iex.exs so that they're executed in the IEx process: you mean still defining the module in a different file, right?

defmodule Foo do
  def foo, do: 1
end

import Foo

should always fail, right?

[alco]

@whatyouhide I meant that this should work:

defmodule Foo do
  def foo, do: 1
end

quote do: import Foo

In other words, the last expression in .iex.exs is evaluated as a quoted expression in the context of the IEx shell (more specifically, as the first thing in IEx server's loop, I think), after the file itself has been evaluated normally, by IEx.Evaluator.eval_dot_iex.

[whatyouhide]

@alco ah, I see. I'm not a fan of this solution from a usability stand point as it looks a little bit cryptic IMO (you need to roughly understand what's going on in order to understand why you're quoting the last expression). That said, I think the solution could work and I can't come up with any better one.

[josevalim]

I am not a fan either and since I can't think of anything better too, I think we should roll with it.

@alco shouldn't we also change import_file as the whole goal of import_file is to load stuff like this?

[josevalim]

Gah, I guess we can't make it work with import_file because it works as a macro... :(

[whatyouhide]

@josevalim yeah :( we could extract the last quote expression from the file when we use import_file and keep it separated from the rest, so that we can defer its execution, but it's a (bad) hack which would be a mess to maintain and would include some limitations on .iex.exs that users will found weird for sure.

It's a tricky issue for how things are designed right now :(

[alxndr]

I'd love to see this feature as well; anything I can do to help?

[hunterboerner]

Status?

[josevalim]

There is no update. If there was one, it would likely be here. So please avoid pinging issues. :)

[ericentin]

I'm going to do some r&d on this issue unless anyone opposes. :)

[whatyouhide]

@antipax go ahead, this is a really nasty one. :)