prepending a module into a class results in Unexpected Super

[jubishop]

module DateTimeDurationExtensions
  def -(other)
    # @type self: DateTime
    super(other)
  end
end

class DateTime < Date
  prepend DateTimeDurationExtensions
end

this results in Unexpected Super, but there is a - method inside the Date class.

[jubishop]

found my answer:

# @type module: DateTime.class

[jubishop]

actually then all type checking stops. hmm.

[soutaro]

@jubishop Could you give me the RBS files?

Seems like you need to add module-self-type constraint in the module declaration:

interface _DateTimeMinus
  def -: (untyped) -> DateTime
end

module DateTimeDurationExtensions : _DateTimeMinus     # The module have to be mixed to classes compatible to _DateTimeMinus
  def -: (untyped) -> DateTime
end

[jubishop]

thanks @soutaro for looking into this! I've attached the 3 files to show a simple example of my problem.

PrependExample.zip

[jubishop]

actually your interface hint worked for me. :) I don't fully understand why tho, and wish I did :) why does having the module adhere to the interface allow super() calls, whereas just defining it in the module itself doesn't?

[jubishop]

Here's a more interesting example with the Time class. Even tho my + and - signatures match the core Time signatures precisely, I still get an error that UnexpectedError: undefined method `add_call' for nil:NilClass at the super(other) line.

However if I remove the second definition for -, which is | (Time other) -> Float , then it passes just fine.

[jubishop]

PrependExample.zip

[soutaro]

In the second example, the error around add_call is a bug of Steep. The module declaration in RBS is empty and the module implementation in .rb has. That inconsistency caused that unexpected error. The add_call error is a bug, but I'm not sure how it should be...

Unfortunately, type checking super calls in modules is not straightforward, and I don't think it's good enough.

I assume that you want to extend DateTime#+ to support other types, String for example. And the RBS would be something like the following:

interface _DateTimeAdd
  def +: (DateTime) -> DateTime
end

module DateTimeDurationExtensions : _DateTimeAdd
  include _DateTimeAdd

  def +: (String) -> DateTime
       | ...
end

class DateTime
  include DateTimeDurationExtensions
end

[jubishop]

I can't put any combination together that works for me. I'm trying to reuse the Time signatures here, but add Duration as an option, I have this:

interface _TimeDurationExtensions
  def -: (Numeric other) -> Time
       | (Time other) -> Float
  def +: (Numeric other) -> Time
end

module TimeDurationExtensions : _TimeDurationExtensions
  def -: (Duration other) -> Time
  def +: (Duration other) -> Time
end

class Time
  prepend TimeDurationExtensions
end

And any steep check is happy if I pass a Duration but doesn't see the Numeric and Time options coming from the Interface.

[jubishop]

If I try to include _TimeDurationExtensions I get an error about duplicate method definitions..

[soutaro]

How about this?

class Duration
end

interface _TimeDurationExtensions
  def +: (Numeric other) -> Time
end

module TimeDurationExtensions : _TimeDurationExtensions
  def +: (Duration other) -> Time
       | ...
end

class Time
  prepend TimeDurationExtensions
end

module TimeDurationExtensions
  def +(other)
    case other
    when Duration
      Time.new
    else
      super(other)
    end
  end
end

class Time
  prepend TimeDurationExtensions
end

Time.new + Duration.new
Time.new + 3

[jubishop]

ohh I didn't understand ... was actual syntax. that works for the + case, but for the - case, which has two built in options (Numeric or Time), it fails with the undefined method "add_call", like this:

class Duration
end

interface _TimeDurationExtensions
  def -: (Numeric other) -> Time
       | (Time other) -> Float
end

module TimeDurationExtensions : _TimeDurationExtensions
  def -: (Duration other) -> Time
       | ...
end

class Time
  prepend TimeDurationExtensions
end

require 'time'

module TimeDurationExtensions
  def -(other)
    case other
    when Duration
      Time.new
    else
      super(other)
    end
  end
end

class Time
  prepend TimeDurationExtensions
end

Time.new - Time.new
Time.new - 3
Time.new - Duration.new

[soutaro]

Hmm, the add_call error looks like a bug of Steep. Or, we can just say Steep is completely broken for prepend... (Changing the prepend to include reports some errors...)

[jubishop]

Haha well thanks for investigating. I wish I could commit a fix but the code is beyond me 🥲