Type refinement with method calls

[soutaro]

Proposal: https://hackmd.io/cQ75yJxcT_Ka5nHjQySWtg?view

Assume we have a class with the following RBS.

class Account
  attr_reader email: String
  
  attr_reader phone: String?
end

email always exists, but phone may be nil. So, a Ruby code accesing phone number needs to test if phone number exists explicitly.

account = Account.find(email)

if account.phone                # Test if `phone` returns a `String`
    send_sms_to(account.phone)  # Sending a SMS to the phone number
end

This doesn't work with Steep right now, because account.phone is a method call that may return different value in future. This proposal is to make it work by assuming pure methods returns the same value next time.

[soutaro]

I found that the invalidation of type refinements on method call cannot be safe.

a = [0]
b = a

if a[0].is_a?(Integer)        # This refines the type to `Integer`
  a[0] = ""                   # This invalidates a[0] refinement
end

if a[0].is_a?(String)         # This refines the type to `String`
  b[0] = :foo                 # How about this!?!?
end

I will consider dropping type refinements invalidation by method calls. 🤔
Will keep the invalidation on local variable assignments.

[soutaro]

As of Steep 1.1,

• It invalidates on non pure method call of the same receiver node
• We don't provide an option to disable the feature