Dry-struct: Sum structs class interface

Support
1

Are sum types and types classes supposed to implement the same interface? Eg. .[], .call methods.

If yes, does this apply also to sum structs and structs classes? Because, sum structs classes does not have .new class meth as struct classes have.

2

What do you mean by “sum structs classes”?

3

From dry-rb - dry-struct v1.0 - Introduction
“Struct classes quack like dry-types, which means you can use them in hash schemas, as array members or sum them”.

I will provide an example shortly.

4

“Summing” structs gives you a type, not a class/struct, there’s no .new method, it’s not part of the type API. You can .[] or .call.

5

Yes, that is, but below I have an usecase which highlights a possible API inconsistency. I think from a client code perspective, struct classes and sum structs classes make more sense to be interchangeable. If this aspect is not important, but mapping a relation result to a sum struct is desired, then Relation#map_to needs to use .call or .[]. What do you think?

class Movie < Dry::Struct
  attribute :type, Types.Value('movie')
  attribute :title, Types::String
end

class Episode < Dry::Struct
  attribute :type, Types.Value('episode')
  attribute :title, Types::String
  attribute :number, Types::Integer
end

VOD = Movie | Episode

class VODRepo < Repository[:vods]
   def all
     root.map_to(VOD).to_a # => NoMethodError: undefined method `new' for #<Dry::Struct::Sum:0x00007fe53b9deae0>
   end
end
6

Hi! I also ran into something similar, but I used .[] method.

All over my codebase I use .new for instantiating structs, but at some point I needed a sum struct:

class UserBase < Dry::Struct
  attribute :id, Types::String
end    

class ActiveUser < UserBase
  attribute :name, Types::String
end

class BannedUser < UserBase
  attribute :name, Types::String
  attribute :reason, Types::String
end

class DeletedUser < UserBase
  attribute :deleted_at, Types::Time
end

User = ActiveUser | BannedUser | DeletedUser

Somewhere else I use User class like any other struct. To be consistent with which method to use to instantiate the structs, I have to use .new, which is missing. I like to be consistent, but I have to change a lot of parts to use .[], so I sticked with .[] only for instantiating Users