Shouldn't Kleisli::Maybe::None be understood as nil in some cases? about dry-types HOT 5 CLOSED

The only way to add custom types is to use public type definition API. It's a new thing, ie:

class Email
end

# shortcut which assumes `Email.method(:new)` is the constructor
Dry::Types.register_class(Email)

# this expands to:
type = Dry::Types::Definition.new(Email).constructor(Email.method(:new))
Dry::Types.register('email', type)

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Thanks for reporting this. I'd say it's a bug. I'll address this in the current refactoring.

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I had a problem that led me to this and it's basically that I cannot make the output of a type be a valid input for itself given this situation. This had the issue that I would take an object and coerce it at the entry of an HTTP call and then would pass that data to a series of steps that in the end get coerced again with the exact same coercer at the exit point. This is used for consistency so that we guarantee the format in both directions.

I solved the problem by defining a new type Nothing that can be nil or None:

require 'kleisli'
module Types
  class Nothing
    extend Dry::Data::TypeBuilder
    class << self
      def call(input)
        input.nil? ? Kleisli::Maybe::None.new : input
      end

      def try(input)
        call(input)
      end

      def valid?(input)
        #binding.pry
        input.nil? || input.is_a?(Kleisli::Maybe::None)
      end

      alias_method :[], :call
    end
  end
end

Dry::Data.register "types.nothing", Types::Nothing

class Kleisli::Maybe::None
  def to_json
    nil.to_json
  end
end

class Kleisli::Maybe::Some
  def to_json
    value.to_json
  end
end

Then instead of using #optional I created sum types by hand, e.g.: OptionalString = Nothing | String and this solved all my issues. I wanted to share this because maybe it can help the conversation by showing why another type besides Nil could be needed.

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I'm not sure this is the best way to create custom typed, but it's what I managed to figure out by reading the code since there were no docs about building custom types/coercers without recurring to sums or fmap.

from dry-types.

@solnic thanks for the PR and for the help with custom types!

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