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Simple Ruby implementations of some common monads.

Home Page: https://tomstu.art/refactoring-ruby-with-monads

License: MIT License

Ruby 100.00%

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monads's Issues

Optional plays nice with Hashie::Mash. Should it play nice with ActionController::Parameters and OpenStruct too?

I was experimenting the use of Monads::Optional with chains of calls that imply hash access, such as when dealing with Rails parameters:

params[:profile][:resume].tempfile.path

Initially I thought that wrapping an OpenStruct around a Hash would've been a good fit for Monads::Optional but then I discovered that it doesn't work.
Here's a list of tests that I've run:

require 'monads'
require 'hashie'
require 'ostruct'

require 'action_pack'
require 'rack/test'
require 'action_controller'

params = {
  foo: {
    bar: 42
  }
}
rails_params = ActionController::Parameters.new(params)
ostruct_params = OpenStruct.new(params)
hashie_params = Hashie::Mash.new(params)
hashie_rails_params = Hashie::Mash.new(rails_params)
ostruct_rails_params = OpenStruct.new(rails_params)

Monads::Optional.new(ostruct_params).foo
 # => #<struct Monads::Optional value={:bar=>42}>

Monads::Optional.new(hashie_params).foo
 # => #<struct Monads::Optional value=#<Hashie::Mash bar=42>>

Monads::Optional.new(ostruct_rails_params).foo
 # => #<struct Monads::Optional value={"bar"=>42}>

Monads::Optional.new(hashie_rails_params).foo
 # => #<struct Monads::Optional value=#<Hashie::Mash bar=42>>

Monads::Optional.new(ostruct_params).foo.bar
# NoMethodError: undefined method `bar' for {:bar=>42}:Hash
  # from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:11:in `public_send'
  # from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:11:in `block in method_missing'
  # from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:5:in `call'
  # from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:5:in `block in within'
  # from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:21:in `call'
  # from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:21:in `block in ensure_monadic_result'
  # from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/optional.rb:13:in `call'
  # from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/optional.rb:13:in `and_then'
  # from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:4:in `within'
  # from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:10:in `method_missing'
  # from (irb):23
  # from /Users/olistik/.rvm/rubies/ruby-2.2.2/bin/irb:11:in `<main>'

Monads::Optional.new(hashie_params).foo.bar
# => #<struct Monads::Optional value=42>

Monads::Optional.new(ostruct_rails_params).foo.bar
# NoMethodError: undefined method `bar' for {"bar"=>42}:ActionController::Parameters
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:11:in `public_send'
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:11:in `block in method_missing'
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:5:in `call'
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:5:in `block in within'
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:21:in `call'
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:21:in `block in ensure_monadic_result'
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/optional.rb:13:in `call'
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/optional.rb:13:in `and_then'
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:4:in `within'
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:10:in `method_missing'
#   from (irb):25
#   from /Users/olistik/.rvm/rubies/ruby-2.2.2/bin/irb:11:in `<main>'

Monads::Optional.new(hashie_rails_params).foo.bar
# => #<struct Monads::Optional value=42>

Monads::Optional.new(ostruct_params).foo.bar.value
# NoMethodError: undefined method `bar' for {:bar=>42}:Hash
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:11:in `public_send'
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:11:in `block in method_missing'
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:5:in `call'
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:5:in `block in within'
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:21:in `call'
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:21:in `block in ensure_monadic_result'
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/optional.rb:13:in `call'
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/optional.rb:13:in `and_then'
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:4:in `within'
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:10:in `method_missing'
#   from (irb):27
#   from /Users/olistik/.rvm/rubies/ruby-2.2.2/bin/irb:11:in `<main>'

Monads::Optional.new(hashie_params).foo.bar.value
# => 42

Monads::Optional.new(ostruct_rails_params).foo.bar.value
# NoMethodError: undefined method `bar' for {"bar"=>42}:ActionController::Parameters
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:11:in `public_send'
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:11:in `block in method_missing'
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:5:in `call'
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:5:in `block in within'
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:21:in `call'
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:21:in `block in ensure_monadic_result'
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/optional.rb:13:in `call'
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/optional.rb:13:in `and_then'
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:4:in `within'
#   from /Users/olistik/.rvm/gems/ruby-2.2.2/gems/monads-0.0.1/lib/monads/monad.rb:10:in `method_missing'
#   from (irb):29
#   from /Users/olistik/.rvm/rubies/ruby-2.2.2/bin/irb:11:in `<main>'

Monads::Optional.new(hashie_rails_params).foo.bar.value
# => 42

It looks like that the only viable solution is to wrap params into a Hashie::Mash and then pass it to Monads::Optional.

Would you like/expect to have an implementation that supports OpenStruct and ActionController::Parameters?

Many seems to be broken

First of all, thanks for your talk about Ruby monads, it blew my mind ^^

I've been trying to imitate your logic as a TDD exercise, and was comparing what I was writing to your code and found a difference puzzling me in the "Many" monad.

When I'm trying to use yours, I find that:

Blaher = Struct.new(:blahs)
Bloher = Struct.new(:blohs)
blahers = [Blaher.new([Bloher.new([1, 2, 3, 4])])]
Monads::Many.new(blahers).blahs.blohs.values

Ends up in a:

NoMethodError: undefined method `bloh' for [#<struct bloh=[1, 2, 3, 4]>]:Array

I thought it had something to do with the Many.from_value method and tried a quick-and-dirty fix with:

def from_value(value)
  Many.new(value)
end

It makes the method_missing do its job and return the subelements expected, though I find one test (spec/monads/many_spec.rb:49 the one that verifies that Many.from_value wraps the value in an array before wrapping it in the monad) fails.

I'm a bit unsure on what to do to fix this, so I thought I'd let you know ^^°

Again, thanks for the awesome talk and ideas.

Optional.new([1, 2, 3]) - calling .first gives me the whole array, calling .last gives me Optional 3

Optional.new([1, 2, 3]).first # => [1, 2, 3]
Optional.new([1, 2, 3]).last # => #<struct Monads::Optional value=3>

Optional.new([1, 2, 3]).last(2) # => #<struct Monads::Optional value=[2, 3]>
Optional.new([1, 2, 3]).first(2) # => [[1, 2, 3]]

I'm not knowledgable enough about the behavior of the Maybe monad to tell if this is the correct behavior or if this is a bug.

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