dry-types 1.7

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Monads

The :monads extension provides a #to_monad method that returns a Result compatible with dry-monads.

The .try method returns a simple Result that's defined within dry-types (i.e. Dry::Types::Result). If you want to use the result with dry-monads, you can load this extension and call #to_monad on the Dry::Types::Result to get a Result that's defined in dry-monads (i.e. Dry::Monads::Result). This will let you use the dry-monads methods on the result.

To enable the :monads extension:

require 'dry/types'
Dry::Types.load_extensions(:monads)
Types = Dry.Types()

After loading the extension, you can leverage the .to_monad method:

result = Types::String.try('Jane')
result.class            # => Dry::Types::Result::Success
monad = result.to_monad # => Success("Jane")
monad.class             # => Dry::Monads::Result::Success
monad.value!            # => 'Jane'

result = Types::String.try(nil)
result.class            # => Dry::Types::Result::Failure
monad = result.to_monad # => Failure([...])
monad.class             # => Dry::Monads::Result::Failure
monad.failure           # => [#<Dry::Types::ConstraintError: ...>, nil]
monad
  .fmap { |result| puts "passed: #{result.inspect}" }
  .or   { |error, input| puts "input '#{input.inspect}' failed with error: #{error.to_s}" }

Note that you must use the .try method and not the .[] method, since that skips the intermediate Result object and just returns the value. If you want to use the .[] method and also have errors be raised rather than captured in Failure, then you can consider using the :maybe extension instead.

dry-monads Do notation

This can be useful with do notation in dry-monads.

require 'dry/types'
require 'dry/monads'
Types = Dry.Types()
Dry::Types.load_extensions(:monads)

class AddTen
  include Dry::Monads[:result, :do]

  def call(input)
    integer = yield Types::Coercible::Integer.try(input)

    Success(integer + 10)
  end
end

add_ten = AddTen.new

add_ten.call(10)
# => Success(20)

add_ten.call('integer')
# => Failure([#<Dry::Types::CoercionError: invalid value for Integer(): "integer">, "integer"])

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