Going over https://medium.com/@roman01la/understanding-transducers-in-javascript-3500d3bd9624 in Ruby

transducers.rb

# Do note that I'm being purposely verbose to make this easier to read through,
# as transducers are already a bit hard to grok without shorthanding things.

def maps(&function)
  proc { |accumulator, value|
    accumulator << function.call(value)
  }
end

def filters(&function)
  proc { |accumulator, value|
    if function.call(value)
      accumulator << value
    else
      accumulator
    end
  }
end

[1,2,3,4,5]
  .reduce([], &maps    { |v| v * 3   })
  .reduce([], &filters { |v| v.even? })
  # => [6, 12]

def mapping(&function)
  proc { |reducing_function|
    proc { |accumulator, value|
      reducing_function.call(accumulator, function.call(value))
    }
  }
end

def filtering(&function)
  proc { |reducing_function|
    proc { |accumulator, value|
      if function.call(value)
        reducing_function.call(accumulator, value)
      else
        accumulator
      end
    }
  }
end

concats = proc { |accumulator, value| accumulator << value }

[1,2,3,4,5]
  .reduce([], &mapping   { |v| v * 3   }.call(concats))
  .reduce([], &filtering { |v| v.even? }.call(concats))
  # => [6, 12]

# gem install ramda-ruby
require 'ramda'

[1,2,3,4,5].reduce([], &Ramda.pipe(
  mapping   { |v| v + 1   },
  filtering { |v| v.even? },
  mapping   { |v| v * 3   }
).call(concats))
# => [7, 13]