A little spike on implementing currying for keyword arguments after reading: https://bugs.ruby-lang.org/issues/16113

keyword_curry.rb

# frozen_string_literal: true

module KeywordCurry
  refine Method do
    def curry(curry_arity = nil)
      to_proc.curry(curry_arity)
    end
  end

  refine Proc do
    def curry(curry_arity = nil)
      params = parameters
      min_args = params.count { |type, _| type == :req }

      if curry_arity && lambda?
        unless arity.negative? && arity != curry_arity
          raise ArgumentError, "Wrong number of arguments (#{curry_arity} for #{arity})"
        end

        max_args = params.count { |type, _| type != :block }
        no_rest = params.none? { |type, _| type == :rest }

        if curry_arity < min_args || no_rest && curry_arity > max_args
          additional = no_rest ? "..#{max_args}" : '+'

          raise ArgumentError, 'Wrong number of arguments ' \
                               "(given  #{curry_arity}, expected #{min_args}#{additional})"
        end
      end

      req_kwargs = params.filter_map { |type, name| name if type == :keyreq }

      -> *args, **kwargs do
        currying_done = !curry_arity&.-(args.size)&.nonzero?
        args_met = args.size >= min_args
        kwargs_met = req_kwargs.all? { |req_kwarg| kwargs.key? req_kwarg }

        if currying_done && args_met && kwargs_met
          call(*args + kwargs.map { |key, value| {key => value} })
        else
          -> *curry_args, **curry_kwargs do
            curry(curry_arity).call(*args, *curry_args, **kwargs, **curry_kwargs)
          end
        end
      end
    end
  end
end

require 'json'
require 'minitest/autorun'
require 'minitest/pride'

describe KeywordCurry do
  using KeywordCurry

  it 'curries keyword arguments' do
    assert_equal JSON.:parse.curry.call(symbolize_names: true).call('{"aim":true}'), aim: true
  end

  it 'accepts a curried arity' do
    h = {first: {second: {third: 42}}}

    assert_equal h.:dig.curry(3).call(:first).call(:second).call(:third), 42
  end
end