Ruby semigroups

example.rb

class LotsOfData < Struct.new(:field1, :field2, :field3, :field4, :value)
  def as_accumulation
    # Group by field1, then the pair of field2 and field3, then field4, finally summing all the values
    { field1 => { [field2, field3] => { field4 => Plus.new(value) } } }
  end
end

many_data = [
  LotsOfData.new(:a, :b, :c, :d, 15),
  LotsOfData.new(:a, :b, :c, :d, 20),
  LotsOfData.new(:a, :b, :c, :e, 15)
  ]
  
many_data.fold_map({}) { |v| v.as_accumulation }

# => {:a=>{[:b, :c]=>{:d=>#<struct Plus underlying=35>, :e=>#<struct Plus underlying=15>}}}  

semigroups.rb

# >> Plus.new(3).mappend(Plus.new(5)).underlying
# => 8
class Plus < Struct.new(:underlying)
  def mappend other
    Plus.new(underlying + other.underlying)
  end
end

class Product < Struct.new(:underlying)
  def mappend other
    Product.new(underlying * other.underlying)
  end
end

class Hash
  # Hashes are straight up merged, resolving key clashes by mappend'ing the values.
  # Values must have mappend. 
  # ie. Semigroup a => Semigroup (Hash k a)
  #
  # doctest: hash mappend
  # >> { :a => Plus.new(3) }.mappend({ :a => Plus.new(1) })
  # => { :a => Plus.new(4) }
  def mappend other
    merge(other) { |_, a, b| a.mappend(b) }
  end
  
  # Oh god what have I become, a mutable mappend.
  def mappend! other
    merge!(other) { |_, a, b| a.mappend(b) }
  end
end

class ZipList < Struct.new(:underlying)
  # A zip list combines values pair-wise.
  # Values must have #mappend
  # ie. Semigroup a => Semigroup (ZipList [a])
  #
  # doctest: zip list mappend
  # >> def pluses(*xs); xs.map { |x| Plus.new(x) }; end
  # >> ZipList.new(pluses(1,2)).mappend(ZipList.new(pluses(3,4))).underlying
  # => pluses(4,6)
  def mappend other
    ZipList.new(underlying.zip(other.underlying).map { |(a,b)| a.mappend(b) })
  end
end

module Enumerable
  def fold_map(initial)
    inject(initial) { |m, v| m.mappend(yield v) }
  end

  def mconcat(initial)
    fold_map(initial) { |v| v }
  end

  def mconcat1
    inject { |m, v| m.mappend(v) }
  end
end