functional practice

data.rb

students = [
  {age: 18, first_name: "Shane", last_name: "Osbourne"},
  {age: 22, first_name: "John", last_name: "Doe"},
  {age: 12, first_name: "John", last_name: "Barker"},
  {age: 40, first_name: "Ben", last_name: "Haskell"},
  {age: 34, first_name: "Ben", last_name: "Barker"},
  {age: 9, first_name: "Shane", last_name: "Doe"},
  {age: 33, first_name: "Lee", last_name: "Ruby"}
]

with_group_by.exs

students
|> Enum.filter(fn s -> s.age > 20 end)
|> Enum.group_by(fn s -> s.first_name end)
|> Enum.map(fn {k, v} ->
  last_names = Enum.map(v, fn s -> s.last_name end)
  {k, last_names}
end)
|> Enum.into(%{})
|> IO.inspect

with_group_by.js

let result = R.compose(
  R.fromPairs,
  R.map(([k, v]) => {
      let last_names = R.map(s => s.last_name, v)
      return [k, last_names]
    }),
  R.toPairs,
  R.groupBy(s => s.first_name),
  R.filter(s => s.age > 20)
)(students);

p result

with_group_by.rb

result = 
  students.filter { |s| s[:age] > 20}
          .group_by { |s| s[:first_name] }
          .map do |key, val| # 也可以改用 hash.transform_values
            last_names = val.map { |s| s[:last_name] }
            [key, last_names]
          end
          .to_h
p result

with_reduce.exs

students
|> Enum.filter(fn s -> s.age > 20 end)
|> Enum.reduce(%{}, fn %{first_name: first_name, last_name: last_name}, acc ->
  prev = acc |> Map.get(first_name) |> List.wrap()
  Map.put(acc, first_name, [last_name | prev])
end)
|> IO.inspect()

with_reduce.js

let result = R.compose(
  R.reduce((accu, {first_name, last_name}) => {
             let prev = R.propOr([], first_name, accu)
             return R.assoc(first_name, [...prev, last_name], accu);
           }, {}),
  R.filter(s => s.age > 20)
)(students);

console.log(result)

with_reduce.rb

result =
  students.filter { |s| s[:age] > 20 }
          .reduce({}) do |accu, s|
            first_name, last_name = s.values_at(:first_name, :last_name)
            prev = Array(accu[first_name])
            accu[first_name] = [*prev, last_name]
            accu
          end

p result