Feature request: Add `Enum.sum_by/2` (like existing `Enum.max_by/2`)

[Alexey Chernenkov]

I'm missing `Enum.sum_by/2` func. There are several other `xxx_by` methods (e.g. `sort_by` and `max_by`).

[Kelvin Raffael Stinghen]

The thing with `x_by` functions is that they still return the whole list (like in `sort_by`) or an element of it (like in `max_by`). 

Example right from `h Enum.max_by`:

```elixir

iex> Enum.max_by(["a", "aa", "aaa"], fn(x) -> String.length(x) end)

"aaa"

```

`Enum.sum` totally changes the returning value type to another, so I guess there is no point of having them since you can totally use the function `map` on your enumerable to get what you want. For example:

```elixir

iex> [“a”, “aa”, “aaa”] |> Stream.map(&String.length/1) |> Enum.sum()

=> 6

```

Best,
Kelvin Stinghen
kelvin....@me.com

On Feb 2, 2018, at 05:48, Alexey Chernenkov <a.y.che...@gmail.com> wrote:

I'm missing `Enum.sum_by/2` func. There are several other `xxx_by` methods (e.g. `sort_by` and `max_by`).

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[Andrea Leopardi]

Also, sum/1 is easily replaceable with Enum.reduce(enum, &+/2). sum_by(enum, fun) could be written as:

Enum.reduce(enum, 0, &(fun.(&1) + &2))

If you use this a lot in your code, you can extract it to a function and it will maintain the same clarity as sum_by :).

Andrea Leopardi

an.le...@gmail.com

On Fri, Feb 2, 2018 at 11:27 AM, Kelvin Raffael Stinghen <kelvin....@gmail.com> wrote:

The thing with `x_by` functions is that they still return the whole list (like in `sort_by`) or an element of it (like in `max_by`). 

Example right from `h Enum.max_by`:

```elixir

iex> Enum.max_by(["a", "aa", "aaa"], fn(x) -> String.length(x) end)

"aaa"

```

`Enum.sum` totally changes the returning value type to another, so I guess there is no point of having them since you can totally use the function `map` on your enumerable to get what you want. For example:

```elixir

iex> [“a”, “aa”, “aaa”] |> Stream.map(&String.length/1) |> Enum.sum()

=> 6

```

Best,
Kelvin Stinghen
kelvin....@me.com

On Feb 2, 2018, at 05:48, Alexey Chernenkov <a.y.che...@gmail.com> wrote:

I'm missing `Enum.sum_by/2` func. There are several other `xxx_by` methods (e.g. `sort_by` and `max_by`).

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