[elixir-core:8911] Re: [Proposal] identity function

[Alexis Brodeur]

While this is a great idea, and I am overall in favor of an 
`Kernel.identity/1`, let me play devil's advocate for a bit...

According to lib/elixir/src/elixir_locals.erl 
<https://github.com/elixir-lang/elixir/blob/master/lib/elixir/src/elixir_locals.erl#L92>,
 
all `def`s in a module are validated to not clash against an already 
imported function from other modules.  By introducing `Kernel.identity/1`, 
modules in the wild already using a `identity/1` function would cease to 
compile, therefore making this change a breaking one, according to my 
current understanding of the compiler.

A simple `test.exs` module
```ex
defmodule Test do
  def test do
    inspect("term")
  end

  def inspect(term, opts \\ []) do
    IO.puts("my inspect/2")
  end
end

Test.test
```

against the current `master` of elixir yields:
```
== Compilation error in file /Users/brodeuralexis/test.exs ==
** (CompileError) /Users/brodeuralexis/test.exs:6: imported 
Kernel.inspect/1 conflicts with local function
    (elixir) src/elixir_locals.erl:94: 
:elixir_locals."-ensure_no_import_conflict/3-lc$^0/1-0-"/2
    (elixir) src/elixir_locals.erl:95: anonymous fn/3 in 
:elixir_locals.ensure_no_import_conflict/3
    /Users/brodeuralexis/test.exs:1: (file)
```

On Tuesday, July 2, 2019 at 5:23:47 AM UTC-4, Alexey Nikitin wrote:
>
> Like Haskell's `id` or Clojure's `identity` or Ruby's `itself`
>
> It can be very useful sometimes. Instead of writing ugly `&(&1)` it would 
> much more attractive to write `&identity/1` or `&id\1`
> Moreover this function is already used as a default argument for some 
> higher-order function. For example `Enum.group_by(enumerable, key_fun, 
> value_fun \\ fn x -> x end)`
>
> Here are some examples
>
> ```
> #=> [1, 2, 3, true, 1234] 
>
> 'abcdaabccc' |> Enum.sort |> Enum.chunk_by(&identity/1) 
> #=> ['aaa', 'bb', 'cccc', 'd'] 
>
> [1, 1, 2, 3, 3, 1, 1, 5, 5] |> Enum.chunk_by(&identity/1) |> 
> Enum.map(&hd/1) 
> #=> [1, 2, 3, 1, 5] 
>
> Enum.group_by('abracadabra', &identity/1) 
> #=> %{97 => 'aaaaa', 98 => 'bb', 99 => 'c', 100 => 'd', 114 => 'rr'} 
>
> Enum.map([1, 2, 3, 4], &identity/1) 
> #=> [1, 2, 3, 4]
> [1,2,3,nil, true, false, 1234] |> Enum.filter(&identity/1) 
> ```
>

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