Add Kernel.tap/2 and Kernel.then/2

[josevalim]

tap/2 is for side-effects in pipelines, then/2 is for anonymous functions:

def tap(x, fun) do
  fun.(x)
  x
end

def then(x, fun), do: fun.(x)

We should probably improve the error message in Macro.pipe to link to refer to Kernel.then/2.

[v0idpwn]

Can I take this one? Bonus: are these the definitive names?

[Eiji7]

Sorry @v0idpwn, I started working on Kernel.then/2 before you asked. 😈

[lukaszsamson]

I wonder if those functions should handle awaitable %Task{} specially.

Task.async(fn -> some_io_bound_op() end)
|> tap(fn res -> do_sth(res) end)
|> then(fn res -> Task.async(fn -> do_sth(res) end) end)
|> Task.await
# awaited do_sth(res)

[josevalim]

Please see mailing list discussion @lukaszsamson. Overall, It is not the goal of these functions to be polymorphic, modules could provide variants but I don’t think there are reasonable implementations for those in task.