[Proposal] Debug macro like dbg! in Rust 1.32

[Роман Смирнов]

Hi,

Recently I read release notes of Rust 1.32 and was impressed of dbg macro convenience. Therefore I decided to implement the same concept in Elixir.

At the moment I've implemented it as a library: https://github.com/romul/dbg_inspect

But maybe it would be useful to implement it in the Elixir itself to avoid extra require calls each time. What do you think?

[Wojtek Mach]

I just wanted to add that I've been using such macro from the day I saw that Rust blog post [1]  and it had replaced all my usages of `IO.inspect` ever since (and I did use it a lot) and it proved really helpful to me.

To try moving this forward, I'd like to try fleshing out a concrete proposal. It has a bit different feature set than https://github.com/romul/dbg_inspect mentioned above. 

My proposal is to add `Kernel.dbg(expr, options \\ [])` which prints file, line, expression, pretty prints expression result, and prints how long evaluating the expression took.

I've literally added the time measurement feature today so still getting a feel for it; perhaps it should be optional and not be enabled by default if it's not important most of the time.

The options are:

- :label - useful for pipes, see below

- :inspect - passed to Kernel.inspect

Examples

    $ cat lib/a.exs

    dbg Time.utc_now

    $ elixir lib/a.exs

    lib/a.exs:1: Time.utc_now() #=> ~T[12:42:40.809472] (1ms)

    $ iex

    iex(1)> dbg Time.utc_now()

    iex:1: Time.utc_now() #=> ~T[11:52:07.744334] (1ms)

    ~T[11:52:07.744334]

    iex(2)> 1..5 |> Enum.map(&dbg(&1 * 2)) |> Enum.sum()

    iex:2: x1 * 2 #=> 2 (2µs)

    iex:2: x1 * 2 #=> 4 (1µs)

    iex:2: x1 * 2 #=> 6 (1µs)

    iex:2: x1 * 2 #=> 8 (1µs)

    iex:2: x1 * 2 #=> 10 (1µs)

    30

    iex(1)> 1..5 |> Enum.map(&dbg(&1 * 2, inspect: [syntax_colors: [number: :blue]])) |> Enum.sum()

    iex:3: x1 * 2 #=> 2 (6µs)

    iex:3: x1 * 2 #=> 4 (1µs)

    iex:3: x1 * 2 #=> 6 (10µs)

    iex:3: x1 * 2 #=> 8 (1µs)

    iex:3: x1 * 2 #=> 10 (1µs)

    30

Since pipes are expanded we'll get this:

    iex(4)> Time.utc_now() |> to_string() |> String.slice(0, 5) |> dbg()

    iex:4: String.slice(to_string(Time.utc_now()), 0, 5) #=> "11:53" (28µs)

    "11:53"

We can overwrite the label as following:

    iex(5)> Time.utc_now() |> to_string() |> String.slice(0, 5) |> dbg(label: :pipe)

    iex:5: pipe #=> "11:54" (34µs)

    "11:54"

I think it needs to be a function on Kernel to be the most easily accessible, e.g. we wouldn't need to require it.

I've started with calling it `Kernel.debug` however, that could introduce breakage if projects were already importing modules with that function, e.g.:

    ** (CompileError) lib/strategy/dns_poll.ex:121: function debug/2 imported from both Cluster.Logger and Kernel, call is ambiguous

Of course `dbg` suffers from the same problem although it's probably less likely to happen. It might be confused with OTP's :dbg module too.

If that's a deal breaker, or explicitly requiring it would be preferred anyway, I'd propose to call it `IO.debug` instead.

See [2] for example implementation.

Thanks

Wojtek

[1] https://blog.rust-lang.org/2019/01/17/Rust-1.32.0.html#the-dbg-macro

[2] https://github.com/elixir-lang/elixir/commit/9876c141e64af6112050e5a1338cb410fed40e37

[Chris Keathley]

This seems like a really useful addition to the core language. I like the idea of including the time measurements but I don't have a strong opinion on whether they should be shown by default or hidden behind an option. As far as the name goes I don't have a problem with `dbg` or `debug`. Both names communicate the same thing to me personally. I do think it makes sense to put this function in the `IO` module since it's used to output debug information. If it was on `Kernel` it may be more likely to be confused as a function to invoke a breakpoint or debugger or something similar. Thats just my intuition though. Overall this seems great.

- keathley

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