Debugging

printing

op()
|> IO.inspect(label: "op", limit: :infinity)

op()
|> dbg()

tracing

:dbg.tracer()
:dbg.p(:all, :c) # :c for call trace
:dbg.tpl(App1Web.PageController, :home, 2, []) # tpl for trace process local

:dbg.stop_clear()

breaking point

IEx.pry starts an interactive IEx session at a specific point during code execution.

You can insert IEx.pry() into your code.
When execution reaches IEx.pry(), it pauses and drops you into an IEx shell.
Inside the pry session, you can inspect local variables, evaluate expressions, and resume execution.

`IEx.break!`

The iex --dbg pry command line option configures IEx to automatically enter a pry session when a breakpoint is hit or certain errors occur.

iex --dbg pry -S mix

IEx.break! will leads to a pry session when the breakpoint is hit.

IEx.break!(App1Web.PageController, :home, 2)

recon

REmote CONtrol, it can be used in prod env.

defmodule M, do: (def incr(n), do: n + 1)

:recon_trace.calls({M, :incr, :_}, 3)

M.incr(0)

The maching pattern here is {mod, fn, arity}, and it limited to 3 times.

:recon_trace.calls/2 returns 0 if instruction is not successful, this happens when the module is not loaded or the function spec is incorrect.

maching parameters

Only tracing calls where paramters are [1]:

:recon_trace.calls({M, :incr, [{[1], [], []}]}, 3)
M.incr(0)
M.incr(1)

tracing result

:recon_trace.calls({M, :incr, [{[:_], [], [{:return_trace}]}]}, 3)
# or simply:
:recon_trace.calls({M, :incr, :return_trace}, 3)
M.incr(0)

12:24:58.761345 <0.658.0> 'Elixir.M':incr(0)
1
Recon tracer rate limit tripped.

tracing all calls to modules matching a prefix

prefix = "Elixir.App1Web."

:code.all_loaded()
  |> Enum.map(fn {mod, _file} -> mod end)
  |> Enum.filter(&String.starts_with?(Atom.to_string(&1), prefix))
  |> Enum.each(&:recon_trace.calls({&1, :_, :return_trace}, 100))

Phoenix