Enhancements to dbg in elixir 1.18

Published at

12/22/2024

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Elixir 1.18 added some interesting, features but one that went under the radar was extended support for dbg. In 1.17 when you had this code

a = 1
b = 2

if a + b == 3 do
  :equal
else
  :non_equal
end
|> dbg

It evaluated to:

if a + b == 3 do
  :equal
else
  :non_equal
end #=> :equal

In 1.18 this was aligned with what case and cond does:

Case argument:
1 + 2 #=> 3

Case expression (clause #1 matched):
case 1 + 2 do
  3 -> :equal
  _ -> :non_equal
end #=> :equal

if result in 1.18:

If condition:
a + b == 3 #=> true

If expression:
if a + b == 3 do
  :equal
else
  :non_equal
end #=> :equal

if this is not enough you can wrap your code in brackets and pass it to dbg

(
  input = %{a: 1, b: 2}
  c = Map.get(input, :c, 20)

  if input[:a] + c == 3 do
    :equal
  else
    :non_equal
  end
)
|> dbg

results in displaying results for every expression

Code block:
(
  input = %{a: 1, b: 2} #=> %{a: 1, b: 2}
  c = Map.get(input, :c, 20) #=> 20
  if input[:a] + c == 3 do
  :equal
else
  :non_equal
end #=> :non_equal
)

Last missing piece is the with macro which is sometimes a nightmare to debug, in elixir 1.18 we have support for that:

with input = %{a: 1, b: 2},
     {:ok, c} <- Map.fetch(input, :c),
     3 <- input[:a] + c do
  :equal
else
  _ -> :non_equal
end
|> dbg

The result here is:

With clauses:
%{a: 1, b: 2} #=> %{a: 1, b: 2}
Map.fetch(input, :c) #=> :error

With expression:
with input = %{a: 1, b: 2},
     {:ok, c} <- Map.fetch(input, :c),
     3 <- input[:a] + c do
  :equal
else
  _ -> :non_equal
end #=> :non_equal

This should make your debugging journey easier.
If you are stuck in an older elixir versions you can install a package that backports this functionality dbg_mate
Thanks for your attention.

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Enhancements to dbg in elixir 1.18