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Daniel Kukula
Daniel Kukula

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Enhancements to dbg in elixir 1.18

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
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It evaluated to:

if a + b == 3 do
  :equal
else
  :non_equal
end #=> :equal
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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
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if result in 1.18:

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

If expression:
if a + b == 3 do
  :equal
else
  :non_equal
end #=> :equal
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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
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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
)
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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
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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
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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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abreujp profile image
João Paulo Abreu

Excellent.

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The Next Generation Developer Platform

Coherence is the first Platform-as-a-Service you can control. Unlike "black-box" platforms that are opinionated about the infra you can deploy, Coherence is powered by CNC, the open-source IaC framework, which offers limitless customization.

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