elixir-root-cause-only

Elixir Root Cause Only: No Random Fixes

THE IRON LAW

NEVER fix symptoms. ALWAYS trace to the root cause.

No guessing. No "try this". No random changes. No symptom fixes.

TRACE. UNDERSTAND. FIX. VERIFY.

ABSOLUTE PROHIBITIONS

You are NEVER allowed to:

1. Propose random fixes

   • "Try adding this import"
   • "Maybe change this to that"
   • "Let's see if this works"
   • "Could you try restarting the server?"

2. Fix where error appears

   • Error appears in module A
   • Root cause is in module B
   • Don't just patch module A

3. Make multiple changes at once

   • Change A + B + C together
   • Now you don't know which one fixed it
   • Make ONE change, verify, repeat

4. Skip understanding

   • "I don't know why, but this fixes it"
   • If you don't know why, it's not fixed
   • Understanding is mandatory

5. Accept "works on my machine"

   • Reproducibility is required
   • Environmental differences matter
   • Document exact reproduction steps

THE 4-PHASE DEBUGGING PROCESS

Phase 1: REPRODUCE

Objective: Get consistent, repeatable reproduction.

# Required steps:
1. Identify exact steps to trigger the issue
2. Run those steps
3. Confirm issue appears
4. Document steps precisely
5. Verify it reproduces every time

Output required:

## Reproduction Steps

1. Run `mix test test/my_app/accounts_test.exs:42`
2. Error appears: "undefined function User.changeset/2"
3. Reproduces 100% of the time
4. Environment: Elixir 1.15.7, OTP 26

CHECKPOINT: Cannot proceed to Phase 2 until you have consistent reproduction.

Phase 2: TRACE

Objective: Follow the error back to its origin.

# Tracing questions:
1. What function fails?
2. What called that function?
3. What called THAT function?
4. Where did the bad data/state originate?
5. What's the first point where things went wrong?

Tracing tools:

# 1. Add IO.inspect to see data flow
def create_user(attrs) do
  attrs
  |> IO.inspect(label: "Input attrs")
  |> User.changeset(%User{})
  |> IO.inspect(label: "Changeset")
  |> Repo.insert()
end

# 2. Use IEx.pry for interactive debugging
def create_user(attrs) do
  require IEx; IEx.pry()
  # Execution pauses here
  User.changeset(%User{}, attrs)
  |> Repo.insert()
end

# 3. Check the stack trace completely
** (UndefinedFunctionError) function User.changeset/2 is undefined
    (my_app 0.1.0) lib/my_app/accounts/user.ex:42: User.changeset/2
    (my_app 0.1.0) lib/my_app/accounts.ex:15: MyApp.Accounts.create_user/1
    test/my_app/accounts_test.exs:25: (test)