47. Reproducible debugging (pocket version)

TL;DR (5 lines)

Debugging begins with reduction.
A stable failing scenario is better than a huge noisy one.
The contract matters more than the symptom wording.
Reproducibility is a technical asset.
Fixes should be explained against the reduced case.

Concrete Problem

Debugging chapters often list tools without showing how a failure becomes reproducible and explainable.

Coherent example

space demo/debug

proc normalize_port(raw: int) -> int {
  if raw < 0 { give 0 }
  if raw > 65535 { give 65535 }
  give raw
}

export *

Complete examples

Each block below is a complete reading unit with its own boundary, data shape, and observable result.

Primary coherent example

This is the compact chapter anchor used by the surrounding explanation.

space demo/debug

proc normalize_port(raw: int) -> int {
  if raw < 0 { give 0 }
  if raw > 65535 { give 65535 }
  give raw
}

export *

Reduced reproducible case

This keeps the failing boundary small enough to inspect.

space examples/debugging/repro

proc parse_port(raw: int) -> int {
  if raw < 1 { give 11 }
  if raw > 65535 { give 12 }
  give raw
}

proc main(args: list[string]) -> int {
  give parse_port(8080)
}

export *

Traceable correction

This block names each intermediate value that helps debugging.

space examples/debugging/trace

proc normalize(raw: int) -> int {
  let fallback: int = 80
  if raw <= 0 { give fallback }
  give raw
}

proc main(args: list[string]) -> int {
  let port: int = normalize(0)
  give port
}

export *

Immediate work for this chapter

Reduce the failure to one stable repro.
Keep intermediate values named when they aid inspection.
Preserve the evidence that proves the correction.
Keep the first code block complete and aligned with current Vitte docs syntax.
Keep the invalid block focused on one broken contract only.
Replace generic prose with one concrete rule visible in the code.
Keep every example small enough to review from top to bottom.

Chapter override: 47-reproducible-debugging.html

Dedicated problem

47. Reproducible debugging needs a concrete production-grade anchor around symptom, reduced case, and verified correction. The page should keep the examples, diagnostics, and review rules tied to that exact boundary instead of drifting back to generic tutorial prose.

Specific complete examples

47. Reproducible debugging chapter anchor

The primary example is promoted here as the first chapter-specific production reading unit.

space demo/debug

proc normalize_port(raw: int) -> int {
  if raw < 0 { give 0 }
  if raw > 65535 { give 65535 }
  give raw
}

export *

Reduced reproducible case

This keeps the failing boundary small enough to inspect.

space examples/debugging/repro

proc parse_port(raw: int) -> int {
  if raw < 1 { give 11 }
  if raw > 65535 { give 12 }
  give raw
}

proc main(args: list[string]) -> int {
  give parse_port(8080)
}

export *

Traceable correction

This block names each intermediate value that helps debugging.

space examples/debugging/trace

proc normalize(raw: int) -> int {
  let fallback: int = 80
  if raw <= 0 { give fallback }
  give raw
}

proc main(args: list[string]) -> int {
  let port: int = normalize(0)
  give port
}

export *

Risks and diagnostics

Risk	Diagnostic signal	Action
Boundary drift	The chapter loses sight of symptom, reduced case, and verified correction.	Restate the boundary beside the first code block and every invalid case.
Generic prose	A paragraph would still be true in another chapter.	Replace it with a code-specific rule from this page.
Weak diagnostic	The failure does not point back to the chapter contract.	Reduce the invalid case until one failure explains the rule.

Review checklist

The first example is complete and aligned with Vitte docs syntax.
The production section names where this construct belongs in real code.
The risk table connects each failure to a diagnostic or review action.
The avoid list rejects broad misuse without adding quiz-like prompts.

Production use

Use this chapter when a code review needs to preserve symptom, reduced case, and verified correction.
Keep examples small enough to copy into fixtures or docs smoke tests.
Treat the invalid case as regression material for future docs checks.

What to avoid

Do not add a second topic that hides the chapter's main contract.
Do not expand examples by adding unrelated subsystems.
Do not rely on prose when a small Vitte block can show the rule.

Global explanation

Debugging should be taught as reduction and evidence. The useful move is to isolate a contract, reproduce the failure with the smallest still-meaningful case, and keep the path stable while you inspect it.

Invalid case

proc bad_port(raw: int) -> int {
  if raw { give 0 }
  give raw
}

This invalid case stays small so the broken contract remains visible.

Common pitfalls

Keeping bug reports too large to reason about.
Changing too many things before reproducing the failure.
Using tooling pages without a concrete failing story.

Short exercise

Reduce the invalid example to the smallest still-failing shape and write the contract it breaks.

Summary in 5 points

Debugging begins with reduction.
A stable failing scenario is better than a huge noisy one.
The contract matters more than the symptom wording.
Reproducibility is a technical asset.
Fixes should be explained against the reduced case.

Next best action

Keep the example small, reproduce it locally, then continue to the full chapter if you need the broader context.

Chapter deep dive

47. Reproducible debugging turns failure into a reproducible investigation. The chapter is written for a reader preserving evidence while narrowing scope.

The practical boundary is: symptom, reduced case, and verified correction. Keep that boundary in view while reading the example, the invalid case, and the exercise.

Role in the learning path

Debugging chapters often list tools without showing how a failure becomes reproducible and explainable.

One failure is reduced, reproduced, inspected, and fixed through a small stable scenario.

This chapter helps the reader move from vague bug reports to reproducible technical evidence.

Profile-specific deep dive

Repro boundary

Reduce the bug to one stable input and one observable output.
Name intermediate values that explain the failure path.
Keep the corrected case beside the failing shape.

Investigation record

Preserve the symptom, reduced case, and verified correction.
Do not replace evidence with speculation.
Regression coverage should protect the reduced case.

Reading the valid example

space demo/debug: names the ownership boundary before any behavior appears.
proc normalize_port(raw: int) -> int {: states the callable contract: inputs first, result shape last.
if raw < 0 { give 0 }: guards a failure or edge case before the nominal result.
if raw > 65535 { give 65535 }: guards a failure or edge case before the nominal result.
give raw: ends the local path with an explicit result.
}: supports the chapter contract without adding hidden behavior.
export *: supports the chapter contract without adding hidden behavior.

Lesson from the invalid example

proc bad_port(raw: int) -> int {: this line helps isolate the failure because it states the callable contract: inputs first, result shape last.
if raw { give 0 }: this line helps isolate the failure because it guards a failure or edge case before the nominal result.
give raw: this line helps isolate the failure because it ends the local path with an explicit result.
}: this line helps isolate the failure because it supports the chapter contract without adding hidden behavior.

Engineering decisions to preserve

Name the boundary before changing code: Debugging begins with reduction.
Keep the smallest example executable: A stable failing scenario is better than a huge noisy one.
Make the invalid path explain one failure only: The contract matters more than the symptom wording.
Prefer a visible contract over an implied convention: Reproducibility is a technical asset.
Leave a review anchor that another maintainer can verify: Fixes should be explained against the reduced case.

Context-specific review criteria

The page makes the symptom, reduced case, and verified correction boundary visible before the first code block.
The intended reader, a reader preserving evidence while narrowing scope, can follow the valid example through named contracts instead of memorized tokens.
The invalid example fails for the same reason the prose discusses.
The exercise extends the same contract instead of introducing an unrelated concept.
The next chapter can reuse the vocabulary introduced here without redefining it.
The chapter stays specific enough that its title materially changes the meaning of the page.
Every warning connects to a concrete code shape.
The summary leaves one durable engineering rule behind.

Contract matrix

Concern	Chapter rule	Evidence to keep
Ownership	Code belongs behind the boundary named by the chapter.	The chapter keeps ownership visible through symptom, reduced case, and verified correction.
Input contract	The procedure receives a shape that is named before branching.	The valid example names the accepted shape before branching.
Nominal path	The clean path remains readable without hidden state.	The successful result can be found without reading hidden state.
Failure path	The invalid case isolates one failure reason.	The broken example has one main reason to fail.
Naming	Names explain the domain rather than only the mechanism.	Names remain tied to the chapter goal.
Types	Types remove ambiguity from values and results.	Fields and return values carry domain meaning.
Control flow	Branches stay traceable from guard to result.	Guards appear before the result they protect.
Module boundary	The public surface stays smaller than implementation detail.	The public surface remains smaller than the implementation detail.
Diagnostic value	The failure path points back to the exact contract.	The invalid example points back to the exact contract.
Test value	Regression evidence covers one passing path and one failing path.	One passing case and one failing case cover the lesson.
Refactor value	Implementation cleanup preserves the result shape.	The result shape stays stable during local cleanup.
Publication value	The chapter leaves one concrete engineering rule.	The chapter leaves one concrete engineering rule.

Rewrite path for this chapter

Rewrite the opening paragraph so it names symptom, reduced case, and verified correction before naming syntax.
Keep the valid example small enough that the full contract fits on screen.
Move any broad claim back to a specific line in the example.
Preserve one invalid case that fails for the chapter's main reason.
Add one sentence explaining why the invalid case is not a random error.
Make every pitfall actionable by naming the code shape it damages.
Keep the exercise inside the same domain as the example.
Avoid introducing a second unrelated project just to show variety.
Use the summary to restate the chapter rule, not the table of contents.
Check that the next chapter can build on this vocabulary.
Remove any sentence that would still be true in every other chapter.
Keep the last action small, local, and testable.

Diagnostic anchors

The first inspected line is the one that declares the chapter's main contract.
The central type, field, procedure, or branch carries the chapter's main idea.
The invalid example includes a sentence-level explanation of its failure.
Refactors preserve the detail that would otherwise mislead a future reader.
The behavior that must stay stable is named before implementation changes begin.
Vague names are replaced before they become review friction.
Regression coverage protects the chapter's main contract.
Implementation details stay out of public API unless the chapter explicitly teaches that surface.
Beginner-facing diagnostics point to the contract, not to a random syntax detail.
The next chapter can assume one clearly named concept from this page.

When extending this chapter

Extend toward a reader preserving evidence while narrowing scope, not toward a broader catalog of features.
Add a second example only if it sharpens the same contract.
Prefer a small variant over a new subsystem.
Keep prose close to code; every abstract claim should point to a visible shape.
Do not hide a new concept in the exercise.
If a paragraph explains policy, add the concrete code boundary it protects.
If a paragraph explains syntax, add the semantic reason the syntax matters.
If a paragraph explains architecture, identify the owner of each boundary.
If a paragraph explains failure, keep the failing line close to the explanation.
Stop expanding when the chapter has one complete, testable lesson.

Failure modes to avoid

Keeping bug reports too large to reason about.
Changing too many things before reproducing the failure.
Using tooling pages without a concrete failing story.

Practice scenario

Start from the coherent example in 47. Reproducible debugging. Change one identifier, one guard, and one returned value. After each change, write down whether the public contract is still the same contract or a new one.

If the contract changed, update the type or result shape first. If only the implementation changed, keep the external name stable and add one regression note explaining what should not change again.

Before moving on

You can state the chapter role: turns failure into a reproducible investigation.
You can point to the main boundary: symptom, reduced case, and verified correction.
You can connect the invalid case to the problem statement: Debugging chapters often list tools without showing how a failure becomes reproducible and explainable.
You can perform the exercise: Reduce the invalid example to the smallest still-failing shape and write the contract it breaks.