14. Macros

Level: beginner · Reading time: 19 min · Prerequisite: book/chapters/13-generiques.html · Track: essential · Maturity: reviewed · Last review: 2026-05-09

TL;DR (5 lines)

Macros should pay their readability cost.
A bounded use case teaches better than a flashy demo.
Generated surface still needs a clear contract.
Debuggability matters as much as expressiveness.
Macro pages should teach restraint, not only power.

Frequent mistakes

Using macros to hide ordinary control flow.
Expanding syntax when a procedure would stay clearer.
Skipping the readability and debugging cost of generated surfaces.

Prerequisites: book/chapters/13-generiques.html. See also: book/chapters/13-generiques.html, book/chapters/27-grammar.html, book/chapters/31-build-errors.html.

Concrete Problem

Macro chapters become misleading when they present expansion power without naming the readability cost.

Red Thread (Single Project)

One small macro-like helper exists to remove duplication while preserving a stable public shape.

For what

This chapter helps the reader keep macro use intentional and bounded.

Work in this chapter

You will inspect one use case where a macro-like surface is justified, then compare it to a noisy misuse.

Coherent example

space demo/macros

macro trace_value(name, value) {
  emit name
  emit value
}

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/macros

macro trace_value(name, value) {
  emit name
  emit value
}

export *

Bounded generated surface

This macro-like block emits only the repeated surface it owns.

space examples/macros/trace

macro emit_trace(name, code) {
  emit name
  emit code
}

proc run() -> int {
  give 0
}

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

export *

Prefer procedure when logic is ordinary

This example keeps normal behavior in procedures and reserves generation for shape.

space examples/macros/restraint

proc trace_status(code: int) -> int {
  if code != 0 { give code }
  give 0
}

proc main(args: list[string]) -> int {
  give trace_status(0)
}

export *

Immediate work for this chapter

Show the generated surface and the readability cost.
Prefer a procedure when ordinary control flow is enough.
Keep macro examples bounded to one repeated shape.
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: 14-macros.html

Dedicated problem

14. Macros needs a concrete production-grade anchor around input pattern, generated surface, and reviewable output. 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

14. Macros chapter anchor

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

space demo/macros

macro trace_value(name, value) {
  emit name
  emit value
}

export *

Bounded generated surface

This macro-like block emits only the repeated surface it owns.

space examples/macros/trace

macro emit_trace(name, code) {
  emit name
  emit code
}

proc run() -> int {
  give 0
}

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

export *

Prefer procedure when logic is ordinary

This example keeps normal behavior in procedures and reserves generation for shape.

space examples/macros/restraint

proc trace_status(code: int) -> int {
  if code != 0 { give code }
  give 0
}

proc main(args: list[string]) -> int {
  give trace_status(0)
}

export *

Risks and diagnostics

Risk	Diagnostic signal	Action
Boundary drift	The chapter loses sight of input pattern, generated surface, and reviewable output.	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 input pattern, generated surface, and reviewable output.
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

Macros should be taught as controlled power tools. The important criterion is whether generation preserves or destroys the clarity of the resulting program surface.

Invalid case

macro broken {
  give 0
}

This invalid case is intentionally small. It exists to isolate the contract failure that the chapter is trying to teach.

Common pitfalls

Using macros to hide ordinary control flow.
Expanding syntax when a procedure would stay clearer.
Skipping the readability and debugging cost of generated surfaces.

Short exercise

Take one repetitive pattern from another chapter and argue whether it deserves a macro or a plain procedure.

Summary in 5 points

Macros should pay their readability cost.
A bounded use case teaches better than a flashy demo.
Generated surface still needs a clear contract.
Debuggability matters as much as expressiveness.
Macro pages should teach restraint, not only power.

Next best action

Extend the coherent example by one small, justified step and keep the same contract visible from input to output.

Chapter deep dive

14. Macros treats generation as a contract, not a shortcut. The chapter is written for a reader deciding whether generated code preserves clarity.

The practical boundary is: input pattern, generated surface, and reviewable output. Keep that boundary in view while reading the example, the invalid case, and the exercise.

Role in the learning path

Macro chapters become misleading when they present expansion power without naming the readability cost.

One small macro-like helper exists to remove duplication while preserving a stable public shape.

This chapter helps the reader keep macro use intentional and bounded.

Profile-specific deep dive

Generation boundary

Macros should generate a narrow, reviewable surface.
The generated shape should be easier to inspect than duplicated hand-written code.
Ordinary logic should stay in procedures unless generation changes the structure.

Debuggability

The macro call should make the generated intent visible.
Diagnostics should point back to the input pattern or generated surface.
Avoid examples where macro power hides the actual control path.

Reading the valid example

space demo/macros: names the ownership boundary before any behavior appears.
macro trace_value(name, value) {: supports the chapter contract without adding hidden behavior.
emit name: supports the chapter contract without adding hidden behavior.
emit value: supports the chapter contract without adding hidden behavior.
}: supports the chapter contract without adding hidden behavior.
export *: supports the chapter contract without adding hidden behavior.

Lesson from the invalid example

macro broken {: this line helps isolate the failure because it supports the chapter contract without adding hidden behavior.
give 0: 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: Macros should pay their readability cost.
Keep the smallest example executable: A bounded use case teaches better than a flashy demo.
Make the invalid path explain one failure only: Generated surface still needs a clear contract.
Prefer a visible contract over an implied convention: Debuggability matters as much as expressiveness.
Leave a review anchor that another maintainer can verify: Macro pages should teach restraint, not only power.

Context-specific review criteria

The page makes the input pattern, generated surface, and reviewable output boundary visible before the first code block.
The intended reader, a reader deciding whether generated code preserves clarity, 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 input pattern, generated surface, and reviewable output.
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 input pattern, generated surface, and reviewable output 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 deciding whether generated code preserves clarity, 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

Using macros to hide ordinary control flow.
Expanding syntax when a procedure would stay clearer.
Skipping the readability and debugging cost of generated surfaces.

Practice scenario

Start from the coherent example in 14. Macros. 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: treats generation as a contract, not a shortcut.
You can point to the main boundary: input pattern, generated surface, and reviewable output.
You can connect the invalid case to the problem statement: Macro chapters become misleading when they present expansion power without naming the readability cost.
You can perform the exercise: Take one repetitive pattern from another chapter and argue whether it deserves a macro or a plain procedure.