13-generics

Level: beginner · Reading time: 19 min · Prerequisite: book/chapters/12-pointers.html · Track: essential · Maturity: reviewed · Last review: 2026-05-09

TL;DR (5 lines)

Generics justify themselves through reusable contracts.
The reader should see the gain, not only the syntax.
A tiny but real reusable helper is the best teaching unit.
Malformed abstractions fail at the contract boundary.
Use abstraction when it clarifies, not when it decorates.

Frequent mistakes

Adding type parameters with no real reuse benefit.
Teaching generic call syntax before generic design intent.
Mixing valid generic syntax with invalid return contracts.

Prerequisites: book/chapters/12-pointers.html. See also: book/chapters/12-pointers.html, book/chapters/27-grammar.html, book/chapters/31-build-errors.html.

Concrete Problem

Generic pages often show parameter syntax without explaining why a reusable contract is worth the extra abstraction.

Red Thread (Single Project)

One reusable container-like helper works for multiple element shapes while keeping the contract readable.

For what

This chapter helps the reader use abstraction only when reuse really needs it.

Work in this chapter

You will inspect one generic helper, then compare it to a malformed or unnecessary abstraction.

Coherent example

space demo/generics

proc identity[T](value: T) -> T {
  give 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/generics

proc identity[T](value: T) -> T {
  give value
}

export *

Reusable value wrapper

This generic code keeps the input and output type identical.

space examples/generics/value

proc keep[T](value: T) -> T {
  give value
}

proc main(args: list[string]) -> int {
  let code: int = keep[int](7)
  give code
}

export *

Generic pair shape

This variant gives abstraction a concrete data shape.

space examples/generics/pair

form Pair[T] {
  left: T
  right: T
}

proc first[T](pair: Pair[T]) -> T {
  give pair.left
}

proc main(args: list[string]) -> int {
  let pair: Pair[int] = Pair[int] { left: 1, right: 2 }
  give first[int](pair)
}

export *

Immediate work for this chapter

Show the concrete reuse payoff before introducing type parameters.
Keep input and output type contracts consistent.
Avoid generic examples that only rename `int` without adding reuse.
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: 13-generics.html

Dedicated problem

13-generics needs a concrete production-grade anchor around specific operation, reusable shape, and concrete call site. 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

13-generics chapter anchor

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

space demo/generics

proc identity[T](value: T) -> T {
  give value
}

export *

Reusable value wrapper

This generic code keeps the input and output type identical.

space examples/generics/value

proc keep[T](value: T) -> T {
  give value
}

proc main(args: list[string]) -> int {
  let code: int = keep[int](7)
  give code
}

export *

Generic pair shape

This variant gives abstraction a concrete data shape.

space examples/generics/pair

form Pair[T] {
  left: T
  right: T
}

proc first[T](pair: Pair[T]) -> T {
  give pair.left
}

proc main(args: list[string]) -> int {
  let pair: Pair[int] = Pair[int] { left: 1, right: 2 }
  give first[int](pair)
}

export *

Risks and diagnostics

Risk	Diagnostic signal	Action
Boundary drift	The chapter loses sight of specific operation, reusable shape, and concrete call site.	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 specific operation, reusable shape, and concrete call site.
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

A generics chapter should make the reuse payoff explicit. Weak payoff means the generic surface is probably too abstract for the reader and the code.

Invalid case

proc bad_identity[T](value: T) -> int {
  give value
}

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

Common pitfalls

Adding type parameters with no real reuse benefit.
Teaching generic call syntax before generic design intent.
Mixing valid generic syntax with invalid return contracts.

Short exercise

Add one second generic procedure that reuses the same abstraction for a clearly different value shape.

Summary in 5 points

Generics justify themselves through reusable contracts.
The reader should see the gain, not only the syntax.
A tiny but real reusable helper is the best teaching unit.
Malformed abstractions fail at the contract boundary.
Use abstraction when it clarifies, not when it decorates.

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

13-generics asks what reuse is worth paying for. The chapter is written for a reader avoiding abstraction without a visible payoff.

The practical boundary is: specific operation, reusable shape, and concrete call site. Keep that boundary in view while reading the example, the invalid case, and the exercise.

Role in the learning path

Generic pages often show parameter syntax without explaining why a reusable contract is worth the extra abstraction.

One reusable container-like helper works for multiple element shapes while keeping the contract readable.

This chapter helps the reader use abstraction only when reuse really needs it.

Profile-specific deep dive

Reuse boundary

Type parameters should preserve a relationship between input and output.
The call site should show at least one concrete instantiation.
Generic code should remove duplication without hiding meaning.

Abstraction cost

Add a generic only when two concrete uses share the same real contract.
Keep constraints near the operation that needs them.
Invalid generic examples should break the reusable promise directly.

Reading the valid example

space demo/generics: names the ownership boundary before any behavior appears.
proc identity[T](value: T) -> T {: states the callable contract: inputs first, result shape last.
give value: 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_identity[T](value: T) -> int {: this line helps isolate the failure because it states the callable contract: inputs first, result shape last.
give value: 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: Generics justify themselves through reusable contracts.
Keep the smallest example executable: The reader should see the gain, not only the syntax.
Make the invalid path explain one failure only: A tiny but real reusable helper is the best teaching unit.
Prefer a visible contract over an implied convention: Malformed abstractions fail at the contract boundary.
Leave a review anchor that another maintainer can verify: Use abstraction when it clarifies, not when it decorates.

Context-specific review criteria

The page makes the specific operation, reusable shape, and concrete call site boundary visible before the first code block.
The intended reader, a reader avoiding abstraction without a visible payoff, 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 specific operation, reusable shape, and concrete call site.
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 specific operation, reusable shape, and concrete call site 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 avoiding abstraction without a visible payoff, 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

Adding type parameters with no real reuse benefit.
Teaching generic call syntax before generic design intent.
Mixing valid generic syntax with invalid return contracts.

Practice scenario

Start from the coherent example in 13-generics. 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: asks what reuse is worth paying for.
You can point to the main boundary: specific operation, reusable shape, and concrete call site.
You can connect the invalid case to the problem statement: Generic pages often show parameter syntax without explaining why a reusable contract is worth the extra abstraction.
You can perform the exercise: Add one second generic procedure that reuses the same abstraction for a clearly different value shape.