Keyword trait

Definition

trait is a declaration keyword that changes the shape of a module, type, or executable contract.

The keyword trait defines a program shape: procedure, type, variant, entry point, namespace, or another structural boundary. It should therefore be read architecturally before it is read locally.

Grammatical role

Introduces a behavior or interface contract intended to be satisfied by other forms.

This grammatical role is essential: if a reader understands the structural place of trait, they already understand much of the diagnostics that will appear when it is moved or truncated.

Canonical syntax

Canonical form: `trait Name { ... }`.

The canonical form matters because it gives the compiler and the reader the same reference structure. A large share of diagnostics related to trait come from an abbreviated, displaced, or incomplete form.

Detailed semantics

Semantically, trait changes the shape of the program before execution even begins. It introduces an entity that other blocks will name, call, instantiate, or reference.

In an encyclopedic reading, trait should not be reduced to a dictionary definition. Its effect on scope, block shape, value visibility, control progression, and the diagnostic family it activates when misused must also be considered.

Effect on execution

trait acts first on the shape of the program. Its main effect appears in the entities it makes callable, instantiable, or visible during execution.

In other words, the presence of trait is not merely syntactic: it helps the reader predict what will be executed, produced, exposed, or forbidden from this point in the program.

Valid variants

• `trait Name { ... }`.

These variants are not free synonyms. They indicate the legitimate forms from which one can reason about diagnostics, scope differences, or contract readability.

Vitte example

trait demo_contract {
  # adapt this declaration to the concrete construct
}

This example shows trait in a nominal context. It should be read globally: where the contract begins, which values are constrained, which output becomes observable, and why the presence of the keyword is justified.

Guided reading of the example

1. First locate the full construction that contains trait, not the isolated word.
2. Then identify which contract becomes visible because of trait: type, branch, binding, module, exit, or advanced boundary.
3. Finish by checking the observable effect produced by the construction that contains trait.
4. For a declaration keyword, verify which stable entity is created and how it will be referenced later.

This guided reading is intentionally closer to a reference page than to a tutorial: it helps reconstruct the exact role of trait in a complete block.

Comparison with C

/* C comparison: declaration shape is usually expressed with a different surface. */

For this keyword, the parallel with C remains approximate. The comparison mainly indicates that in C the same idea is often spread across file conventions, operators, or less explicit control structures.

The source of truth remains Vitte grammar and semantics. The comparison with C should be read as a cultural marker, not as a parallel specification.

Recommended uses

trait deserves to appear when it simplifies the reading of the block's global contract, not when it merely adds one more surface form.

When to use it

• When trait makes the block contract more explicit at first reading.
• When it reduces the number of implicit assumptions the reader must reconstruct mentally.
• When the program must introduce a stable entity that will be reused elsewhere.

When to avoid it

• Avoid trait when another, more precise keyword already carries the block's intent.
• Avoid trait when it adds only surface noise without clarifying the contract.
• Avoid reading or teaching it as an isolated token with no relation to the full structure.

Common pitfalls

• Using trait in a grammatical layer where it does not belong.
• Confusing the role of the keyword with the role of the full surrounding block.
• Showing only the nominal form and never how the contract fails.

Invalid example and diagnostic

trait

The declaration surface is incomplete or misplaced.

The counter-example is not merely wrong: it is wrong in an instructive way. It shows which grammar or execution-contract assumption is no longer accepted when trait is moved, truncated, or combined with the wrong context. Concretely, the declaration is incomplete or moved into an invalid grammatical layer.

A good encyclopedic counter-example does not show arbitrarily broken code: it isolates the precise reason why trait can no longer support the expected contract. Its teaching value is diagnostic before it is syntactic.

Common compilation errors

This table does not replace the compiler's exact diagnostics. It serves as a mental map: when trait fails, the problem usually comes from an invalid grammatical form, an incoherent type contract, or an incomplete construction.

Neighbor keywords

Comparison with neighboring keywords is essential on a wiki-style page: trait is better understood when one knows precisely what it does not do.

Common misreadings

• Reducing trait to a local token instead of reading it as part of a full construction.
• Explaining only the syntax and forgetting the reading or diagnostic contract it imposes.

Implementation and diagnostic notes

• Useful diagnostics for this family often concern incomplete signatures, constituent ordering, or declarative scope.
• In a compiler, these keywords primarily feed symbol tables and the structural representation of the program.

Presence in the book

• docs/book/chapters/06-procedures.html
• docs/book/chapters/09-modules.html
• docs/book/chapters/27-grammar.html

See also

• docs/book/chapters/keywords/erreurs-compilateur.html
• docs/book/chapters/keywords/type.html
• docs/book/glossary.html