Keyword not

Definition

not is an operator keyword that changes how values are combined, tested, or traversed.

The keyword not participates in a relation between values or in the logical reading of an expression. It should not be read in isolation, but as an operator that changes the meaning of the whole expression.

Grammatical role

Inverts the logical polarity of a boolean expression.

This grammatical role is essential: if a reader understands the structural place of not, they already understand much of the diagnostics that will appear when it is moved or truncated.

Canonical syntax

Canonical form: `not value`.

The canonical form matters because it gives the compiler and the reader the same reference structure. A large share of diagnostics related to not come from an abbreviated, displaced, or incomplete form.

Detailed semantics

Semantically, not only makes sense with its operands and the surrounding form. The full expression must be read to understand the logical or structural contract it imposes.

In an encyclopedic reading, not 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

not acts at expression level by changing the logical or structural result computed at that exact point.

In other words, the presence of not 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

• `not value`.

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

proc invert(a: bool) -> bool {
  give not a
}

This example shows not 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 not, not the isolated word.
2. Then identify which contract becomes visible because of not: type, branch, binding, module, exit, or advanced boundary.
3. Finish by checking the observable effect produced by the construction that contains not.
4. For an operator keyword, reread the full expression to avoid an overly local interpretation.

This guided reading is intentionally closer to a reference page than to a tutorial: it helps reconstruct the exact role of not in a complete block.

Comparison with C

/* C comparison: this role is usually expressed with operators or explicit loops and conditionals. */

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

not 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 not makes the block contract more explicit at first reading.
• When it reduces the number of implicit assumptions the reader must reconstruct mentally.
• When a logical or structural relation must be read as an integral part of the expression.

When to avoid it

• Avoid not when another, more precise keyword already carries the block's intent.
• Avoid not 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 not 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

proc bad_not() -> int {
  give not
}

The operator surface is incomplete because it is missing the operands or the valid surrounding construct.

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 not is moved, truncated, or combined with the wrong context. Concretely, the logical inversion does not receive the expression it is supposed to invert.

A good encyclopedic counter-example does not show arbitrarily broken code: it isolates the precise reason why not 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 not 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: not is better understood when one knows precisely what it does not do.

Common misreadings

• Reducing not 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

• The most useful diagnostics signal missing operands, an incompatible value category, or a reading ambiguity.
• In the front-end, these keywords influence expression hierarchy and sometimes precedence rules.

Presence in the book

• docs/book/chapters/05-types.html
• docs/book/chapters/07-control.html
• docs/book/chapters/27-grammar.html

See also

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