Keyword when

Definition

when is a control-flow keyword that changes the program's execution path.

The keyword when structures execution. It determines when a block opens, when a branch is chosen, when a loop continues or stops, or when an exit becomes observable.

Grammatical role

Introduces a specialized condition inside a selection construction.

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

Canonical syntax

Canonical form: `when condition { ... }` inside a selection surface.

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

Detailed semantics

Semantically, when reorganizes the execution path. The right question is not only 'where is it placed?' but 'which path becomes possible, impossible, or preferred from this point onward?'

In an encyclopedic reading, when 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

when immediately changes the execution path, whether by choosing a branch, repeating, interrupting, or terminating a flow.

In other words, the presence of when 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

• `when condition { ... }` inside a selection surface.

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 clamp_low(x: int) -> int {
  if x < 0 { give 0 }
  give x
}

This example shows when 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 when, not the isolated word.
2. Then identify which contract becomes visible because of when: type, branch, binding, module, exit, or advanced boundary.
3. Finish by checking the observable effect produced by the construction that contains when.
4. For a control keyword, mentally reconstruct the possible paths and the ones that become impossible.

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

Comparison with C

int clamp_low(int x) {
  if (x < 0) return 0;
  return x;
}

This C comparison is structural: it aligns the role of the keyword with a familiar surface without claiming that the two languages carry exactly the same contracts.

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

when 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 when makes the block contract more explicit at first reading.
• When it reduces the number of implicit assumptions the reader must reconstruct mentally.
• When a branch choice, repetition, or flow exit must be made visible.

When to avoid it

• Avoid when when another, more precise keyword already carries the block's intent.
• Avoid when 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 when 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_branch(x: int) -> int {
  if x { give 0 }
  give x
}

The branch contract is broken because the condition surface is not valid.

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 when is moved, truncated, or combined with the wrong context. Concretely, the specialized condition does not appear inside the selection surface that gives it meaning.

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

Common misreadings

• Reducing when 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 usually signal an incomplete branch shape, a mistyped condition, or an illegal placement in control flow.
• In a compiler, these keywords directly influence control-flow graph construction or intermediate representation building.

Presence in the book

• docs/book/chapters/07-control.html
• docs/book/chapters/27-grammar.html
• docs/book/chapters/31-build-errors.html

See also

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