Keyword for

Definition

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

The keyword for 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 structured iteration loop in which a local variable successively receives the elements of a source.

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

Canonical syntax

Canonical form: `for value in values { ... }`.

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

Detailed semantics

Semantically, for 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, for 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

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

In other words, the presence of for 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

• `for value in values { ... }`

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 sum_all(values: list[int]) -> int {
  let acc: int = 0
  for value in values {
    set acc = acc + value
  }
  give acc
}

This example shows for 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 for, not the isolated word.
2. Then identify which contract becomes visible because of for: type, branch, binding, module, exit, or advanced boundary.
3. Finish by checking the observable effect produced by the construction that contains for.
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 for in a complete block.

Comparison with C

/* C comparison: use an explicit loop and keep the branch purpose visible. */

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

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

The control-flow surface is broken because the keyword is not used in a valid loop context.

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 for is moved, truncated, or combined with the wrong context. Concretely, the iteration loop loses either its variable, its source, or its block.

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

Common misreadings

• Reducing for 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/loop.html
• docs/book/glossary.html