Unused Closure Parameter

ID

swift.unused_closure_parameter

Severity

low

Remediation Complexity

medium

Remediation Risk

low

Remediation Effort

medium

Resource

Code Smell

Language

Swift

Tags

code-smell, suspicious-code

Description

Reports a named closure parameter whose name is never referenced from inside the closure body. The closure would behave identically with the parameter renamed to the wildcard _, which already signals "intentionally ignored" and avoids misleading the reader into thinking the binding matters.

[1, 2, 3].map { x, y in            // FLAW on `y`
    x * 2
}

[1, 2, 3].map { _, y in            // OK — explicitly ignored
    y * 2
}

Rationale

A named parameter is a contract with the reader: "this value is used below." When the body never reads it, that contract is broken — the name becomes noise that survives refactors and obscures what the closure actually consumes. Swift offers _ as the conventional "this position is required by the closure signature but is not consumed" marker, and code review should require its use.

References from inside a nested closure that re-declares the same name are not counted — a parameter that is only "used" by a shadow binding is still reported as unused.

Remediation

Rename the unused parameter to _ to keep the closure signature arity while making the intent explicit:

[1, 2, 3].map { _, y in
    y * 2
}

[1, 2, 3].forEach { _ in
    print("tick")
}

References