Reuse narrowing checks

Use a const when a condition needs a clear name. Use a type predicate when several functions need the same check.

Name a condition#

Bind the check with const, then branch on that name:

fn label(value: string | int) -> string {
  const kind = type_of(value)
  const is_text = kind == "string"
  if is_text {
    return value.upper()
  }
  return to_string(value + 1)
}

Use const, not let. A mutable value may change after the check.

Name the parameter and its narrower type after the return arrow:

fn is_text(value: unknown) -> value is string {
  return type_of(value) == "string"
}

Call the helper in an if, guard, or another condition:

fn normalize(value: string | int) -> string {
  if is_text(value) {
    return value.upper()
  }
  return to_string(value)
}

A false result also rules out string when the input is a closed union.

Add implies when false does not rule out the type:

fn is_nonempty_text(value: unknown) -> implies value is string {
  return type_of(value) == "string" && len(value) > 0
}

Here, false may mean an empty string. Harn narrows only the true branch.

Keep the helper body simple. It may contain const aliases followed by one return condition. Harn reports HARN-TYP-029 when the condition does not prove the declared contract.

See Type annotations for the full rules.

Reuse narrowing checks

Name a condition#

Share a two-sided check#

Share a one-sided check#