Why does EK9 flag checking empty on a freshly constructed list?

← Code Quality · Ref: Q865

EK9 uses data flow analysis to detect redundant empty checks. A freshly created collection is always empty, so checking 'if items empty' immediately after creation is a tautology — the condition is always true.

REDUNDANT (E08092)

  items <- List() of String
  if items empty           // ALWAYS true — nothing was added
    result: "nothing here"

VALID CHECK

  items <- List() of String
  items += "hello"
  if items empty           // valid — state changed since creation
    result: "nothing here"

Adding items between creation and the empty check makes the check meaningful because the collection state is no longer provably empty.

See Q820 for redundant empty check overview. See Q310 for code quality.

Example

defines module qa.quality.redundant.empty

  defines function

    checkEmptiness()
      -> greeting as String
      <- result as String: String()

      items <- List() of String
      items += greeting
      if items empty
        result: "nothing here"
      else
        result: "has content"

  defines program

    RedundantEmptyDemo()
      stdout <- Stdout()

      outcome <- checkEmptiness("hello")
      stdout.println(`Result: ${outcome}`)

Common mistakes

E08092 — Without adding any items, the list is provably empty. Checking 'if items empty' is always true — the compiler detects this tautology. Add items before checking. See ek9 -h E08092 for details.

Incorrect:

      if items empty

Correct:

      items += greeting
      if items empty

Coming from another language?

Java: no detection — isEmpty() on new ArrayList compiles silently. Python: no detection. Rust: no detection. Go: no detection. EK9: compile-time error for provably redundant empty checks via data flow analysis.

Keywords: empty, flow, list, collection, redundant, E08092, data, tautology