How does EK9 detect conditions that are always true or always false?

← Code Quality · Ref: Q735

EK9 tracks variable assignments through code paths and detects when a condition is guaranteed to produce the same result every time.

CONDITION ALWAYS TRUE (E08086)

If you assign a literal and then compare to the same value, the compiler knows the result. 'x <- 5; if x == 5' is always true. The else branch is dead code. Also detected when an outer condition implies an inner one: 'if count > 10; if count > 6' is always true since count > 10 implies count > 6.

CONDITION ALWAYS FALSE (E08087)

Similarly, 'x <- 5; if x == 10' is always false. The if body is dead code. Also detected for contradictory ranges: 'if count > 10 and count < 3' is always false.

WHEN TRACKING RESETS

The compiler stops tracking when a variable is reassigned from a function call or mutated by a non-pure call, since the value becomes unknown.

See Q557 for dead code detection overview. See Q558 for tautological conditions.

Example

defines module qa.codequality.conditiontruth

  defines function

    classifyScore() as pure
      -> score as Integer
      <- label as String: "average"

      highThreshold <- 90
      lowThreshold <- 40

      if score >= highThreshold
        label: "excellent"
      else if score < lowThreshold
        label: "poor"

    categorizeRange() as pure
      -> measurement as Integer
      <- category as String: "normal"

      upperBound <- 100
      lowerBound <- 10

      if measurement > upperBound
        category: "above"
      else if measurement < lowerBound
        category: "below"

  defines program

    ConditionTruthDemo()
      stdout <- Stdout()

      stdout.println(classifyScore(95))
      stdout.println(classifyScore(30))
      stdout.println(classifyScore(60))

      stdout.println(categorizeRange(150))
      stdout.println(categorizeRange(5))
      stdout.println(categorizeRange(50))

Common mistakes

E50060 — String has no toUpperCase() method in EK9. Use upperCase() instead. See ek9 -h E50060 for details.

Incorrect:

stdout.println(classifyScore(95).toUpperCase())

Correct:

stdout.println(classifyScore(95))

E50060 — String has no toUpperCase() method in EK9. Use upperCase() instead. See ek9 -h E50060 for details.

Incorrect:

stdout.println(categorizeRange(150).toUpperCase())

Correct:

stdout.println(categorizeRange(150))

Coming from another language?

Java: SpotBugs detects some constant conditions. C/C++: compiler warnings for tautological comparisons with -Wtautological-compare. Rust: clippy detects some always-true conditions. Go: go vet detects some unreachable code. EK9: mandatory compiler error with full flow-sensitive tracking across branches.

Keywords: E08087, E08086, dead, always, true, code, false, condition, quality, tracking, flow, constant