Write a pure validation function that checks if an age is between 0 and 150.

Define a pure function with a Boolean return:

  isValidAge() as pure
    -> age as Integer
    <- rtn as Boolean: age >= 0 and age <= 150

Pure functions cannot modify parameters or access external state. They are ideal for validation logic because they are testable, composable, and safe to use in stream pipelines. See Q596 for function vs method. See Q568 for pure basics.

Example

defines module qa.functionsandmethods.validation

  defines function

    isValidAge() as pure
      -> age as Integer
      <- rtn as Boolean?
      maxAge <- 150
      rtn: age >= 0 and age <= maxAge

    isValidPercentage() as pure
      -> pct as Integer
      <- rtn as Boolean?
      maxPercent <- 100
      rtn: pct >= 0 and pct <= maxPercent

    isNonBlank() as pure
      -> text as String
      <- rtn as Boolean: text? and text.length() > 0

  defines program

    ValidationDemo()
      stdout <- Stdout()

      // Validate ages
      ages <- [25, -1, 0, 150, 151, 42]
      for age in ages
        if isValidAge(age)
          stdout.println(`Age ${age}: valid`)
        else
          stdout.println(`Age ${age}: invalid`)

      // Validate percentages
      scores <- [0, 50, 100, 101, -5]
      for score in scores
        if isValidPercentage(score)
          stdout.println(`Score ${score}: valid`)
        else
          stdout.println(`Score ${score}: invalid`)

      // Validate strings
      names <- ["Alice", "", "Bob"]
      for name in names
        if isNonBlank(name)
          stdout.println(`Name '${name}': valid`)
        else
          stdout.println(`Name '${name}': blank`)

Common mistakes

E01072 — EK9 has no return statement; assign the result to the declared return variable instead. See ek9 -h E01072 for details.

Incorrect:

return age >= 0 and age <= maxAge

Correct:

rtn: age >= 0 and age <= maxAge

Other ways to ask this

Write code to validate a numeric range using a pure function
I have an age value and need to verify it falls within a valid range
Given an integer, produce a Boolean indicating whether it is a valid age
In Java I'd write a static boolean isValidAge(int age). Write the EK9 equivalent

Coming from another language?

Java: static boolean isValidAge(int age) { return age >= 0 && age <= 150; }. Python: def is_valid_age(age): return 0 <= age <= 150. Rust: fn is_valid_age(age: i32) -> bool. EK9: isValidAge() as pure -> age as Integer <- rtn as Boolean.

Keywords: predicate, age, range, validation, pure, check, function, boolean