Why must stream functions have exactly one parameter in EK9?

← Streams and Pipelines · Ref: Q824

EK9 stream operators like filter, uniq, sort, and map require functions with exactly one parameter matching the stream element type.

THE RULE

Stream pipeline functions must have a single parameter:

  isPositive() as pure
    -> amount as Integer       // ONE parameter: correct
    <- rtn as Boolean: amount > 0

A function with zero or multiple parameters triggers E07460:

  isInRange() as pure
    -> lo as Integer           // TWO parameters: wrong
    -> hi as Integer
    <- rtn as Boolean: true

WHY THIS RULE

Stream operators call the function once per element, passing the current element as the single argument. A function with 0 or 2+ parameters cannot receive a single stream element.

HOW TO FIX

- Use a single parameter matching the stream type
- If you need additional context, use a dynamic function that captures extra values

See Q812 for stream errors. See Q126 for stream reference.

Example

defines module qa.streams.function.arity

  defines function

    isPositive() as pure
      -> amount as Integer
      <- rtn as Boolean: amount > 0

    isInRange() as pure
      ->
        lowerBound as Integer
        upperBound as Integer
      <- rtn as Boolean: lowerBound < upperBound

  defines program

    StreamFunctionArityDemo()
      stdout <- Stdout()

      // === CORRECT: filter with single-parameter predicate ===
      cat [3, -1, 5, -2, 8]
        | filter by isPositive
        > stdout

Common mistakes

E07460 — Stream uniq requires a function with exactly one parameter matching the stream type. isInRange has two parameters so cannot receive a single stream element. See ek9 -h E07460 for details.

Incorrect:

        | uniq isInRange

Correct:

        | filter by isPositive

Coming from another language?

Java: Stream.filter() takes Predicate<T> (single arg). Python: filter(func, iterable) passes one item. Rust: .filter(|x| ...) takes one reference. Go: no built-in stream, but range gives one item. EK9: compile-time enforcement that stream functions have exactly one parameter.

Keywords: sort, uniq, function, single, stream, arity, filter, parameter, E07460