How do I pass a function as a parameter in EK9?

← Functions and Methods · Ref: Q942

EK9 supports first-class functions. Define an abstract function as the parameter type, then pass a concrete function that matches its signature.

DEFINE THE FUNCTION TYPE

An abstract function acts as a type signature:

  Transformer() as pure abstract
    -> incoming as Integer
    <- rtn as Integer

ACCEPT IT AS A PARAMETER

  applyTransform()
    ->
      value as Integer
      fn as Transformer
    <- answer as Integer: fn(value)

PASS A MATCHING FUNCTION

  doubleIt() is Transformer as pure
    -> incoming as Integer
    <- rtn as Integer: incoming * 2

  answer <- applyTransform(5, doubleIt)

The concrete function must match the abstract function's signature exactly. If it does not, the compiler raises E07480.

STREAMS USE THIS PATTERN

Stream operations like 'filter by' and 'map with' accept function parameters internally. The predicate you pass to filter must match the expected signature.

See Q597 for function parameters. See Q601 for dynamic functions. See Q596 for function vs method.

Example

defines module qa.functions.asparameter

  defines function

    Transformer() as pure abstract
      -> incoming as Integer
      <- rtn as Integer?

    doubleIt() is Transformer as pure
      -> incoming as Integer
      <- rtn as Integer: incoming * 2

    tripleIt() is Transformer as pure
      -> incoming as Integer
      <- rtn as Integer: incoming * 3

    applyTransform()
      ->
        operand as Integer
        fn as Transformer
      <- answer as Integer: fn(operand)

  defines program

    FunctionParamDemo()
      stdout <- Stdout()

      doubled <- applyTransform(7, doubleIt)
      stdout.println(`7 doubled: ${doubled}`)

      tripled <- applyTransform(7, tripleIt)
      stdout.println(`7 tripled: ${tripled}`)

Common mistakes

E06270 — Function parameters expect a matching function delegate, not a String literal — pass the function name (doubleIt) without quotes so the argument types match. See ek9 -h E06270 for details.

Incorrect:

applyTransform(7, "doubleIt")

Correct:

applyTransform(7, doubleIt)

Coming from another language?

Java: Functional interfaces (Function, Predicate, Consumer). Python: functions are objects, pass directly. Rust: Fn/FnMut/FnOnce trait bounds. Go: func types as parameters. Kotlin: lambda and function references. EK9: abstract functions as type signatures, concrete functions with 'is' relationship.

Keywords: delegate, E07480, abstract, pass, higher-order, first-class, parameter, function