Create a dynamic function that captures a greeting and prepends it to any input.

Define an abstract function type, then extend it dynamically:

  Greeter as abstract
    -> name as String
    <- message as String?

  greet <- (greeting: greetingText) extends Greeter as function
    message: `${greeting} ${name}`

Use 'as abstract' on the function definition. Dynamic functions use 'extends TypeName as function'. Named capture (greeting: greetingText) copies greetingText into field 'greeting'.

See Q1125 for dynamic function capture. See Q1126 for dynamic classes.

Example

defines module qa.advancedtypes.dynamicfunctioncorrect

  defines function

    Greeter as abstract
      -> name as String
      <- message as String?

  defines program

    DynamicFunctionCorrectDemo()
      stdout <- Stdout()

      greetingText <- "Hello"

      //Create dynamic function with named capture
      greet <- (greeting: greetingText) extends Greeter as function
        message: `${greeting} ${name}`

      stdout.println(greet("Alice"))
      stdout.println(greet("Bob"))

      //Another dynamic function with different captured state
      farewellText <- "Goodbye"
      farewell <- (farewell: farewellText) extends Greeter as function
        message: `${farewell} ${name}`

      stdout.println(farewell("Charlie"))

Other ways to ask this

Write code for a closure that captures a local variable as state
I need a dynamic function extending an abstract function type with named capture
In JavaScript I'd use a closure over a variable. Write the EK9 dynamic function equivalent
Build a Greeter dynamic function that captures a greeting string and prepends it to input

Coming from another language?

JavaScript: const greet = (name) => greeting + name (closure). Java: Function<String,String> greet = name -> greeting + name. Python: lambda name: greeting + name. EK9: (field: source) extends AbstractType as function.

Keywords: function, greeting, capture, extends, closure, abstract, dynamic