Write a function that returns a set or unset String depending on input.

String() creates an unset return; caller guards with ?:

  <- colour as String: String()
  if name == "sky"
    colour: "blue"

If the condition doesn't match, the return stays unset. Caller uses if-guard: if found <- findColour("sky"). See Q1146, Q1088.

Example

defines module qa.functionsandmethods.functionoptionalreturn

  defines function

    findColour() as pure
      -> name as String
      <- colour as String: String()
      if name == "sky"
        colour: "blue"
      else if name == "grass"
        colour: "green"

  defines program

    FunctionOptionalReturnDemo()
      stdout <- Stdout()

      //Found: the guard sets the variable and runs the block. Not found: the return is unset, so the else runs
      for colourName in ["sky", "ocean"]
        if found <- findColour(colourName)
          stdout.println(`Found ${colourName}: ${found}`)
        else
          stdout.println(`No colour found for ${colourName}`)

Common mistakes

E08180 — A return variable must be initialised (or declared with ?). The return here has no initialiser. Optional is a valid type, but the simpler EK9 idiom is to initialise with an unset String() and let the caller guard with ?. See ek9 -h E08180 for details.

Incorrect:

      <- colour as Optional of String

Correct:

      <- colour as String: String()

Other ways to ask this

I need a function that sometimes returns a value and sometimes returns nothing
In Java I'd return Optional<String>. Write the EK9 function with optional return
Given a lookup function, return the found value or leave the return unset
Create a function whose return can be checked with ? by the caller

Coming from another language?

Java: Optional<String>. Kotlin: String?. Rust: Option<String>. EK9: return String() (unset) or a value — caller uses ? or guard.

Keywords: function, optional, lookup, unset, return, guard