E07895: cannot assign to a method-call result - not an assignable target; only mutating operators (+=, -=, :~:, :^:, :=:) apply to a call result

What went wrong

You're trying to assign to the RESULT of a method call. A call result is a temporary value, not a storage location, so there is nothing for a simple (:=) or guarded (:=?) assignment to store into.

A mutating operator IS allowed on a call result, because it modifies the returned object in place rather than storing into a slot: 'holder.current() += 1' compiles, while 'holder.current() := 99' has no location to assign into (both are in the [EXAMPLE] section below).

Worked examples

Compilable examples demonstrating this error and the correct fix:

• ek9 -q 1363 (Can I apply a mutating operator like += to a method-call ...)

How to fix it

1. To mutate the returned object, use a mutating operator (+=, -=, :~:, :^:, :=:) rather than an assignment
2. If you meant to replace what the method returns, add a setter method and call it: repository.setItems(List())
3. If you want to work on the returned value, take it into a variable first and work on that variable

Common causes

• Using := / :=? where a mutating operator (+=, :~:, ...) was intended
• Treating a getter's result as an assignable field
• Expecting an accessor to behave like a settable property

See also

• E07895: cannot assign to a method-call result (this error)
• E07890: not mutable (mutating a constant or a void call result)

Examples

Every example is compiled by the EK9 compiler: the first fails exactly as captioned, the others fix it.

Fails with E07895: cannot assign to a method-call result - not an assignable target; only mutating operators (+=, -=, :~:, :^:, :=:) apply to a call result

#!ek9
defines module error.e07895.bad

  defines class
    Holder
      count <- 0

      current()
        <- rtn as Integer: count

      default operator ?

  defines function

    assignToCallResult()
      -> holder as Holder

      //A call result is a value, not somewhere to store one
      holder.current() := 99

Fixed: Mutate it in place, or assign it to a variable first

#!ek9
defines module error.e07895.good

  defines class
    Holder
      count <- 0

      current()
        <- rtn as Integer: count

      default operator ?

  defines function

    useCallResult()
      -> holder as Holder
      <- rtn as Integer: 0

      //Mutate the returned object in place with a mutating operator
      holder.current() += 1

      //Or take it into a variable and work on that
      rtn: holder.current()
      rtn += 99

Questions and answers

These Q&A entries show E07895 and its fix:

• Q1363: Can I apply a mutating operator like += to a method-call result?