E07950: '++' and '--' are statement-only operators in EK9 - they cannot be used in expressions

What went wrong

The '++' and '--' operators are STATEMENT-ONLY in EK9. They CANNOT be used in expressions where a value is expected.

Worked examples

Compilable examples demonstrating this error and the correct fix:

• ek9 -q 780 (Why can't I use ++ or -- in an expression in EK9?)

Rationale

In C/Java, 'y <- x++' has confusing and bug-prone semantics:

• In C: y gets the OLD value before increment (surprising!)
• In Java: y becomes an ALIAS to x (both reference same object!)

The Java case is particularly dangerous - subsequent changes to x unexpectedly affect y. EK9 eliminates this class of bugs.

Key principles

• '++' and '--' MUTATE the variable in-place
• They return VOID (no value to assign)
• Use them only as standalone statements

How to fix it

1. Use '++' or '--' as standalone statement
2. Or use explicit arithmetic (x + 1) for expression context

See also

• E07950: increment/decrement in expression (this error)
• E07510: mutating operator with pure modifier

Examples

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

Fails with E07950: '++' and '--' are statement-only operators in EK9 - they cannot be used in expressions

#!ek9
defines module error.e07950.bad

  defines function

    countUp()
      <- rtn as Integer: 5

      //A statement on its own
      rtn++

    nextOf()
      -> start as Integer
      //The number after start
      <- rtn as Integer: start++

Fixed: Use '++' and '--' as statements

#!ek9
defines module error.e07950.good

  defines function

    countUp()
      <- rtn as Integer: 5

      //A statement on its own
      rtn++

    nextOf()
      -> start as Integer
      //The number after start
      <- rtn as Integer: start + 1

Questions and answers

These Q&A entries show E07950 and its fix:

• Q664: Why are ++ and -- statement-only operators in EK9?
• Q780: Why can't I use ++ or -- in an expression in EK9?
• Q905: What happens when I use mutating operators in EK9?
• Q940: Can I use ++ in an assignment like y <- x++ in EK9?
• Q967: Does EK9 have ++ and -- operators? What are the rules?
• Q996: Show me ++ and -- being used in a real EK9 program.
• Q1057: Does ++ return a value in EK9? Can I write x <- y++?
• Q1103: Increment an Integer, a Float, and a Character value.
• Q1259: Show me how to use the ++ and -- operators on an Integer counter, demonstrating that they are statement-only and modify the variable in place.
• Q1260: Show me how to use ++ and -- on a Float temperature reading, demonstrating that floating-point types also support in-place increment and decrement.