E07640: use <> for the not equal operator

What went wrong

A type declares 'operator !='. EK9's not-equal operator is '<>', and a type never writes it: '<>' is generated from the type's sameness - its '==' (equatable only) or its '<=>' (ordered) - so the two ways of saying "equal" cannot disagree (E07181). In an EXPRESSION 'a != b' is accepted and means 'a <> b'; it is only declaring an operator named '!=' that is refused.

How to fix it

1. Remove 'operator !='
2. Give the type its sameness instead: 'default operator ==' (or '<=>') - or write '==' (or '<=>') yourself - and '<>' comes with it
3. Compare with '<>' (or '!=') at the call site as before

The [EXAMPLE] section below shows 'operator !=' declared, and a Point whose '<>' is generated from its '==' and used in a comparison.

Worked examples

Compilable examples demonstrating this error and the correct fix:

• ek9 -q 1320 (Why can't I declare 'operator !=' on my EK9 type?)

Rationale

A type defines sameness once, and every comparison is generated from it. An operator called '!=' would be a second, hand-written not-equal beside the generated '<>', free to disagree with '=='. '<>' is the operator's one name - the form used in Pascal, SQL and BASIC - and composes with '<' and '>' as a family.

Common causes

• Habit from C++ or Kotlin, where 'operator!=' is declared by hand
• Porting a type that implemented equals() and a separate not-equals
• Copy-paste from a non-EK9 source

See also

• E07650: use ~ for the not operator
• E07181: a comparison written by hand that is generated from sameness

Examples

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

Fails with E07640: use <> for the not equal operator

#!ek9
defines module error.e07640.bad

  defines class

    Point

      //'<>' is EK9's not-equal
      operator != as pure
        -> other as Point
        <- rtn as Boolean: true

Fixed: Use '<>', generated from the type's sameness

#!ek9
defines module error.e07640.good

  defines class

    Point
      x <- 0
      y <- 0

      default private Point()

      Point()
        ->
          across as Integer
          down as Integer

        this.x: across
        this.y: down

      //Equatable sameness: '<>' is generated from '=='
      default operator ==

      default operator ?

      default operator #?

  defines program

    ComparePoints()
      stdout <- Stdout()
      first <- Point(1, 2)
      second <- Point(2, 1)
      if first <> second
        stdout.println("different points")

Questions and answers

These Q&A entries show E07640 and its fix:

• Q1320: Why can't I declare 'operator !=' on my EK9 type?