E07180: 'default' of operators requires this type to have appropriate operator

What went wrong

You're using 'default operator X' but this type doesn't have a required prerequisite operator that the default implementation depends on.

Worked examples

Compilable examples demonstrating this error and the correct fix:

• ek9 -q 260 (What types can be used as Dict keys?)

Operator dependency chain

A type defines its SAMENESS once - '<=>' (ordered) or '==' (equatable only) - and the other comparisons are GENERATED from it, never requested:

from '<=>':  ==  <>  <  <=  >  >=
from '==':   <>

So asking for '<' on a type with no '<=>' has nothing to be generated from (this error). Declare the '<=>' instead - generated with 'default' or written by hand - and '<' comes with it. Asking for '<' beside a '<=>' is E07181 (redundant). A bare 'default operator' over a field that cannot be ordered (a Dict, a List) gives '==' compared field by field and no ordering operators.

Semantics note

• 'default operator ?' returns true only if ALL fields are set (one unset field is enough to make the whole object unset)
• 'default operator #?'/'$'/'$$' return UNSET if object is completely empty
• Custom '?' does NOT affect how 'default #?'/'$'/'$$' behave

How to fix it

1. Replace 'default operator <' with 'default operator <=>' (or a hand-written '<=>')
2. Or use 'default operator' to generate all at once
3. For an ordering by some other rule, write a comparator function ('sort by f')

Common causes

• Asking for '<' (or '<=', '>', '>=') instead of declaring '<=>'
• Not aware that comparisons are generated from the type's sameness

See also

• E07180: default operator missing prerequisite (this error)
• E07190: default operator missing in super class
• E07181: comparisons are generated from the sameness, never requested

Examples

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

Fails with E07180: 'default' of operators requires this type to have appropriate operator

#!ek9
defines module error.e07180.bad

  defines class

    Reading
      taken <- Time()

      //An ordering operator is generated from '<=>', and there is none
      default operator <

      default operator ?

Fixed: Default '<=>' instead: the comparisons come from it

#!ek9
defines module error.e07180.good

  defines class

    Reading
      taken <- Time()

      //Choose ordered sameness: '<', '<=', '>', '>=', '==' and '<>' are all generated from it
      default operator <=>

      default operator ?