E07520: must return a Boolean

What went wrong

A function or method is required to return Boolean but returns a different type.

Worked examples

Compilable examples demonstrating this error and the correct fix:

• ek9 -q 24 (How do I convert between types?)
• ek9 -q 25 (What is the promote operator?)
• ek9 -q 22 (How do I declare a variable in EK9?)
• ek9 -q 5 (How are EK9 source files structured?)
• ek9 -q 16 (How does indentation work in EK9?)

How to fix it

1. Ensure return type is Boolean

Common causes

• Predicate function with wrong return
• Condition implementation error

See also

• E07520: Boolean return required (this error)
• E07530: Boolean type required in context

Examples

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

Fails with E07520: must return a Boolean

#!ek9
defines module error.e07520.bad

  defines class

    Reading
      celsius <- 0.0

      //'?' answers yes or no
      override operator ? as pure
        <- rtn as Integer: 1

Fixed: Return a Boolean

#!ek9
defines module error.e07520.good

  defines class

    Reading
      celsius <- 0.0

      override operator ? as pure
        <- rtn as Boolean: celsius?

Questions and answers

These Q&A entries show E07520 and its fix:

• Q5: How are EK9 source files structured?
• Q16: How does indentation work in EK9?
• Q22: How do I declare a variable in EK9?
• Q24: How do I convert between types?
• Q25: What is the promote operator?
• Q28: How do I declare multiple variables at once?
• Q89: How do I use streams and stream pipelines in EK9?
• Q122: How do I use collect as to aggregate stream results in EK9?
• Q131: What is the EK9 equivalent of Python list comprehensions?
• Q184: How do I find the first item matching a condition in EK9?
• Q237: When should I use a stream pipeline vs a loop in EK9?
• Q238: What operators does EK9 support and why is the set fixed?
• Q239: How do comparison and ordering operators work in EK9?
• Q245: How do I implement all the operators my custom type needs?
• Q596: What is the difference between a function and a method in EK9?
• Q779: Why must certain operators return a value in EK9?
• Q1142: Sum the prices of all products over 20 using a stream pipeline.