E08120: mutating variables is not allowed when scope is marked as 'pure'

What went wrong

You're calling a MUTATING operation (like +=, :=:, ++, --) inside a PURE context where mutations are forbidden.

Worked examples

Compilable examples demonstrating this error and the correct fix:

• ek9 -q 54 (What are pure functions and what is the difference betwee...)
• ek9 -q 273 (How does purity create security boundaries?)
• ek9 -q 52 (What are dynamic functions and how do they differ from la...)
• ek9 -q 53 (How do closures and variable capture work in EK9?)
• ek9 -q 268 (How does EK9 prevent common security vulnerabilities?)

Mutating operators

'++', '--', '+=', '-=', '*=', '/=', ':=:', ':~:', ':^:'

How to fix it

1. Use non-mutating alternatives (+ instead of +=)
2. Create new values instead of mutating
3. If mutation needed, remove 'pure' modifier

Common causes

• Using compound assignment in pure function
• Calling mutating method in pure scope

See also

• E08120: mutating operation in pure context (this error)
• E08130: non-pure call in pure context

Examples

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

Fails with E08120: mutating variables is not allowed when scope is marked as 'pure'

#!ek9
defines module error.e08120.bad

  defines function

    withReviewedTag() as pure
      -> tags as List of String
      <- tagged as List of String: tags
      tags += "reviewed"

Fixed: Return a new value instead of changing one

#!ek9
defines module error.e08120.good

  defines function

    withReviewedTag() as pure
      -> tags as List of String
      <- tagged as List of String: tags + "reviewed"

Questions and answers

These Q&A entries show E08120 and its fix:

• Q52: What are dynamic functions and how do they differ from lambdas?
• Q53: How do closures and variable capture work in EK9?
• Q54: What are pure functions and what is the difference between Consumer and Acceptor?
• Q268: How does EK9 prevent common security vulnerabilities?
• Q272: How do I build defense in depth in EK9?
• Q273: How does purity create security boundaries?
• Q560: What does 'as pure' mean on a method in EK9?
• Q589: How do I implement an abstract function with multiple implementations?
• Q590: Can dynamic functions extend abstract functions?
• Q592: What happens if a function extension has a signature mismatch?
• Q593: Can functions form multi-level inheritance chains?
• Q594: What are the rules for when a function can or cannot be extended?
• Q595: How does purity interact with abstract function extension?
• Q600: How does method overloading work compared to function dispatching?
• Q635: What operations are forbidden inside a pure method?
• Q690: How do pure function call chains work and what restrictions apply?
• Q790: Why can't I reassign variables in a pure function?
• Q1109: Mutate a Counter component's state through increment and reset methods.
• Q2000: If a parameter cannot be reassigned, is it read-only? How do aliasing and copying work in EK9?