E01084: prefix operator used in suffix position - place BEFORE expression

What went wrong

You're using a PREFIX operator in SUFFIX position. PREFIX operators must come BEFORE the expression they operate on.

EK9 has several prefix operators that extract values or transform expressions:

• #< (first/prefix) - extracts first element: #< list
• #> (last/suffix) - extracts last element: #> list
• $ (string) - converts to string: $value
• $$ (JSON) - converts to JSON: $$value
• #? (hashcode) - gets the hash code: #? value (the length is 'length')
• #^ (promote) - type promotion: #^ value
• ~ (negate/reverse) - negates or reverses: ~ list
• not (boolean negate) - negates boolean: not condition
• abs (absolute) - absolute value: abs number
• sqrt (square root) - square root: sqrt number
• empty (empty check) - checks if empty: empty collection
• length (length) - gets length: length collection

Worked examples

Compilable examples demonstrating this error and the correct fix:

• ek9 -q 794 (Why does EK9 care about operator placement (prefix vs suf...)

Common causes

• Coming from languages where such operators are postfix (Ruby's .first, .last)
• Confusing '#<' (prefix/first) naming with its syntactic position
• Muscle memory from other operator positions

How to fix it

Move the operator to BEFORE the expression:

• list#< --> #< list
• value$ --> $value
• collection#? --> #? collection

Or use the equivalent method call if available.

See also

• E01085: Suffix operator used in prefix position
• https://ek9.io/operators.html

Examples

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

Fails with E01084: prefix operator used in suffix position - place BEFORE expression

#!ek9
defines module error.e01084.bad

  defines function

    firstLetter()
      -> name as String
      <- rtn as Character: name #<

Fixed: Put the prefix operator before the expression: #< name

#!ek9
defines module error.e01084.good

  defines function

    firstLetter()
      -> name as String
      <- rtn as Character: #< name

Questions and answers

These Q&A entries show E01084 and its fix:

• Q794: Why does EK9 care about operator placement (prefix vs suffix)?