Operators have a job: + - * / % do arithmetic on numbers, & | ^ << >> work on whole numbers, && and || combine two bools, ! flips a bool, ~ flips the bits of an int. If a value isn’t the kind the operator works on – a string, a struct, a bool where a number belongs – Turf can’t tell what the operation is supposed to mean, so it stops and names the operator and the two types.
This works:
mut sum: int = 3 + 4;mut avg: double = 7.0 / 2.0;This doesn’t, because "4" is text, not a number:
mut bad: int = 3 + "4"; // cannot apply '+' to 'int' and 'string'The same goes for the shorthand forms (x += "4"), x++ on a string, n && m or !n on ints (write n != 0), and ** or & on a type that isn’t a number.
To fix it, make both sides genuinely the kind the operator wants. If you have a number stored as text you need an actual numeric value first – Turf doesn’t parse text into numbers on its own.
If you wanted to join two pieces of text, + on two strings does that; it is a number and a string that can’t be mixed.
A struct can get its own + (or ==, [], …) by declaring an operator; without one, the operators above don’t apply to it.
(Division or remainder by a literal zero is a different thing: Turf warns about it at compile time – “this will crash at runtime” – and the program stops when it runs. It is not this error.)
Run turf --explain E057 in a terminal to read this again.