In Turf, when you declare a variable with immutable (or its shorter spelling immut), you’re making a promise that its value will never change again after that line. Turf holds you to that promise at compile time.
This is fine – counter was declared mut, so it’s allowed to change:
mut counter: int = 0;counter = counter + 1;This isn’t, because pi was declared immut:
immut pi: double = 3.14159;pi = 3.14;Turf tells you exactly which variable you tried to reassign, so there’s no guessing which line caused it.
If you genuinely need this value to change later in your program, the fix is simple: go back to where it was declared and use mut (or mutable) instead of immut. If it really shouldn’t change, then the assignment itself is the mistake – you probably meant to introduce a brand new variable with a different name, rather than overwrite this one.
immutable-by-default thinking takes a little getting used to if you’re coming from a language where every variable can always change, but it’s a genuinely useful safety net: it means once you see immut, you know for certain that value is never going to shift under you later in the code.
Run turf --explain E060 in a terminal to read this again.