E4087

The variable is not mutable.

MoonBit requires programmers to explicitly declare mutable variables. Notice that mutability in MoonBit only refers to the variable itself, not the value it holds. This means for Array[T], Ref[T], and mutable fields in struct, you don't have to declare them as mutable.

Erroneous Example

fn main {
  let a = 0
  a = 1 // Error: The variable a is not mutable.
  println(a)
}

Suggestion

To fix this error, you need to declare the variable as mutable by adding the mut keyword before the variable name.

fn main {
  let mut a = 0
  a = 1
  println(a)
}