E4087

变量是不可变的。

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.

错误示例：

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

建议

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)
}