E4094

E4094#

Cannot modify a read-only field.

For a read-only struct, you cannot modify the value of a field using assignment outside of the package where the type is defined.

Erroneous example#

Suppose you have a package a in module username/hello:

a/moon.pkg:

a/a.mbt:

///|
pub struct T {
  mut value : Int
}

///|
pub fn T::new() -> T {
  T::{ value: 0 }
}

///|
pub fn T::set(self : T, value : Int) -> Unit {
  self.value = value
}

And you have another package b in the same module:

b/moon.pkg:

import {
  "moonbit-community/E4094/a",
}

b/b.mbt:

///|
test {
  let a = @a.T::new()
  a.value = 3 // Error: Cannot modify a read-only field: value
}

Suggestion#

If you have control over the package where the type is defined, expose mutation through a method in that package.

a/a.mbt:

///|
pub struct T {
  mut value : Int
}

///|
pub fn T::new() -> T {
  T::{ value: 0 }
}

///|
pub fn T::set(self : T, value : Int) -> Unit {
  self.value = value
}

b/b.mbt:

///|
test {
  let a = @a.T::new()
  a.set(3)
}
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