E0007

Warning name: unused_field

Field is never read. This include fields in structs and fields in enum constructors.

Erroneous example

///|
priv enum E {
  A(Int)
  B(value~ : Int)
}

///|
priv struct S {
  value : Int
}

///|
test {
  ignore(B(value=1))
  match A(1) {
    A(_) => println("A")
    B(_) => println("B")
  }
  ignore(S::{ value : 1 })
}

Suggestion

• If the fields in enum constructors are unused, you can expand them in the pattern to use them:

  
  ///|
  priv enum E {
    A(Int)
    B(value~ : Int)
  }
  
  ///|
  priv struct S {
    value : Int
  }
  
  ///|
  test {
    ignore(B(value=1))
    match A(1) {
      A(x) => inspect("A(\{x})", content="A(1)")
      B(value~) => inspect("B(\{value})")
    }
    let s = S::{ value: 1 }
    match s {
      { value } => println("S(\{value})")
    }
    println("S(\{s.value})")
  }
  

• If the fields are indeed useless, you can remove the field from the constructor:

  
  ///|
  priv enum E2 {
    A2
    B2
  }
  
  ///|
  priv struct S2 {}
  
  ///|
  test {
    ignore(E2::B2)
    match E2::A2 {
      A2 => println("A")
      B2 => println("B")
    }
    ignore(S2::{  })
  }
  

• If the fields are expected to be used by the others, you can mark the type as pub or pub(all).

  
  ///|
  pub enum E3 {
    A3(Int)
    B3(value~ : Int)
  }
  
  ///|
  pub struct S3 {
    value : Int
  }