E0007

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
}
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